Mollusca Temporal range: Cambrian Stage 2–Recent
Tonicella lineata, a polyplacophoran or chiton, anterior end towards the right
Scientific classification
Kingdom:	Animalia
Superphylum:	Lophotrochozoa
Phylum:	Mollusca Linnaeus, 1758
Classes
See text.
Diversity[1]
85,000 recognized living species.

The Annelids – Phylum Annelida The annelids, or segmented worms, occupy marine, aquatic and aquatic habitats. Annelids are coelomate protostomes and typically, are elongated ‘worm-like’ animals. By far, the most distinguishing characteristic of annelids is metamerism, the serial repetition of body parts which gives them a segmented appearance.

Serial Repetition Of Segments Is A Defining Characteristic Of Which Worm Phylum

Mollusca is the second-largest phylum of invertebrate animals after the arthropoda. The members are known as molluscs or mollusks^{[note 1]} (/ˈmɒləsk/). Around 85,000 extantspecies of molluscs are recognized.^[2] The number of fossil species is estimated between 60,000 and 100,000 additional species.^[3] The proportion of undescribed species is very high. Many taxa remain poorly studied.^[4]

Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrialhabitats. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. The phylum is typically divided into 8 or 9 taxonomicclasses, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species.

The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a 'hypothetical ancestral mollusc' (see image below). This has a single, 'limpet-like' shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular 'foot'. Although molluscs are coelomates, the coelom tends to be small.The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The 'generalized' mollusc's feeding system consists of a rasping 'tongue', the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered 'hairs' called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophorelarvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.

Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

Molluscs have been and still are an important food source for anatomically modern humans. A risk of food poisoning exists from toxins that can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies.

Mollusc species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis, or snail fever) is transmitted to humans by water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.

• 4Hypothetical ancestral mollusc
• 5Ecology
• 7Evolutionary history
• 8Human interaction
  • 8.1Uses by humans
  • 8.2Harmful to humans

Etymology[edit]

The words mollusc and mollusk are both derived from the French mollusque, which originated from the Latinmolluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκιαta malákia (the soft ones; < μαλακόςmalakós 'soft'), which he applied inter alia to cuttlefish.^[5][6] The scientific study of molluscs is accordingly called malacology.^[7]

The name Molluscoida was formerly used to denote a division of the animal kingdom containing the brachiopods, bryozoans, and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs. As now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned.^[8]

Definition[edit]

The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell.^[9]

Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult.^[10] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.^[11] The following are present in all modern molluscs:^[12][13]

• The dorsal part of the body wall is a mantle (or pallium) which secretescalcareousspicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity.
• The anus and genitals open into the mantle cavity.
• There are two pairs of main nerve cords.^[13]

Other characteristics that commonly appear in textbooks have significant exceptions:

Whether characteristic is found in these classes of Molluscs
Supposed universal Molluscan characteristic[12]	Aplacophora[14]	Polyplacophora[15]	Monoplacophora[16]	Gastropoda[17]	Cephalopoda[18]	Bivalvia[19]	Scaphopoda[20]
Radula, a rasping 'tongue' with chitinous teeth	Absent in 20% of Neomeniomorpha	Yes	Yes	Yes	Yes	No	Internal, cannot extend beyond body
Broad, muscular foot	Reduced or absent	Yes	Yes	Yes	Modified into arms	Yes	Small, only at 'front' end
Dorsal concentration of internal organs (visceral mass)	Not obvious	Yes	Yes	Yes	Yes	Yes	Yes
Large digestive ceca	No ceca in some Aplacophora	Yes	Yes	Yes	Yes	Yes	No
Large complex metanephridia ('kidneys')	None	Yes	Yes	Yes	Yes	Yes	Small, simple
One or more valves/ shells	Primitive forms, yes; modern forms, no	Yes	Yes	Snails, yes; slugs, mostly yes (internal vestigial)	Octopuses, no; cuttlefish, nautilus, squid, yes	Yes	Yes
Odontophore	Yes	Yes	Yes	Yes	Yes	No	Yes

Diversity[edit]

Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.^[1] In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were about 12,000 fresh-water gastropods and 35,000 terrestrial. The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of the living molluscs.^[22] In 2009, Chapman estimated the number of described living species at 85,000.^[1] Haszprunar in 2001 estimated about 93,000 named species,^[23] which include 23% of all named marine organisms.^[24] Molluscs are second only to arthropods in numbers of living animal species^[21]—far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.^[25] About 200,000 living species in total are estimated,^[1][26] and 70,000 fossil species,^[12] although the total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today.^[27]

Molluscs have more varied forms than any other animal phylum. They include snails, slugs and other gastropods; clams and other bivalves; squids and other cephalopods; and other lesser-known but similarly distinctive subgroups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes.^[10] About 80% of all known mollusc species are gastropods.^[21]Cephalopoda such as squid, cuttlefish, and octopuses are among the neurologically most advanced of all invertebrates.^[28] The giant squid, which until recently had not been observed alive in its adult form,^[29] is one of the largest invertebrates, but a recently caught specimen of the colossal squid, 10 m (33 ft) long and weighing 500 kg (1,100 lb), may have overtaken it.^[30]

Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of nonmarine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered nonmarine molluscs. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of nonmarine species.^[31]

Hypothetical ancestral mollusc[edit]

Because of the great range of anatomical diversity among molluscs, many textbooks start the subject of molluscan anatomy by describing what is called an archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc (HAM) to illustrate the most common features found within the phylum. The depiction is visually rather similar to modern monoplacophorans.^{[10][13][16][32]}

The generalized mollusc is bilaterally symmetrical and has a single, 'limpet-like' shell on top. The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular 'foot'.^[13] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.^[11]

Mantle and mantle cavity[edit]

The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat, to sea, fresh water or air. The cavity was at the rear in the earliest molluscs, but its position now varies from group to group. The anus, a pair of osphradia (chemical sensors) in the incoming 'lane', the hindmost pair of gills and the exit openings of the nephridia ('kidneys') and gonads (reproductive organs) are in the mantle cavity.^[13] The whole soft body of bivalves lies within an enlarged mantle cavity.^[11]

Shell[edit]

