28.3F: Classification of Phylum Mollusca - Biology

28.3F: Classification of Phylum Mollusca - Biology

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The phylum Mollusca includes a wide variety of animals including the gastropods (“stomach foot”), the cephalopods (“head foot”), and the scaphopods (“boat foot”).

Learning Objectives

  • Differentiate among the classes in the phylum mollusca

Key Points

  • Mollusks can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. These classes are distinguished by, among other criteria, the presence and types of shells they possess.
  • Class Aplacophora includes worm-like animals with no shell and a rudimentary body structure.
  • Members of class Monoplacophora have a single shell that encloses the body.
  • Members of class Polyplacophora are better known as “chitons;” these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side.
  • Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels.
  • Members of class Gastropoda have an asymmetrical body plan and usually have a shell, which can be planospiral or conispiral. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling.
  • Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey.

Key Terms

  • ctenidium: a respiratory system, in the form of a comb, in some molluscs
  • captacula: the foot of a Scaphalopod, modified into tentacles for capturing prey
  • nephridium: a tubular excretory organ in some invertebrates

Classes in Phylum Mollusca

Phylum Mollusca is a very diverse (85,000 species ) group of mostly marine species, with a dramatic variety of form. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.

Class Aplacophora

Class Aplacophora (“bearing no plates”) includes worm-like animals primarily found in benthic marine habitats. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs).

Class Monoplacophora

Members of class Monoplacophora (“bearing one plate”) posses a single, cap-like shell that encloses the body. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. Today, scientists have identified nearly two dozen extant species.

Class Polyplacophora

Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear an armor-like, eight-plated dorsal shell. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Chitons live worldwide, in cold water, warm water, and the tropics. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods.

Class Bivalvia

Bivalvia is a class of marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts. Bivalves include clams, oysters, mussels, scallops, and numerous other families of shells. The majority are filter feeders and have no head or radula. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed, while others lie on the sea floor or attach themselves to rocks or other hard surfaces.

The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge by a flexible ligament that, in conjunction with interlocking “teeth” on each of the valves, forms the hinge.

Class Gastropoda

Animals in class Gastropoda (“stomach foot”) include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell. Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or conispiral (like a spiral staircase), commonly seen in marine conches.

The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling. Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia (singluar: ctenidium) as well as a pair of nephridia (singular: nephridium).

Class Cephalopoda

Class Cephalopoda (“head foot” animals) includes octopi, squids, cuttlefish, and nautilus. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell. They display vivid coloration, typically seen in squids and octopi, which is used for camouflage. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well as a closed circulatory system. The foot is lobed and developed into tentacles and a funnel, which is used as the mode of locomotion. Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion (“jet” propulsion). Cephalopods, such as squids and octopi, also produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway. Suckers are present on the tentacles in octopi and squid. Ctenidia are enclosed in a large mantle cavity serviced by blood vessels, each with its own associated heart. The mantle has siphonophores that facilitate exchange of water.

A pair of nephridia is present within the mantle cavity. Sexual dimorphism is seen in this class of animals. Members of a species mate, then the female lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages.

Class Scaphopoda

Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident when examining Dentalium, one of the few remaining scaphopod genera. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals.

Phylum Mollusca: Characters and Classification (With Diagram) | Animals

They are mostly marine. Many, however, occur in fresh water and some even in damp soil.

The body of molluscs is un-segmented with a distinct head, muscular foot and visceral hump. Neopilina is a segmented mollusc.

They usually show bilateral symmetry. In some molluscs like Pila, due to torsion (twisting) during growth, the adults become asymmetrical.

Shell is secreted by mantle. It is made up of calcium carbonate. Shell may be external (e.g., most of molluscs), internal (e.g., slug, cuttle fish, squid) or absent (e.g., Octopus).

It is a thin, fleshy fold of dorsal body wall more or less covering the body. It encloses a space, which is called mantle cavity (= pallial cavity).

Single layered epidermis is usually ciliated. Muscles are un-striped and occur in bundles.

Coelom is greatly reduced. It is restricted to pericardial cavity (space around the fieart), and to small spaces within kidneys and gonads (testes and ovaries). Spaces amongst the viscera (soft organs) contain blood and form haemocoel.

9. Blood vascular system:

It is open type. It includes dorsal heart, arteries that open into sinuses (spaces) and veins. Blood is usually blue due to the presence of a copper- containing blue respiratory pigment called haemocyanin. Among the molluscs, cuttle fish are exceptional in having closed blood vascular system.

These are gills (cteoidia). Mantle and pulmonary sae (in semi-terrestrial form).

Excretory organs are one or two pairs of sac- like kidneys. Gills are also excretory in function. Ammonia is chief excretory matter.

The nervous system comprises paired cerebral, pleural, pedal and visceral ganglia joined by the nerve connectives and conumssures. Connectives con­nect dissimilar ganglia however, commissures connect similar ganglia

In many molluscs, eyes are present over stalks called ommatophores. Statocysts (balancing organs) may be present. Osphradium is present in some molluscs for testing chemical and physical nature of water.

The sexes are generally separate but some are hermaphrodite.

They are oviparous. The development is either direct or indirect (metamorphosis). When the development is indirect it includes a characteristic larva, liger, trochophore or glochidium. Asexual reproduction is absent.