The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs^[11]) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate),^[13][33] except the outermost layer, which in almost all cases is all conchiolin (see periostracum).^[13] Molluscs never use phosphate to construct their hard parts,^[34] with the questionable exception of Cobcrephora.^[35]While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.^[36]

The shell consists of three layers: the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite, and an inner layer consisting of laminated calcite, often nacreous.^[11]

In some forms the shell contains openings. In abalones there are holes in the shell used for respiration and the release of egg and sperm, in the nautilus a string of tissue called the siphuncle goes through all the chambers, and the eight plates that make up the shell of chitons are penetrated with living tissue with nerves and sensory structures.^[37]

Foot[edit]

The underside consists of a muscular foot, which has adapted to different purposes in different classes.^[38]:4 The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.^[13] In bivalves, the foot is adapted for burrowing into the sediment;^[38]:4 in cephalopods it is used for jet propulsion,^[38]:4 and the tentacles and arms are derived from the foot.^[39]

Circulatory system[edit]

Most molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton.^[11] The blood of these molluscs contains the respiratory pigmenthemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.^[13] The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine. A pair of nephridia ('little kidneys') to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.^[13]

Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood.

Respiration[edit]

Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.^[13]

Eating, digestion, and excretion[edit]

Members of the mollusc family use intracellular digestion to function. Most molluscs have muscular mouths with radulae, 'tongues', bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks, and is associated with the odontophore, a cartilaginous supporting organ.^[11] The radula is unique to the molluscs and has no equivalent in any other animal.

Molluscs' mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny 'hairs') drive the mucus towards the stomach, so the mucus forms a long string called a 'food string'.^[13]

At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it.^[13]

The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect.^[13]

Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus, in the part of the mantle cavity, is swept by the outgoing 'lane' of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.^[13]

As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus.^[11]

Nervous system[edit]

The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar.^[13]

The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of the three functioning as the principal center of 'thinking'. Some such as the scallops have eyes around the edges of their shells which connect to a pair of looped nerves and which provide the ability to distinguish between light and shadow.

Reproduction[edit]

The simplest molluscan reproductive system relies on external fertilization, but with more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to the coelom, a small cavity that surrounds the heart, into which they shed ova or sperm. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some molluscs use internal fertilization and/or are hermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems.^[13]

The most basic molluscan larva is a trochophore, which is planktonic and feeds on floating food particles by using the two bands of cilia around its 'equator' to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of mesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the 'equatorial' band of cilia nearest the apical tuft, develops into the velum ('veil'), a pair of cilia-bearing lobes with which the larva swims. Eventually, the larva sinks to the seafloor and metamorphoses into the adult form. While metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult.^[41]

Ecology[edit]

Feeding[edit]

Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as a 'rake' to comb up filaments from the sea floor. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts.^[42]Filter feeders are molluscs that feed by straining suspended matter and food particle from water, typically by passing the water over their gills. Most bivalves are filter feeders.

Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in the usual fashion, but its diet includes protists such as the xenophyophoreStannophyllum.^[43]Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls,^[44] whereas doridnudibranchs and some Vetigastropoda feed on sponges^[45][46] and others feed on hydroids.^[47] (An extensive list of molluscs with unusual feeding habits is available in the appendix of GRAHAM, A. (1955). 'Molluscan diets'. Journal of Molluscan Studies. 31 (3–4): 144..)

Classification[edit]

Opinions vary about the number of classes of molluscs; for example, the table below shows seven living classes,^[23] and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora.^[14][32] Two of the commonly recognized 'classes' are known only from fossils.^[21]

Class	Major organisms	Described living species[23]	Distribution
Gastropoda[48]	All the snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterfly	70,000	marine, freshwater, land
Bivalvia[49]	clams, oysters, scallops, geoducks, mussels	20,000	marine, freshwater
Polyplacophora[15]	chitons	1,000	rocky tidal zone and seabed
Cephalopoda[50]	squid, octopus, cuttlefish, nautilus, spirula	900	marine
Scaphopoda[20]	tusk shells	500	marine 6–7,000 metres (20–22,966 ft)
Aplacophora[14]	worm-like organisms	320	seabed 200–3,000 metres (660–9,840 ft)
Monoplacophora[16]	An ancient lineage of molluscs with cap-like shells	31	seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)
Rostroconchia †[51]	fossils; probable ancestors of bivalves	extinct	marine
Helcionelloida †[52]	fossils; snail-like organisms such as Latouchella	extinct	marine

Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora.^[53] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. Tentaculita may also be in Mollusca (see Tentaculites).

Evolutionary history[edit]

Fossil record[edit]

Good evidence exists for the appearance of gastropods (e.g. Aldanella), cephalopods (e.g. Plectronoceras, ?Nectocaris) and bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian period, c. 500 million years ago, though arguably each of these may belong only to the stem lineage of their respective classes.^[54] However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated.

Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about 555 million years ago, has been described by some paleontologists as 'mollusc-like',^[55][56] but others are unwilling to go further than 'probable bilaterian',^[57][58] if that.^[59]

There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms.^[57][60] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from are fragments of a genuinely mollusc-like radula.^[61] This appears to contradict the concept that the ancestral molluscan radula was mineralized.^[62]

The tiny Helcionellid fossil Yochelcionella is thought to be an early mollusc[52]

Spirally coiled shells appear in many gastropods.[17]

However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China,^[63][64] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites.^[52] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults.^[65]

Some analyses of helcionellids concluded these were the earliest gastropods.^[66] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head.^[17][67][68]

Volborthella, some fossils of which predate 530 million years ago, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthella's classification is uncertain.^[69] The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a 'ballast' of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface.^[70] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago.^[71] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.^[72]

The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves.^{[73][74][75][76]} 'Modern-looking' bivalves appeared in the Ordovician period, .^[77] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event.^[78] Even so, bivalves remain abundant and diverse.

The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life.^[79]

Phylogeny[edit]

Lophotrochozoa

Mollusca Monoplacophorans ('limpet-like', 'living fossils') Gastropods (snails, slugs, limpets, sea hares) Cephalopods (nautiloids, ammonites, squid, etc.) Scaphopods (tusk shells) Aplacophorans (spicule-covered, worm-like) Polyplacophorans (chitons) Halwaxiids

The phylogeny (evolutionary 'family tree') of molluscs is a controversial subject. In addition to the debates about whether Kimberella and any of the 'halwaxiids' were molluscs or closely related to molluscs,^{[56][57][60][61]} debates arise about the relationships between the classes of living molluscs.^[58] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.^[82]

Molluscs are generally regarded members of the Lophotrochozoa,^[80] a group defined by having trochophore larvae and, in the case of living Lophophorata, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla.^[83] The diagram on the right summarizes a phylogeny presented in 2007.

Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs.^[84] For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm.^[85] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,^[10] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a 'shell' that was mineralized.^[58] In one particular branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans; but this is difficult to reconcile with the embryological origins of spicules.^[84]

The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills.^[52] Eventually, the cuticle would have become mineralized,^[52] using the same genetic machinery (engrailed) as most other bilaterian skeletons.^[85] The first mollusc shell almost certainly was reinforced with the mineral aragonite.^[33]

The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely supported reconstructions:

Molluscs Aculifera Conchifera

Molluscs Testaria

Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,^[86] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups.^[87]

However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.^[82] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated.^[88] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree.^[89] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.^[90]

Human interaction[edit]

For millennia, molluscs have been a source of food for humans, as well as important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, and chemical compounds. Their shells have also been used as a form of currency in some preindustrial societies. A number of species of molluscs can bite or sting humans, and some have become agricultural pests.

Uses by humans[edit]

Molluscs, especially bivalves such as clams and mussels, have been an important food source since at least the advent of anatomically modern humans, and this has often resulted in overfishing.^[91] Other commonly eaten molluscs include octopuses and squids, whelks, oysters, and scallops.^[92] In 2005, China accounted for 80% of the global mollusc catch, netting almost 11,000,000 tonnes (11,000,000 long tons; 12,000,000 short tons). Within Europe, France remained the industry leader.^[93] Some countries regulate importation and handling of molluscs and other seafood, mainly to minimize the poison risk from toxins that can sometimes accumulate in the animals.^[94]

Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable.^[17][19] The best natural pearls are produced by marine pearl oysters, Pinctada margaritifera and Pinctada mertensi, which live in the tropical and subtropical waters of the Pacific Ocean. Natural pearls form when a small foreign object gets stuck between the mantle and shell.

The two methods of culturing pearls insert either 'seeds' or beads into oysters. The 'seed' method uses grains of ground shell from freshwater mussels, and overharvesting for this purpose has endangered several freshwater mussel species in the southeastern United States.^[19] The pearl industry is so important in some areas, significant sums of money are spent on monitoring the health of farmed molluscs.^[95]

Other luxury and high-status products were made from molluscs. Tyrian purple, made from the ink glands of murex shells, 'fetched its weight in silver' in the fourth century BC, according to Theopompus.^[96] The discovery of large numbers of Murex shells on Crete suggests the Minoans may have pioneered the extraction of 'imperial purple' during the Middle Minoan period in the 20th–18th centuries BC, centuries before the Tyrians.^[97][98]Sea silk is a fine, rare, and valuable fabric produced from the long silky threads (byssus) secreted by several bivalve molluscs, particularly Pinna nobilis, to attach themselves to the sea bed.^[99]Procopius, writing on the Persian wars circa 550 CE, 'stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from lana pinna. Apparently, only the ruling classes were allowed to wear these chlamys.'^[100]

Mollusc shells, including those of cowries, were used as a kind of money (shell money) in several preindustrial societies. However, these 'currencies' generally differed in important ways from the standardized government-backed and -controlled money familiar to industrial societies. Some shell 'currencies' were not used for commercial transactions, but mainly as social status displays at important occasions, such as weddings.^[101] When used for commercial transactions, they functioned as commodity money, as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or 'goldrush' behavior appeared.^[102]

Bioindicators[edit]

Bivalve molluscs are used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments. Their population status or structure, physiology, behaviour or the level of contamination with elements or compounds can indicate the state of contamination status of the ecosystem. They are particularly useful since they are sessile so that they are representative of the environment where they are sampled or placed.^[103]Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial agriculture.

Harmful to humans[edit]

Stings and bites[edit]

Some molluscs sting or bite, but deaths from mollusc venoms total less than 10% of those from jellyfish stings.^[105]

All octopuses are venomous,^[106] but only a few species pose a significant threat to humans. Blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked,^[104] but their venom kills 25% of human victims. Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly,^[107] and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated.^[108]

All species of cone snails are venomous and can sting painfully when handled, although many species are too small to pose much of a risk to humans, and only a few fatalities have been reliably reported. Their venom is a complex mixture of toxins, some fast-acting and others slower but deadlier.^{[109][105][110]} The effects of individual cone-shell toxins on victims' nervous systems are so precise as to be useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them.^[109][111]

Disease vectors[edit]

Schistosomiasis (also known as bilharzia, bilharziosis or snail fever), a disease caused by the fluke worm Schistosoma, is 'second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease – 100 million in Africa alone.'^[112] The parasite has 13 known species, two of which infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts.^[113]

Pests[edit]

Some species of molluscs, particularly certain snails and slugs, can be serious crop pests,^[114] and when introduced into new environments, can unbalance local ecosystems. One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean and Pacific Ocean. In the 1990s, this species reached the West Indies. Attempts to control it by introducing the predatory snail Euglandina rosea proved disastrous, as the predator ignored Achatina fulica and went on to extirpate several native snail species, instead.^[115]

Notes[edit]

1. ^The formerly dominant spelling mollusk is still used in the U.S. — see the reasons given in Gary Rosenberg's 'Mollusckque - Mollusk vs Mollusc'. For the spelling mollusc, see the reasons given in: Brusca & Brusca. Invertebrates (2nd ed.).