(ii) Mantle may be surrounded by shell, and

(iii) Nervous system consists of cerebral, visceral, pleural and pedal ganglia.

Advancement over Annelids:

(i) Shell is present in many individuals,

(ii) In some forms, a lungs is present for pulmonary respiration, and

(iii) Better developed sense organs such as eyes, statocysts and osphradia.

Classification of Phylum Mollusca:

Phylum Mollusca are divided into six classes.

Class 1. Monoplacophora (Gk. monas- one, plax- plate, pherein- bearing):

The shell is spoon or cup shaped. They have die characters of both the phylum Annelida and phylum Mollusca.

Class 2. Amphineura (Gk. amphi- both + two neuron = nerve):

There is a present non-ganglionated nerve ring around mouth with two pairs of interconnected nerve cord.

Class 3. Scaphopoda (Gk. scapha- boat, podos-foot):

Shell is tubular and open at both ends.

Class 4. Gastropoda (Gk. gaster- belly, podos- foot):

Shell is made up of one piece.

The early embryo is symmetrical but during development the body twists showing torsion so that the body becomes asymmetrical. It includes the largest number of molluscs e.g., Pila, Umax, Cypraea (Cowrie), Helix (garden snail), Aplysia (sea hare), Doris (sea lemon), Limnaea, (pond snail), Planorbis, Patella (true limpet), Turbinella (Shankh), Creseis (Sea butterfly).

Class 5. Pelecypoda or Lamellibranchiate or Bivalvia (Gk. pelekus- hatchet Podos foot):

Shells is made up of two halves.

Unio, Mytilus (Sea mussel), Teredo SpwoS EnZ (razor shell or razor clam), Solen (razor fish or razor shell), Ostrea (edible oyster), Pecten (scallop), Pinctada (Pearl oyster).

Class 6. Cephalopoda (Gk. kephale- head, podos- foot):

Head and foot region combined and modified into a structure which has eyes and eight tentacles, hence the name cephalopod or ‘head foot’. Cephalopods are regarded at the top of invertebrates evolution in terms of learned behaviour they exihibit. Shell is external (Nautilus), internal (sepia) or absent (octopus).

Sepia, Loligo, octopus, nautilus, (pearly nautilus). Some cephalopods are the largest invertebrates.

Evolutionary Precursor of Molluscs:

A “living fossil” named Neopilina discovered in 1952 from the Pacific Ocean, shows metameric segmentation which is not a characteristic of molluscs. Neopilina has 8 pairs of muscles, 5 or 6 pairs of gills, and 5 pairs of nephridia.

Metameric segmentation and presence of the trochophore larva in both annelids and molluscs suggest that molluscs have descended from the annelids. Thus, the annelids are the evolutionary precursor of molluscs. Neopilina is a “connecting link” between Annelida and Mollusca.

Chiton-The coat of mail shell:

Chiton is marine and found attached to rocks by its foot the shell consists of a row of eight plates.

Dentalium— The Elephant’s tusk-shell:

It is a marine mollusc. Dentalium is found burrowing in sand. The shell is slightly curved, tubular and opens at both the ends. There are present filamentous tentacles called the captacula, which are useful in feeding.

Unio (Fresh Water Mussel):

It is found in rivers, lakes and ponds. The animal is omnivorous feeding on microscopic organisms. Its larva lives as parasite on fish. Its shell consists of two halves, called the valves. A whitish elevation in each valve is called umbo.

The lines of growth indicate the age of the individual. There are present two siphons posteriorly. It acts as scavenger and keeps water clean. Its shell yields an excellent quantity of lime the shells of the fresh water mussels are used in the manufacture of buttons.

It inhabits ponds, paddy fields, sometimes streams and rivers It is chiefly hetmvorous and feeds on aquatic plants like Pistia and Valusnena. It leads an amohibious life respiring by means of gill in water and by a pulmonary sac an and. Thus it is adapted for both aquatic and terrestrial life.

The mouth or aperture of the shell is closed by a flat and oval plate, the operculum. Pila has osphradium which is meant for testing chemical and physical nature of water. The buccal cavity of Pila contains a rasping organ, the radula, with transverse rows of teeth for cutting the grasses.

It is terrestrial and is abundantly found in gardens, cultivated lands and over damp soil. It is nocturnal and herbivorous. Shell is internal. It is a hermaphrodite animal. The slug is a plant pest. It damages seedlings, tender shoots and leaves.

It is a marine and a good swimmer. In male the left arm is spoon shaped and is called hectocotylized which is used to transfer sperms into the female. The shell is internal. A pear-shaped ink-sac containing the ink-like fluid is present.

When the animal is attacked ink-like fluid is ejected through the funnel to form a smoke cloud. Sepia is edible. Sepia ink obtained from this animal is used by artists. The shell of sepia is used as a source of calcium for pet birds.

Like Sepia, it is also found in the warm seas. It also ejects a dark ink to form a smoke cloud to escape from .the enemy. Its body resembles that of Sepia in form except that it is narrower than that of Sepia. It also has internal shell invertebrate is giant squid (Architeuthis) up to 18 meters long. LxAigo is edible. The giant squid has the largest eye in the animal kingdom.