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101. ^Maurer, B. (October 2006). 'The Anthropology of Money'(PDF). Annual Review of Anthropology. 35: 15–36. doi:10.1146/annurev.anthro.35.081705.123127. Archived from the original(PDF) on 2007-08-16.Cite uses deprecated parameter |dead-url= (help)
102. ^Hogendorn, J. & Johnson, M. (2003). The Shell Money of the Slave Trade. Cambridge University Press. ISBN978-0521541107. Particularly chapters 'Boom and slump for the cowrie trade' (pages 64–79) and 'The cowrie as money: transport costs, values and inflation' (pages 125–147)
103. ^Université Bordeaux; et al. 'MolluSCAN eye project'. Retrieved 2017-01-28.
104. ^ ^abAlafaci, A. (5 June 2018). 'Blue ringed octopus'. Australian Venom Research Unit. Retrieved 2008-10-03.
105. ^ ^abWilliamson, J.A.; Fenner, P.J.; Burnett, J.W.; Rifkin, J. (1996). Venomous and Poisonous Marine Animals: A Medical and Biological Handbook. UNSW Press. pp. 65–68. ISBN978-0-86840-279-6.
106. ^Anderson, R.C. (1995). 'Aquarium husbandry of the giant Pacific octopus'. Drum and Croaker. 26: 14–23.
107. ^Brazzelli, V.; Baldini, F.; Nolli, G.; Borghini, F.; Borroni, G. (March 1999). 'Octopus apollyon bite'. Contact Dermatitis. 40 (3): 169–70. doi:10.1111/j.1600-0536.1999.tb06025.x. PMID10073455.
108. ^Anderson, R.C. (1999). 'An octopus bite and its treatment'. The Festivus. 31: 45–46.
109. ^ ^abcConcar, D. (19 October 1996). 'Doctor snail—Lethal to fish and sometimes even humans, cone snail venom contains a pharmacopoeia of precision drugs'. New Scientist. Retrieved 2008-10-03.
110. ^Livett, B. 'Cone Shell Mollusc Poisoning, with Report of a Fatal Case'. Department of Biochemistry and Molecular Biology, University of Melbourne.
111. ^Haddad Junior, V.; Paula Neto, J.O.B.D.; Cobo, V.L.J. (September–October 2006). 'Venomous mollusks: The risks of human accidents by conus snails (gastropoda: Conidae) in Brazil'. Revista da Sociedade Brasileira de Medicina Tropical. 39 (5): 498–500. doi:10.1590/S0037-86822006000500015. PMID17160331.
112. ^'The Carter Center Schistosomiasis Control Program'. The Carter Center. Retrieved 2008-10-03.
113. ^Brown, D.S. (1994). Freshwater Snails of Africa and Their Medical Importance. CRC Press. p. 305. ISBN978-0-7484-0026-3.
114. ^Barker, G.M. (2002). Molluscs As Crop Pests. CABI Publications. ISBN978-0-85199-320-1.
115. ^Civeyrel, L.; Simberloff, D. (October 1996). 'A tale of two snails: is the cure worse than the disease?'. Biodiversity and Conservation. 5 (10): 1231–1252. doi:10.1007/BF00051574.

Bibliography[edit]

• Ruppert, E.E.; Fox, R.S.; Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. ISBN978-0-03-025982-1.

Further reading[edit]

• Sturm, C.; Pearce, T.A. & Valdes, A. The Mollusks: A Guide to their Study, Collection, and Preservation. Universal Publishers. 2006. 454 pages. ISBN1581129300
• Trigo, J.E.; Díaz Agras, G.J.; García-Álvarez, O.L.; Guerra, A.; Moreira, J.; Pérez, J.; Rolán, E.; Troncoso, J.S. & Urgorri, V. (2018). Troncoso, J.S., Trigo, J.E. & Rolán, E., ed. Guía de los Moluscos Marinos de Galicia. Vigo: Servicio de Publicacións da Universidade de Vigo. 836 pages. ISBN978-84-8158-787-6

External links[edit]

Wikimedia Commons has media related to Mollusca.

The Wikibook Dichotomous Key has a page on the topic of: Mollusca

Wikisource has the text of the 1911 Encyclopædia Britannica article Mollusca.

• Data related to Mollusca at Wikispecies
• 'Mollusca' at the Encyclopedia of Life
• Rotterdam Natural History Museum Shell Image Gallery
• Online biomonitoring of bivalve activity, 24/7: MolluSCAN eye

Annelida Temporal range: Early Ordovician–Recent[1]
Glycera sp.
Scientific classification
Kingdom:	Animalia
Superphylum:	Lophotrochozoa
Phylum:	Annelida Lamarck, 1809
Classes and subclasses
Class Polychaeta (paraphyletic?) Class Clitellata (see below) Oligochaeta – earthworms, etc. Branchiobdellida Hirudinea – leeches Sipuncula (previously a phylum) Class Machaeridia†

The annelids (Annelida, from Latinanellus, 'little ring'^[2][a]), also known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extantspecies including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.

The annelids are bilaterally symmetrical, triploblastic, coelomate, invertebrate organisms. They also have parapodia for locomotion. Most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a 'super-phylum' of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.

The basic annelid form consists of multiple segments. Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in others, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles that act as hearts. The septa of such species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ('ripples' that pass along the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Earthworms are oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-letting is used less frequently by doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.

Since annelids are soft-bodied, their fossils are rare – mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago. Palaeontologists disagree about whether some body fossils from the mid Ordovician, about , are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the Tertiary period, which began 66 million years ago.^[4]

• 3Description
  • 3.7Reproduction and life cycle
• 6Evolutionary history

Classification and diversity[edit]

There are over 22,000 living annelid species,^[5][6] ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus (Cognetti, 1922), which can both grow up to 3 metres (9.8 ft) long.^[6][7][8] Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids,^[9][10] most textbooks use the traditional classification into the following sub-groups:^[7][11]

• Polychaetes (about 12,000 species^[5]). As their name suggests, they have multiple chetae ('hairs') per segment. Polychaetes have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors.^[7] Most are marine animals, although a few species live in fresh water and even fewer on land.^[12]

Serial Repetition Of Segments Is A Defining Characteristic Of Which Worm Phylum

• Clitellates (about 10,000 species ^[6]). These have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum ('pack saddle') around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch ^[11][13] or, in moniligastrids, yolky eggs that provide nutrition for the embryos.^[6] The clitellates are sub-divided into:^[7]
  • Oligochaetes ('with few hairs'), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth.^[7] Most are burrowers that feed on wholly or partly decomposed organic materials.^[12]
  • Hirudinea, whose name means 'leech-shaped' and whose best known members are leeches.^[7] Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators.^[12] They have suckers at both ends of their bodies, and use these to move rather like inchworms.^[14]

The Archiannelida, minute annelids that live in the spaces between grains of marine sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes.^[11] Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:

• Pogonophora / Siboglinidae were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate phylum, Pogonophora, or as two phyla, Pogonophora and Vestimentifera. More recently they have been re-classified as a family, Siboglinidae, within the polychaetes.^[12][15]
• The Echiura have a checkered taxonomic history: in the 19th century they were assigned to the phylum 'Gephyrea', which is now empty as its members have been assigned to other phyla; the Echiura were next regarded as annelids until the 1940s, when they were classified as a phylum in their own right; but a molecular phylogenetics analysis in 1997 concluded that echiurans are annelids.^[5][15][16]
• Myzostomida live on crinoids and other echinoderms, mainly as parasites. In the past they have been regarded as close relatives of the trematodeflatworms or of the tardigrades, but in 1998 it was suggested that they are a sub-group of polychaetes.^[12] However, another analysis in 2002 suggested that myzostomids are more closely related to flatworms or to rotifers and acanthocephales.^[15]
• Sipuncula was originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters. The phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida.^[17] Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Myzostomida and Annelida (including echiurans and pogonophorans).^[18] It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.^[19]

Distinguishing features[edit]

No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive combination of features. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and internally by septa ('partitions') at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent.^[7][11] Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen^[7] and does not molt^[20] – on the other hand arthropods' cuticles are made of the more rigid α-chitin,^[7][21] and molt until the arthropods reach their full size.^[22] Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.^[20]

Summary of distinguishing features

Annelida[7]	Recently merged into Annelida[9]		Closely related	Similar-looking phyla
	Echiura[23]	Sipuncula[24]	Nemertea[25]	Arthropoda[26]	Onychophora[27]
External segmentation	Yes	no	no	Only in a few species	Yes, except in mites	no
Repetition of internal organs	Yes	no	no	Yes	In primitive forms	Yes
Septa between segments	In most species	no	no	No	No	No
Cuticle material	collagen	collagen	collagen	none	α-chitin	α-chitin
Molting	Generally no;[20] but some polychaetes molt their jaws, and leeches molt their skins[28]	no[29]	no[29]	no[29]	Yes[22]	Yes
Body cavity	Coelom; but this is reduced or missing in many leeches and some small polychaetes[20]	two coelomata, main and in proboscis	two coelomata, main and in tentacles	Coelom only in proboscis	Hemocoel	Hemocoel
Circulatory system	Closed in most species	Open outflow, return via branched vein	Open	Closed	Open	Open

Description[edit]

Segmentation[edit]

Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external chaetae (Greek χαιτη, meaning 'hair') and, in some species, appendages. The frontmost and rearmost sections are not regarded as true segments as they do not contain the standard sets of organs and do not develop in the same way as the true segments. The frontmost section, called the prostomium (Greek προ- meaning 'in front of' and στομα meaning 'mouth') contains the brain and sense organs, while the rearmost, called the pygidium (Greek πυγιδιον, meaning 'little tail') or periproct contains the anus, generally on the underside. The first section behind the prostomium, called the peristomium (Greek περι- meaning 'around' and στομα meaning 'mouth'), is regarded by some zoologists as not a true segment, but in some polychaetes the peristomium has chetae and appendages like those of other segments.^[7]

The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is called teloblastic growth.^[7] Some groups of annelids, including all leeches,^[14] have fixed maximum numbers of segments, while others add segments throughout their lives.^[11]

The phylum's name is derived from the Latin word annelus, meaning 'little ring'.^[5]

Body wall, chetae and parapodia[edit]

Annelids' cuticles are made of collagen fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis (outermost skin layer). A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, and mucus-secreting glands in the epidermis protect their skins.^[7] Under the epidermis is the dermis, which is made of connective tissue, in other words a combination of cells and non-cellular materials such as collagen. Below this are two layers of muscles, which develop from the lining of the coelom (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips,^[20] whose contractions make the segment shorter and fatter.^[7] Some annelids also have oblique internal muscles that connect the underside of the body to each side.^[20]

The setae ('hairs') of annelids project out from the epidermis to provide traction and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. The parapodia ('limbs') of annelids that have them often bear more complex chetae at their tips – for example jointed, comb-like or hooked.^[7] Chetae are made of moderately flexible β-chitin and are formed by follicles, each of which has a chetoblast ('hair-forming') cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by forming microvilli, fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta.^[7] Hence annelids' chetae are structurally different from the setae ('bristles') of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle.^[7]

Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri (fused bundles of cilia) and gills.^[20]

Nervous system and senses[edit]

The brain generally forms a ring round the pharynx (throat), consisting of a pair of ganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it.^[7] The brains of polychaetes are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the prostomium.^[30] In some very mobile and active polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections.^[20] The rest of the central nervous system, the ventral nerve cord, is generally 'ladder-like', consisting of a pair of nerve cords that run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection. From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body.^[7] However, in most polychaetes the two main nerve cords are fused, and in the tube-dwelling genusOwenia the single nerve chord has no ganglia and is located in the epidermis.^[11][31]

As in arthropods, each muscle fiber (cell) is controlled by more than one neuron, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons. Vertebrates have a different system, in which one neuron controls a group of muscle fibers.^[7] Most annelids' longitudinal nerve trunks include giant axons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement.^[7]

The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body.^[7] Nuchal ('on the neck') organs are paired, ciliated structures found only in polychaetes, and are thought to be chemosensors.^[20] Some polychaetes also have various combinations of ocelli ('little eyes') that detect the direction from which light is coming and camera eyes or compound eyes that can probably form images.^[31] The compound eyes probably evolved independently of arthropods' eyes.^[20] Some tube-worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withdraw into their tubes.^[31] Some burrowing and tube-dwelling polychaetes have statocysts (tilt and balance sensors) that tell them which way is down.^[31] A few polychaete genera have on the undersides of their heads palps that are used both in feeding and as 'feelers', and some of these also have antennae that are structurally similar but probably are used mainly as 'feelers'.^[20]

Coelom, locomotion and circulatory system[edit]