It is also marine. It kills its prey with poisonous saliva. One of the arms of male is spoon shaped and is called hectocotylized which is used to transfer sperms into the female. The shell is absent. Octopus ejects an inky fluid in water and forms a screen for defence from its enemies. Octopus can change its colour.

Phylum Mollusca: General Characteristics and Its Classification

Mollusca makes the second largest phylum of non-chordate animals including snails, octopuses, sea slugs, squid, and bivalves such as clams, oysters, and mussels. This phylum contains about 100,000 described species. Among all known marine species, 23% are mollusks. But some species live in freshwater and terrestrial habitats. This group displays a broad range of morphological features such as a muscular foot, a mantle and visceral mass containing internal organs. Calcium carbonate secretes from the mantle and forms outer calciferous body shell in most of the mollusks. The size of the molluscans varies from less than one millimeter to 20 meters. They play an important role in the lives of humans because they are the source of food for many people as well as jewellery. Many molluscans are not good for human’s lives. Some cause diseases or acts as pests like the snails and slugs. Generally, the hard calciferous shells of mollusks are used to build awesome jewellery pieces. Some mollusks such as bivalves and gastropods produce valuable pearls. Natural pearls are produced when a small foreign object gets trapped in between the mollusk's body shell and mantle. Besides these, many scientists use bivalve mollusks as bioindicators of the freshwater and marine environments.


Of all molluscs, cephalopods display the highest level of both physical and mental development. The cephalopods include octopus, squid, cuttlefish and the shelled Nautilus. They have eight arms and two tentacles. The tentacles are used for capturing prey which is then killed and torn apart using poison glands and a parrot-like beak. Each species from within this group are found free living in water rather than attached to rock or beneath the sea floor. The octopus, however, has adapted a bottom-dwelling lifestyle using its tentacles to crawl along the seafloor.

The Nautilus is the only cephalopod to have kept its characteristic mollusc shell, whereas in the others it has been either highly reduced or completely lost. Cuttlefish have a flattened internal cuttlebone, squid have an internal rod-like pen and octopus have no shell at all. The Nautilus shell is divided into chambers which are used to maintain buoyancy at different depths by increasing or reducing the amount of air in each chamber.

The cephalopods have developed a form of jet propulsion to help them avoid predators using their mantle and siphon to blow water behind themselves and propel themselves forward the majority of time, however, they simply use their fins to slowly swim through the water. The subclass Coleoidea, which includes squid, cuttlefish and octopus, have a number of other unique adaptations among the molluscs. They have highly developed eyes, a complex nervous system, ink glands and skin cells with the ability to change color (known as chromatophores).

6 Main Classes of Phylum Mollusca | Zoology

The following points highlight the six main classes of phylum mollusca. The classes are: 1. Monoplacophora 2. Amphineura 3. Gastropoda 4. Scaphopoda 5. Pelecypoda or Bivalvia or Lamellibranchiata 6. Cephalopoda.

Class # 1. Monoplacophora:

1. The body is bilaterally symmetrical and covered by a mantle.

2. The single dorsal shell is thin and sub circular.

3. The metamerism is apparent and not represented externally.

4. The coelom is well formed.

5. A flat creeping ventral foot is present.

6. The mouth and anus are situated at the anteromedian and posteromedian ends of the foot.

Example: Neopilina Galatea.

Class # 2. Amphineura:

It is a small class of marine molluscs dating back from the Cambrian period.

1. The typically elongated form, retain bilateral symmetry having terminal mouth and anus.

2. The nervous system is primitive, with longitudinal pallial and pedal cords with cross anastomoses.

The two subclasses of the class Amphineura sharing a similar primitive ner­vous system are very different in appear­ance, habitat and development.

Subclass i. Aplacophora or Solenogastres:

1. The body is elongated and worm-like and enveloped by the mantle.

2. The shell is absent and the body is covered with a cuticular mantle enclosing spicules of calcified material.

3. A ventral longitudinal groove is present, which is connected anteriorly with a ciliated groove and posteriorly with the cavity of the cloaca. In Chaetoderma the ventral “foot- groove” is absent and cloaca is a discrete bell-like mantle cavity which also contains a pair or a series of ctenidia.

4. Digestive glands are not distinct sali­vary gland is usually present.

5. A ventral foot is absent.

6. The nervous system is ladder-like.

7. Neometiia is hermaphroditic whereas Chaetoderma is dioecious.

8. Marine, found in shallow (Neomenia) and deep sea (Chaetoderma), feeding upon hydroids, corals, etc.

Examples: Neometiia, Proneomenia, Chaetoderma, etc.

Subclass ii. Polyplacophora:

1. Marine molluscs with an elongated, bilaterally symmetrical flattened body and a broad foot ventrally.

2. The shell consists of eight separate pieces on dorsal surface and studded later­ally with scales or spines to form a girdle.

3. The mantle bearing cuticular spicules covers at least a great part of the body.

4. The mouth lies centrally in a reduced head, the anal and excretory apertures are posterior.

5. Between the edge of the foot and the girdle on each side is a mantle groove into which project a number of gills or ctenidia.

6. The nervous system is ladder-like, with slightly developed ganglia.

Occur most commonly in the littoral and sub-littoral rocky shores.

Examples: Chiton, Lepidopleurina, etc.

Class # 3. Gastropoda:

1. Asymmetrical molluscs with a spirally coiled mantle and a shell of one piece en­closing a corresponding visceral mass. The asymmetry of visceropallium is its funda­mental feature.

2. A well-developed head bearing eyes and tentacles are usually present.

3. The foot is behind the head and is typically a flat, creeping organ.

4. The buccal cavity contains an odontophore with a radula bearing rows of chitinoid teeth.

5. The respiratory organs consist either of one or two gills, or a gill and a pulmonary sac or a lung.

6. The nervous system consists of cere­bral, pleural, buccal, pedal, visceral and ab­dominal ganglia with their connectives and commissures.

7. The kidney is usually single.

8. The sexes are either separate or united.

9. The larva passes through trochophore and veliger stages.

Subclass 1. Prosobranchia:

1. The mouth of the shell can be closed by an aperculum borne on the foot.

2. Head with a single pair of tentacles.

3. Mantle cavity opens anteriorly and con­tains two ctenidia anterior to heart.

4. Sexes separate, gonad solitary, opening in the right.

5. Larval stages are trochophore and ve­liger.

6. Primarily aquatic gastropods, retaining larval torsion and nervous asymmetry.

Upper Cambrian to recent 25,000 living and 10,000 fossil species.

Examples: Haliotis, Pila, Viviparus, Pa­tella, Acmaea, Fissurella, Cypraea, Littorina, Murex, Buccinum, Conus, etc.

Subclass 2. Opisthobranchia:

1. Marine gastropods displaying reduc­tion or loss of shell (shell sometimes inter­nal).

2. Visceral complex usually incorporated into head-foot complex, leading to second­ary external bilateral symmetry.

3. Respiratory structures—secondary gills, never ctenidia.

4. Untwisting and shortening of visceral loop and pleuroparietal nerve connectives due to de-torsion is pronounced.

5. Hermaphrodite larva is a veliger, Car­boniferous to recent 3,000 living and 300 fossil species.

Examples: Aplysia (sea hare), Actean, Gastropteron (sea slug), Cliona, Bertholimia, Polycera, Doris, Dendonotus, etc.

Subclass 3. Pulmonata:

1. Shell single piece with a simple spiral or none.

2. Head with one or two pairs of tentacles and one pair of eyes.

3. Mantle cavity communicates with the exterior through an oval, closeable pneumostome.

4. Respiratory organ a lung, fused with the mantle in the adult. The mantle cavity acting as the respiratory chamber.

5. Hermaphrodite gonad single, mostly ovoviviparous development direct or with suppressed larval stages.

Upper Cambrian to recent more than 5,000 living and 1,000 fossil species.

Examples Achatina, Helix, Umax, Artalimax, Agriolimax (slug) Gonaxis, Lymnaea, Planorbis, Ferrisia,Physa, etc.

Class # 4. Scaphopoda:

1. Marine molluscs with an elongated worm-like body enclosed in a bilaterally cy­lindrical shell.

2. The shell is a curved tube open at both the ends, the opening being wider at the oral and narrower at the other end.

3. The foot is narrow and trilobed or with a terminal disc which can be protruded through the oral opening of the shell.

4. The mouth opens at the end of a short buccal tube, at the base of which is a circlet of tentacles.

6. The odontophore is with a simple radula.

7. The sexes are separate the reproduc­tive elements pass out through the right excretory aperture.

8. A free-swimming trochophore larva is succeeded by a veliger.

Devonian to recent 300 fossil and 100 living species,

Examples: Dentalium, Entalina, etc.

Class # 5. Pelecypoda or Bivalvia or Lamellibranchiata:

1. Bilaterally symmetrical molluscs with a laterally compressed body enclosed in a bivalved calcareous shell.

2. Shell usually symmetrical, with dorsal hinge and ligament, and closed by 1 or 2 adductor muscles.

3. Head is not distinct and without eyes and tentacles.

4. The foot is ventral and plough-shaped.

5. The gills one or two pairs (ctenidia or branchia), commonly plate-like.

7. The mouth is bounded by two pairs of labial palps acting as lips.

8. The rectum pierces the pericardium and the ventricle, and opens in the exhalant siphon.

9. The heart consists of two auricles and a ventricle.

10. The nervous system consists of paired cerebral, pedal and visceral ganglia with their connectives.

11. Sexes are separate or united develop­ment with a veliger or glochidiam larva.

Ordovician to recent, 11,000 living and 15,000 fossil species.

Examples: Lamellidens, Unio, Anodonta, Mytilus, Teredo, Oyster etc.

Class # 6. Cephalopoda:

1. Bilaterally symmetrical, free-swimming, marine molluscs.

2. The head is surrounded by a series of arms bearing suckers, which are modified part of the foot.

3. The rest of the foot forms a siphon on the ventral surface of the body.

4. The mantle encloses a large mantle cavity in which are situated the gills, the renal, reproductive and the anal apertures.

5. The shell may be absent or rudimen­tary when present may be internal or exter­nal, undivided or divided internally into a series of chambers.

6. The head bears a pair of large, simple eyes.

7. A pair of horny jaws and an odontophore with a radula are present.

8. In majority, there is an ink gland open­ing in the rectum.

9. The nervous system is highly devel­oped the principal nerve ganglia are aggre­gated around the oesophagus and a carti­laginous skeleton supports and protects the nerve centre.