Most annelids have a pair of coelomata (body cavities) in each segment, separated from other segments by septa and from each other by vertical mesenteries. Each septum forms a sandwich with connective tissue in the middle and mesothelium (membrane that serves as a lining) from the preceding and following segments on either side. Each mesentery is similar except that the mesothelium is the lining of each of the pair of coelomata, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it.^[7] The mesothelium is made of modified epitheliomuscular cells;^[7] in other words, their bodies form part of the epithelium but their bases extend to form muscle fibers in the body wall.^[32] The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium, especially on the outside of the gut, may also form chloragogen cells that perform similar functions to the livers of vertebrates: producing and storing glycogen and fat; producing the oxygen-carrier hemoglobin; breaking down proteins; and turning nitrogenous waste products into ammonia and urea to be excreted.^[7]

Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body),^[7] or flex the body while using parapodia to crawl or swim.^[33] In these animals the septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled 'balloon'.^[7] However, the septa are often incomplete in annelids that are semi-sessile or that do not move by peristalsis or by movements of parapodia – for example some move by whipping movements of the body, some small marine species move by means of cilia (fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themselves into it.^[7]

The fluid in the coelomata contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain a respiratory pigment – red hemoglobin in some species, green chlorocruorin in others (dissolved in the plasma)^[20] – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the blood plasma. Species with well-developed septa generally also have blood vessels running all long their bodies above and below the gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks of capillaries in the body wall and around the gut transfer blood between the main blood vessels and to parts of the segment that need oxygen and nutrients. Both of the major vessels, especially the upper one, can pump blood by contracting. In some annelids the forward end of the upper blood vessel is enlarged with muscles to form a heart, while in the forward ends of many earthworms some of the vessels that connect the upper and lower main vessels function as hearts. Species with poorly developed or no septa generally have no blood vessels and rely on the circulation within the coelom for delivering nutrients and oxygen.^[7]

However, leeches and their closest relatives have a body structure that is very uniform within the group but significantly different from that of other annelids, including other members of the Clitellata.^[14] In leeches there are no septa, the connective tissue layer of the body wall is so thick that it occupies much of the body, and the two coelomata are widely separated and run the length of the body. They function as the main blood vessels, although they are side-by-side rather than upper and lower. However, they are lined with mesothelium, like the coelomata and unlike the blood vessels of other annelids. Leeches generally use suckers at their front and rear ends to move like inchworms. The anus is on the upper surface of the pygidium.^[14]

Respiration[edit]

In some annelids, including earthworms, all respiration is via the skin. However, many polychaetes and some clitellates (the group to which earthworms belong) have gills associated with most segments, often as extensions of the parapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.^[20]

Feeding and excretion[edit]

Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular pharynx that can be everted (turned inside out to extend it). In these animals the foremost few segments often lack septa so that, when the muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Two families, the Eunicidae and Phyllodocidae, have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter. On the other hand, some predatory polychaetes have neither jaws nor eversible pharynges. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths. Filter feeders use 'crowns' of palps covered in cilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth.^[20]

The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the anus on the underside of the pygidium.^[7] However, in members of the tube-dwelling family Siboglinidae the gut is blocked by a swollen lining that houses symbioticbacteria, which can make up 15% of the worms' total weight. The bacteria convert inorganic matter – such as hydrogen sulfide and carbon dioxide from hydrothermal vents, or methane from seeps – to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria.^[20]

Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia.^[7] Both of these systems use a two-stage filtration process, in which fluid and waste products are first extracted and these are filtered again to re-absorb any re-usable materials while dumping toxic and spent materials as urine. The difference is that protonephridia combine both filtration stages in the same organ, while metanephridia perform only the second filtration and rely on other mechanisms for the first – in annelids special filter cells in the walls of the blood vessels let fluids and other small molecules pass into the coelomic fluid, where it circulates to the metanephridia.^[34] In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment. As a result, the hindmost segment (before the growth zone and pygidium) has no structure that extracts its wastes, as there is no following segment to filter and discharge them, while the first segment contains an extraction structure that passes wastes to the second, but does not contain the structures that re-filter and discharge urine.^[7]

Reproduction and life cycle[edit]

Asexual reproduction[edit]

Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism.^[7][35] Some oligochaetes, such as Aulophorus furcatus, seem to reproduce entirely asexually, while others reproduce asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding.^[11][36] However, leeches have never been seen reproducing asexually.^[11][37]

Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genera, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.^[11][35] Annelids are the most complex animals that can regenerate after such severe damage.^[38] On the other hand, leeches cannot regenerate.^[37]

Sexual reproduction[edit]

It is thought that annelids were originally animals with two separate sexes, which released ova and sperm into the water via their nephridia.^[7] The fertilized eggs develop into trochophorelarvae, which live as plankton.^[40] Later they sink to the sea-floor and metamorphose into miniature adults: the part of the trochophore between the apical tuft and the prototroch becomes the prostomium (head); a small area round the trochophore's anus becomes the pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the trochophore becomes the peristomium (the segment that contains the mouth).^[7]

However, the lifecycles of most living polychaetes, which are almost all marine animals, are unknown, and only about 25% of the 300+ species whose lifecycles are known follow this pattern. About 14% use a similar external fertilization but produce yolk-rich eggs, which reduce the time the larva needs to spend among the plankton, or eggs from which miniature adults emerge rather than larvae. The rest care for the fertilized eggs until they hatch – some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few species by keeping the eggs within their bodies until they hatch. These species use a variety of methods for sperm transfer; for example, in some the females collect sperm released into the water, while in others the males have a penis that inject sperm into the female.^[40] There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge.^[40]

Some polychaetes breed only once in their lives, while others breed almost continuously or through several breeding seasons. While most polychaetes remain of one sex all their lives, a significant percentage of species are full hermaphrodites or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent gonads, and it is uncertain how they produce ova and sperm. In a few species the rear of the body splits off and becomes a separate individual that lives just long enough to swim to a suitable environment, usually near the surface, and spawn.^[40]

Most mature clitellates (the group that includes earthworms and leeches) are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity. All have well-developed gonads, and all copulate. Earthworms store their partners' sperm in spermathecae ('sperm stores') and then the clitellum produces a cocoon that collects ova from the ovaries and then sperm from the spermathecae. Fertilization and development of earthworm eggs takes place in the cocoon. Leeches' eggs are fertilized in the ovaries, and then transferred to the cocoon. In all clitellates the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. All clitellates hatch as miniature adults rather than larvae.^[40]