10. The sexes are separate and develop­ment is direct.

Subclass i. Nautiloidea (Tetrabranchia):

1. Shell external, many-chambered, siphunculate and coiled or straight.

2. Head with numerous retractile ten­tacular appendages which lack suckers.

3. Siphonal funnel of two separate folds.

4. Two pairs of ctenidia, osphradia, renal organs, and auricles.

6. Eyes are open vesicles without cornea or lens.

Only one living genus with three species of Nautilus in eastern Pacific and Indian Oceans at depths to 560 metres 2,500 fossil species. Example: Nautilus pompilius.

Subclass ii. Coleoidea (Dibranchia):

1. Body cylindrical or globose, often with fins.

2. Shell internal and reduced or none.

4. The head bears eight non-retractile ten­tacles which are provided with suckers in two rows.

5. One pair of ctenidia, osphradia, au­ricles and renal organs present.

Classification of Mollusca

Classification can be great fun and not as utterly boring as it sometimes turns out to be. It’s fun if you can understand the need for it and how its done how it is a passion for some and when you do not want to learn it for an examination! Enjoy it and then you would even be able to score well in an exam. Seriously- no joke being played on you.

To start with, let me tell you the one thing the scientific community who undertake this task < they are called taxonomists. Taxonomy = Science of Classification >and those who use it are all united in- in pulling their hair out of its roots with this fine art and science of Taxonomy. Why? Do you ask? Because it never is static, the taxonomy and the names are every now and then changing. Complicated? Not really. It is an indication of the continuous nature of learning that is going on as we try to understand the magnificent life on this planet.

Just to remind you that all organisms are classified following a hierarchy which is given below. Each hierarchical group is called taxon, ie Phylum is a taxon, class is a taxon etc etc.(plural -Taxa)

Kingdom -> Phylum -> Class -> Order -> Family -> Genus -> Species

Animalia ->Chordata ->Mammalia ->Primates ->Hominidae ->Homo sapiens
( Human beings or Homo sapiens sapiens are classified as given above)

This is an oversimplified version of classification. In between these groups there are other categories such as Sub-class, Suborder, Superfamily & subfamily, Subgenus, and tribes, subspecies and varieties.

But as a beginner it would suffice if you are able to remember the 7 hierarchical categories given above. The other minute details are important if and when you chose to study something scientifically.

In case you have problems remembering the 7 groups in sequence, then you could perhaps remember this sentence, that I remember reading in a biology text book during my school days:

K ings P lay C hess O n F ine G rain S and


No doubt you would know that all molluscs come under the Phylum Mollusca. They are the second largest group of living organisms and are represented by a humongous number of species, more than a 100,000. Consider the fact that they first made their appearance around 500million years ago ( That time period is referred to as Cambrian), it’ s no wonder there are so many of themoccupying a diverse habitat – from land to water The variety reaching its magnificent diversity and beauty in the oceans. How many global warming situations and climate changes they must have been witness to! But they have adapted and evolved and now face another challenge with the onset of the latest round of global warming and climate change. Will they survive? To some extent perhaps they will, but their fate lies in the hands of us human beings!

Well to get on with classification.When it comes to sorting and grouping them into categories we can see one long journey of changes from the days of Aristotle to the present.

Aristotle who lived from 384-322 B.C classified Molluscs broadly into Malachia (present day cephalopods) and Ostracodermata.

Then some of these terms underwent changes. Malcahia was changed to Mollia and Ostacodermata to Testacea by Pliny (the Elder) who lived in the years 23-79AD and finally Mollia was changed to Mollusca by Jonstonus during the 17 th C.

It did not stop there. Classification of molluscs underwent several changes in its hierarchial categories down the years – In 1758 with Linnaeus, in 1795 with Cuvier, in 1806 by Dumeril and from 1801-1819 through Jean Lamarck. All of them modified the classification patterns in some way or other.

These changes became necessary as more and more information about organisms were gathered, learnt. The history of science is fascinating to read. It tells us amongst other things the journey of classification. In the very early years it was exclusively morphology – the external characteristics that dominated classification then anatomy-study of internal characteristics came in. As microscopes and X-ray became available valuable knowledge from physiology was used, and today we study organisms at the minutest levels the level of molecules of genes, DNA and proteins. Today increasingly classification at the species level is driven by the genetic composition of the organisms.

Doubtless the classification that you will learn today would undergo changes as more and more genetic information becomes available. Even the species names would change as we begin to understand their genetic mechanisms and their behavioral characteristics. The data for classification today come from diverse fields of biology from ecology to genetics, sophisticated chemical techniques coupled with evolutionary data as well as time tested facts gathered from morphology and anatomy.

BUT DON’T FEEL DAUNTED BY ALL THIS ! Learn classification to use it as a tool to understand and protect an organism and its habitat. The classification of molluscs given here is seen in that perspective. To enable you to know the mollusc you are likely to find (and if you perceive them to be as fascinating as I have found them to be, then) learn a little more to be able to conserve its habitat for it to flourish. There is nothing more fascinating than the architecture of their shells. So start just there and if you find them fascinating then you can learn more details.

You are likely to come across a whole lot of specialized words in describing the molluscs. Look up the glossary for their meanings.

Here is a commonly used classification for molluscs. Phylum Mollusca is classified into seven classes

  1. GASTROPODA (single shelled cowries, cones etc)
  2. BIVALVIA ( two shelled like clams, mussels etc)
  3. APLACOPHORA (solenogasters)
  4. MONOPLACOPHORA (segmented limpets)
  5. POLYPLACOPHORA ( or Amphineura as it was earlier called- Chitons)
  6. SCAPHOPODA (tusk shells)
  7. CEPHALOPODA (nautilus, squids, Cuttelfish, octopus etc)

The classes are further subdivided into sub-classes, order, family and finally the genus and species. At each level detailed characteristics of the organisms are added in order to find its exact identification. At present I am not giving details of the sub-class or order but am restricting myself to a brief description of each class.

When you open the images of the shells, there would be a brief description of each of them along with the classification. In time to come this page will be updated with a more detailed classification.


This word is a combination of two Greek words : ‘Gastir which means ‘stomach’ and podi which means ‘foot’. So Gastropods can be called ‘stomach-footed’ molluscs!! Funny isn’t it? Try acting out a gastropod. As the name suggests these have a well developed muscular foot, sometimes quite large that is used for crawling. There is a head with well developed eyes and tentacles. ( observe a land snail and you can see the tentacles coming out and going in). Except the slugs all gastropods produce an outer shell which is spirally wound. A kind of spiral coiling is seen as the animal grows. The immature organism is bilaterally symmetrical but acquires an asymmetrical shape as it matures into an adult. The coiling of the visceral mass around the central axis occurs through a process called torsion. This class with 17 orders and many families contain animals whose shell shape and structure also help to identify them. A Cone shell is easily distinguished from a top shell or a cowry. Refer to the illustration for parts of this shell. They live on land, freshwater and marine environment.

Bi means ‘two’ and as the name suggests these molluscs possess tow shells. They are also known as Pelecypoda meaning ‘hatchet-footed’. They are bilaterally symmetrical and show no torsion. Head is greatly reduced, no tentacles but have a foot that can often be seen sticking out of the valves. The two valves are joined by hinges and other structures that help the valves to open and close. Refer to illustration for parts of the shell. They are completely aquatic. Eg. Oysters, clams etc


‘Without plates’ is what this term means. Aplacophorans are small, cylindrical, worm-like creatures that were till 1987 classified under Phylum Echinodermata as Holothurians. These benthic (deep water), exclusively marine molluscs do not have shells but have small calcareous spicules embedded in their mantle. There is no head or specialized excretory organ(nephridia). The foot and mantle cavity are reduced. Sea cucumber is the most familiar example of this class.


The name means ‘with one plate’ (a single shell). Thought to be extinct, there was a single species discovered in 1952 at Costa Rica and subsequently described in 1957.Along with Neopilina sp . there are about tow dozen species found. They are small and have a single cap like shells that make them look like limpets and hence the common name segmented limpets. Head is reduced with no tentacles or eyes they have a rounded foot.Many of the organs like the gills, nephridia etc are paired. Another common name to describe them is ‘gastroverm’


‘Bearing many plates’ is the meaning of the term Polyplacophora. The name comes from the characteristic presence of 8plates called valves that are overlapping and attached to a part of the mantle called girdle. These plates are distributed around and underneath the edges. The mouth and the anus are present at the opposite ends of a bilaterally symmetrical body. Eyes and tentacles are absent. Example – Chitons

Scaphopoda means ‘boat-footed’. This class includes marine molluscs that usually remain buried in snad. The worm-like animal is found within tusk shaped shells that are open at both ends. Thus they are commonly referred to as tusk shells. The animals do not have eyes, tentacles or gills.

The name means ‘head-footed’, these are exclusively marine molluscs that have a prominent and large head, eyes and tentacles. The eyes of these molluscs are fascinating to study as also some of the mechanisms of camouflage developed by the cuttlefishes. The foot has undergone modification and is seen as a set of arms around the mouth. Shell is either internal or absent, the only exception being species of sub-class Nautiloidea eg Nautilus.

5 comments to Classification of Mollusca

hey can i please know the classification of the molluscs? and what is its anatomy and body plan (symmetry etc) and how does it reproduce?

What is Indonesian-speaking molluscs?
I’m from Indonesian student, I ask you please to be translated in Indonesian, because this is important for our learning program.

Please ! can you complete this classification to species? it will better to us

what about African molluscs?
Breeding technology, growth and reproductives performances?

Molluscan biological and chemical diversity: secondary metabolites and medicinal resources produced by marine molluscs

The phylum Mollusca represents an enormous diversity of species with eight distinct classes. This review provides a taxonomic breakdown of the published research on marine molluscan natural products and the medicinal products currently derived from molluscs, in order to identify priority targets and strategies for future research. Some marine gastropods and bivalves have been of great interest to natural products chemists, yielding a diversity of chemical classes and several drug leads currently in clinical trials. Molluscs also feature prominently in a broad range of traditional natural medicines, although the active ingredients in the taxa involved are typically unknown. Overall secondary metabolites have only been investigated from a tiny proportion (<1%) of molluscan species. At the class level, the number of species subject to chemical studies mirrors species richness and our relative knowledge of the biology of different taxa. The majority of molluscan natural products research is focused within one of the major groups of gastropods, the opisthobranchs (a subgroup of Heterobranchia), which are primarily comprised of soft-bodied marine molluscs. Conversely, most molluscan medicines are derived from shelled gastropods and bivalves. The complete disregard for several minor classes of molluscs is unjustified based on their evolutionary history and unique life styles, which may have led to novel pathways for secondary metabolism. The Polyplacophora, in particular, have been identified as worthy of future investigation given their use in traditional South African medicines and their abundance in littoral ecosystems. As bioactive compounds are not always constitutively expressed in molluscs, future research should be targeted towards biosynthetic organs and inducible defence reactions for specific medicinal applications. Given the lack of an acquired immune system, the use of bioactive secondary metabolites is likely to be ubiquitous throughout the Mollusca and broadening the search field may uncover interesting novel chemistry.