Ecological significance[edit]

Charles Darwin's book The Formation of Vegetable Mould through the Action of Worms (1881) presented the first scientific analysis of earthworms' contributions to soil fertility.^[41] Some burrow while others live entirely on the surface, generally in moist leaf litter. The burrowers loosen the soil so that oxygen and water can penetrate it, and both surface and burrowing worms help to produce soil by mixing organic and mineral matter, by accelerating the decomposition of organic matter and thus making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily.^[42][43] Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers, and in some cases conserving earthworms may be essential for conserving endangered birds.^[44]

Terrestrial annelids can be invasive in some situations. In the glaciated areas of North America, for example, almost all native earthworms are thought to have been killed by the glaciers and the worms currently found in those areas are all introduced from other areas, primarily from Europe, and, more recently, from Asia. Northern hardwood forests are especially negatively impacted by invasive worms through the loss of leaf duff, soil fertility, changes in soil chemistry and the loss of ecological diversity. Especially of concern is Amynthas agrestis and at least one state (Wisconsin) has listed it as a prohibited species.

Earthworms migrate only a limited distance annually on their own, and the spread of invasive worms is increased rapidly by anglers and from worms or their cocoons in the dirt on vehicle tires or footwear.

Marine annelids may account for over one-third of bottom-dwelling animal species around coral reefs and in tidal zones.^[41] Burrowing species increase the penetration of water and oxygen into the sea-floor sediment, which encourages the growth of populations of aerobic bacteria and small animals alongside their burrows.^[45]

Although blood-sucking leeches do little direct harm to their victims, some transmit flagellates that can be very dangerous to their hosts. Some small tube-dwelling oligochaetes transmit myxosporeanparasites that cause whirling disease in fish.^[41]

Interaction with humans[edit]

Earthworms make a significant contribution to soil fertility.^[41] The rear end of the Palolo worm, a marine polychaete that tunnels through coral, detaches in order to spawn at the surface, and the people of Samoa regard these spawning modules as a delicacy.^[41]Anglers sometimes find that worms are more effective bait than artificial flies, and worms can be kept for several days in a tin lined with damp moss.^[46]Ragworms are commercially important as bait and as food sources for aquaculture, and there have been proposals to farm them in order to reduce over-fishing of their natural populations.^[45] Some marine polychaetes' predation on molluscs causes serious losses to fishery and aquaculture operations.^[41]

Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.^[41]

Accounts of the use of leeches for the medically dubious practise of blood-letting have come from China around 30 AD, India around 200 AD, ancient Rome around 50 AD and later throughout Europe. In the 19th century medical demand for leeches was so high that some areas' stocks were exhausted and other regions imposed restrictions or bans on exports, and Hirudo medicinalis is treated as an endangered species by both IUCN and CITES. More recently leeches have been used to assist in microsurgery, and their saliva has provided anti-inflammatory compounds and several important anticoagulants, one of which also prevents tumors from spreading.^[41]

Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which are biomineralized with calcium salts. These advantages have attracted the attention of engineers. Investigations showed that ragworm jaws are made of unusual proteins that bind strongly to zinc.^[47]

Evolutionary history[edit]

Fossil record[edit]

Since annelids are soft-bodied, their fossils are rare.^[48]Polychaetes' fossil record consists mainly of the jaws that some species had and the mineralized tubes that some secreted.^[49] Some Ediacaran fossils such as Dickinsonia in some ways resemble polychaetes, but the similarities are too vague for these fossils to be classified with confidence.^[50] The small shelly fossilCloudina, from , has been classified by some authors as an annelid, but by others as a cnidarian (i.e. in the phylum to which jellyfish and sea anemones belong).^[51][52] Until 2008 the earliest fossils widely accepted as annelids were the polychaetes Canadia and Burgessochaeta, both from Canada's Burgess Shale, formed about 505 million years ago in the early Cambrian.^[53]Myoscolex, found in Australia and a little older than the Burgess Shale, was possibly an annelid. However, it lacks some typical annelid features and has features which are not usually found in annelids and some of which are associated with other phyla.^[53] Then Simon Conway Morris and John Peel reported Phragmochaeta from Sirius Passet, about 518 million years old, and concluded that it was the oldest annelid known to date.^[50] There has been vigorous debate about whether the Burgess Shale fossil Wiwaxia was a mollusc or an annelid.^[53] Polychaetes diversified in the early Ordovician, about . It is not until the early Ordovician that the first annelid jaws are found, thus the crown-group cannot have appeared before this date and probably appeared somewhat later.^[1] By the end of the Carboniferous, about 299 million years ago, fossils of most of the modern mobile polychaete groups had appeared.^[53] Many fossil tubes look like those made by modern sessile polychaetes ^[54], but the first tubes clearly produced by polychaetes date from the Jurassic, less than 199 million years ago.^[53] In 2012, a 508 million year old species of annelid found near the burgess shale beds in British Columbia, Kootenayscolex, was found that changed the hypotheses about how the annelid head developed. It appears to have bristles on its head segment akin to those along its body, as if the head simply developed as a specialized version of a previously generic segment.

The earliest good evidence for oligochaetes occurs in the Tertiary period, which began 65 million years ago, and it has been suggested that these animals evolved around the same time as flowering plants in the early Cretaceous, from .^[55] A trace fossil consisting of a convoluted burrow partly filled with small fecal pellets may be evidence that earthworms were present in the early Triassic period from .^[55][56]Body fossils going back to the mid Ordovician, from , have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed.^[55][57]

Family tree[edit]

Annelida

some 'Scolecida' and 'Aciculata' some 'Canalipalpata' Sipuncula, previously a separate phylum Clitellata some 'Oligochaeta' Hirudinea (leeches) some 'Oligochaeta' some 'Oligochaeta' Aeolosomatidae[58] some 'Scolecida' and 'Canalipalpata' some 'Scolecida' Echiura, previously a separate phylum some 'Scolecida' some 'Canalipalpata' Siboglinidae, previously phylum Pogonophora some 'Canalipalpata' some 'Scolecida', 'Canalipalpata' and 'Aciculata'

Annelid groups and phyla incorporated into Annelida (2007; simplified).^[9]
Highlights major changes to traditional classifications.