© 2010 The Author. Biological Reviews © 2010 Cambridge Philosophical Society.

28.4 Superphylum Lophotrochozoa: Molluscs and Annelids

By the end of this section, you will be able to do the following:

  • Describe the unique anatomical and morphological features of molluscs and annelids
  • Describe the formation of the coelom
  • Identify an important extracoelomic cavity in molluscs
  • Describe the major body regions of Mollusca and how they vary in different molluscan classes
  • Discuss the advantages of true body segmentation
  • Describe the features of animals classified in phylum Annelida

The annelids and the mollusks are the most familiar of the lophotrochozoan protostomes. They are also more “typical” lophotrochozoans, since both groups include aquatic species with trochophore larvae, which unite both taxa in common ancestry. These phyla show how a flexible body plan can lead to biological success in terms of abundance and species diversity. The phylum Mollusca has the second greatest number of species of all animal phyla with nearly 100,000 described extant species, and about 80,000 described extinct species. In fact, it is estimated that about 25 percent of all known marine species are mollusks! The annelids and mollusca are both bilaterally symmetrical, cephalized, triploblastic, schizocoelous eucoeolomates They include animals you are likely to see in your backyard or on your dinner plate!

Phylum Mollusca

The name “Mollusca” means “soft” body, since the earliest descriptions of molluscs came from observations of “squishy,” unshelled cuttlefish. Molluscs are predominantly a marine group of animals however, they are also known to inhabit freshwater as well as terrestrial habitats. This enormous phylum includes chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. Molluscs display a wide range of morphologies in each class and subclass, but share a few key characteristics (Figure 28.21). The chief locomotor structure is usually a muscular foot . Most internal organs are contained in a region called the visceral mass . Overlying the visceral mass is a fold of tissue called the mantle within the cavity formed by the mantle are respiratory structures called gills , that typically fold over the visceral mass. The mouths of most mollusks, except bivalves (e.g., clams) contain a specialized feeding organ called a radula , an abrasive tonguelike structure. Finally, the mantle secretes a calcium-carbonate-hardened shell in most molluscs, although this is greatly reduced in the class Cephalopoda, which contains the octopuses and squids.

Visual Connection

Which of the following statements about the anatomy of a mollusc is false?

  1. Most molluscs have a radula for grinding food.
  2. A digestive gland is connected to the stomach.
  3. The tissue beneath the shell is called the mantle.
  4. The digestive system includes a gizzard, a stomach, a digestive gland, and the intestine.

The muscular foot is the ventral-most organ, whereas the mantle is the limiting dorsal organ that folds over the visceral mass . The foot, which is used for locomotion and anchorage, varies in shape and function, depending on the type of mollusk under study. In shelled mollusks, the foot is usually the same size as the opening of the shell. The foot is both retractable and extendable. In the class Cephalopoda (“head-foot”), the foot takes the form of a funnel for expelling water at high velocity from the mantle cavity and the anterior margin of the foot has been modified into a circle of arms and tentacles.

The visceral mass is present above the foot, in the visceral hump. This mass contains digestive, nervous, excretory, reproductive, and respiratory systems. Molluscan species that are exclusively aquatic have gills that extend into the mantle cavity, whereas some terrestrial species have "lungs" formed from the lining of the mantle cavity. Mollusks are schizocoelous eucoelomates, but the coelomic cavity in adult animals has been largely reduced to a cavity around the heart. However, a reduced coelom sometimes surrounds the gonads, part of the kidneys, and intestine as well. This overall coelomic reduction makes the mantle cavity the major internal body chamber.

Most mollusks have a special rasp-like organ, the radula , which bears chitinous filelike teeth. The radula is present in all groups except the bivalves, and serves to shred or scrape food before it enters the digestive tract. The mantle (also known as the pallium) is the dorsal epidermis in mollusks all mollusks except some cephalopods are specialized to secrete a calcareous shell that protects the animal's soft body.

Most mollusks are dioecious animals and fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods. In most aquatic mollusks, the zygote hatches and produces a trochophore larva , with several bands of cilia around a toplike body, and an additional apical tuft of cilia. In some species, the trochophore may be followed by additional larval stages, such as a veliger larvae, before the final metamorphosis to the adult form. Most cephalopods develop directly into small versions of their adult form.