Traditionally the annelids have been divided into two major groups, the polychaetes and clitellates. In turn the clitellates were divided into oligochaetes, which include earthworms, and hirudinomorphs, whose best-known members are leeches.^[7] For many years there was no clear arrangement of the approximately 80 polychaete families into higher-level groups.^[9] In 1997 Greg Rouse and Kristian Fauchald attempted a 'first heuristic step in terms of bringing polychaete systematics to an acceptable level of rigour', based on anatomical structures, and divided polychaetes into:^[59]

• Scolecida, less than 1,000 burrowing species that look rather like earthworms.^[60]
• Palpata, the great majority of polychaetes, divided into:
  • Canalipalpata, which are distinguished by having long grooved palps that they use for feeding, and most of which live in tubes.^[60]
  • Aciculata, the most active polychaetes, which have parapodia reinforced by internal spines (aciculae).^[60]

Also in 1997 Damhnait McHugh, using molecular phylogenetics to compare similarities and differences in one gene, presented a very different view, in which: the clitellates were an offshoot of one branch of the polychaete family tree; the pogonophorans and echiurans, which for a few decades had been regarded as a separate phyla, were placed on other branches of the polychaete tree.^[61] Subsequent molecular phylogenetics analyses on a similar scale presented similar conclusions.^[62]

Updated cladogram of Annelids[63][64][65][66]

Annelida Palaeoannelida Chaetopteriformia Amphinomorpha Pleistoannelida Errantia Protodriliformia Spioniformia Clitellatomorpha Apodadrilida Dorsopharyngea

In 2007 Torsten Struck and colleagues compared three genes in 81 taxa, of which nine were outgroups,^[9] in other words not considered closely related to annelids but included to give an indication of where the organisms under study are placed on the larger tree of life.^[67] For a cross-check the study used an analysis of 11 genes (including the original 3) in ten taxa. This analysis agreed that clitellates, pogonophorans and echiurans were on various branches of the polychaete family tree. It also concluded that the classification of polychaetes into Scolecida, Canalipalpata and Aciculata was useless, as the members of these alleged groups were scattered all over the family tree derived from comparing the 81 taxa. It also placed sipunculans, generally regarded at the time as a separate phylum, on another branch of the polychaete tree, and concluded that leeches were a sub-group of oligochaetes rather than their sister-group among the clitellates.^[9] Rouse accepted the analyses based on molecular phylogenetics,^[11] and their main conclusions are now the scientific consensus, although the details of the annelid family tree remain uncertain.^[10]

In addition to re-writing the classification of annelids and three previously independent phyla, the molecular phylogenetics analyses undermine the emphasis that decades of previous writings placed on the importance of segmentation in the classification of invertebrates. Polychaetes, which these analyses found to be the parent group, have completely segmented bodies, while polychaetes' echiurans and sipunculan offshoots are not segmented and pogonophores are segmented only in the rear parts of their bodies. It now seems that segmentation can appear and disappear much more easily in the course of evolution than was previously thought.^[9][61] The 2007 study also noted that the ladder-like nervous system, which is associated with segmentation, is less universal than previously thought in both annelids and arthropods.^[9][b]

Bilateria

Acoelomorpha (Acoela and Nemertodermatida) Deuterostomia (Echinoderms, chordates, etc.) Protostomia Ecdysozoa (Arthropods, nematodes, priapulids, etc.) Lophotrochozoa Annelida Phoronida and Brachiopoda Platyzoa Other Platyzoa

Relationships of Annelids to other Bilateria:^[62]
(Analysis produced in 2004, before Sipuncula were merged into Annelida in 2007^[9])

Annelids are members of the protostomes, one of the two major superphyla of bilaterian animals – the other is the deuterostomes, which includes vertebrates.^[62] Within the protostomes, annelids used to be grouped with arthropods under the super-group Articulata ('jointed animals'), as segmentation is obvious in most members of both phyla. However, the genes that drive segmentation in arthropods do not appear to do the same in annelids. Arthropods and annelids both have close relatives that are unsegmented. It is at least as easy to assume that they evolved segmented bodies independently as it is to assume that the ancestral protostome or bilaterian was segmented and that segmentation disappeared in many descendant phyla.^[62] The current view is that annelids are grouped with molluscs, brachiopods and several other phyla that have lophophores (fan-like feeding structures) and/or trochophore larvae as members of Lophotrochozoa.^[68]Bryozoa may be the most basal phylum (the one that first became distinctive) within the Lophotrochozoa, and the relationships between the other members are not yet known.^[62] Arthropods are now regarded as members of the Ecdysozoa ('animals that molt'), along with some phyla that are unsegmented.^[62][69]

The 'Lophotrochozoa' hypothesis is also supported by the fact that many phyla within this group, including annelids, molluscs, nemerteans and flatworms, follow a similar pattern in the fertilized egg's development. When their cells divide after the 4-cell stage, descendants of these four cells form a spiral pattern. In these phyla the 'fates' of the embryo's cells, in other words the roles their descendants will play in the adult animal, are the same and can be predicted from a very early stage.^[70] Hence this development pattern is often described as 'spiral determinate cleavage'.^[71]

Notes[edit]

1. ^The term originated from Jean-Baptiste Lamarck's annélides.^[2][3]
2. ^Note that since this section was written, a new paper has revised the 2007 results: Struck, T. H.; Paul, C.; Hill, N.; Hartmann, S.; Hösel, C.; Kube, M.; Lieb, B.; Meyer, A.; Tiedemann, R.; Purschke, G. N.; Bleidorn, C. (2011). 'Phylogenomic analyses unravel annelid evolution'. Nature. 471 (7336): 95–98. doi:10.1038/nature09864. PMID21368831.

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Further reading[edit]

• Dales, R. P. (1967). Annelids (2nd edition). London: Hutchinson University Library.
• 'Annelid Fossils'(Web page). The Virtual Fossil Museum. 2006. Archived from the original on 17 June 2006. Retrieved May 20, 2006.Cite uses deprecated parameter |deadurl= (help) – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range.

External links[edit]

Wikimedia Commons has media related to Annelida.

Wikispecies has information related to Annelida

The Wikibook Dichotomous Key has a page on the topic of: Annelida

• Polychaete Larva – Guide to the Marine Zooplankton of south eastern Australia, Tasmanian Aquacultulre & Fisheries Institute