Classification of Phylum Mollusca

Phylum Mollusca comprises a very diverse group of organisms that exhibits a dramatic variety of forms, ranging from chitons to snails to squids, the latter of which typically show a high degree of intelligence. This variability is a consequence of modification of the basic body regions, especially the foot and mantle. The phylum is organized into eight classes: Caudofoveata, Solenogastres, Monoplacophora, Polyplacophora, Gastropoda, Cephalopoda, Bivalvia, and Scaphopoda. Although each molluscan class appears to be monophyletic, their relationship to one another is unclear and still being reviewed.

Both the Caudofoveata and the Solenogastres include shell-less, worm-like animals primarily found in benthic marine habitats. Although these animals lack a calcareous shell, they get some protection from calcareous spicules embedded in a cuticle that covers their epidermis. The mantle cavity is reduced, and both groups lack eyes, tentacles, and nephridia (excretory organs). The Caudofoveata possess a radula, but the Solenogastres do not have a radula or gills. The foot is also reduced in the Solenogastres and absent from the Caudofoveata.

Long thought to be extinct, the first living specimens of Monoplacophora, Neopilina galatheae, were discovered in 1952 on the ocean bottom near the west coast of Costa Rica. Today there are over 25 described species. Members of class Monoplacophora (“bearing one plate”) possess a single, cap-like shell that covers the dorsal body. The morphology of the shell and the underlying animal can vary from circular to ovate. They have a simple radula, a looped digestive system, multiple pairs of excretory organs, and a pair of gonads. Multiple gills are located between the foot and the edge of the mantle.

Animals in class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear eight limy plates that make up the dorsal shell (Figure 28.22). These animals have a broad, ventral foot that is adapted for suction onto rocks and other substrates, and a mantle that extends beyond the edge of the shell. Calcareous spines on the exposed mantle edge provide protection from predators. Respiration is facilitated by multiple pairs of gills in the mantle cavity. Blood from the gills is collected in a posterior heart, and then sent to the rest of the body in a hemocoel —an open circulation system in which the blood is contained in connected chambers surrounding various organs rather than within individual blood vessels. The radula, which has teeth composed of an ultra-hard magnetite, is used to scrape food organisms off rocky surfaces. Chiton teeth have been shown to exhibit the greatest hardness and stiffness of any biomineral material reported to date, being as much as three-times harder than human enamel and the calcium carbonate-based shells of mollusks.

The nervous system is rudimentary with only buccal or “cheek” ganglia present at the anterior end. Multiple tiny sensory structures, including photosensors, extend from the mantle into channels in the upper layer of the shell. These structures are called esthetes and are unique to the chitons. Another sensory structure under the radula is used to sample the feeding environment. A single pair of nephridia is used for the excretion of nitrogenous wastes.

Class Bivalvia (“two-valves”) includes clams, oysters, mussels, scallops, geoducks, and shipworms. Some bivalves are almost microscopic, while others, in the genus Tridacna, may be one meter in length and weigh 225 kilograms. Members of this class are found in marine as well as freshwater habitats. As the name suggests, bivalves are enclosed in two-part valves or shells (Figure 28.23a) fused on the dorsal side by hinge ligaments as well as shell teeth on the ventral side that keep the two halves aligned. The two shells, which consist of an outer organic layer, a middle prismatic layer, and a very smooth nacreous layer, are joined at the oldest part of the shell called the umbo. Anterior and posterior adductor and abductor muscles close and open the shell respectively.

The overall body of the bivalve is laterally flattened the foot is wedge-shaped and the head region is poorly developed (with no obvious mouth). Bivalves are filter-feeders, and a radula is absent in this class of mollusks. The mantle cavity is fused along the edges except for openings for the foot and for the intake and expulsion of water, which is circulated through the mantle cavity by the actions of the incurrent and excurrent siphons. During water intake by the incurrent siphon, food particles are captured by the paired posterior gills (ctenidia) and then carried by the movement of cilia forward to the mouth. Excretion and osmoregulation are performed by a pair of nephridia. Eyespots and other sensory structures are located along the edge of the mantle in some species. The "eyes" are especially conspicuous in scallops (Figure 28.23b). Three pairs of connected ganglia regulate activity of different body structures.

One of the functions of the mantle is to secrete the shell. Some bivalves, like oysters and mussels, possess the unique ability to secrete and deposit a calcareous nacre or “mother of pearl” around foreign particles that may enter the mantle cavity. This property has been commercially exploited to produce pearls.

Link to Learning

Watch the animations of bivalves feeding: View the process in clams and mussels at these sites.

More than half of molluscan species are in the class Gastropoda (“stomach foot”), which includes well-known mollusks like snails, slugs, conchs, cowries, limpets, and whelks. Aquatic gastropods include both marine and freshwater species, and all terrestrial mollusks are gastropods. Gastropoda includes shell-bearing species as well as species without shells. Gastropod bodies are asymmetrical and usually present a coiled shell (Figure 28.24a). Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or conispiral, (like a spiral staircase), commonly seen in marine conches. Cowrie shells have a polished surface because the mantle extends up over the top of the shell as it is secreted.

A key characteristic of some gastropods is the embryonic development of torsion . During this process, the mantle and visceral mass are rotated around the perpendicular axis over the center of the foot to bring the anal opening forward just behind the head (Figure 28.25), creating a very peculiar situation. The left gill, kidney, and heart atrium are now on the right side, whereas the original right gill, kidney, and heart atrium are on the left side. Even stranger, the nerve cords have been twisted and contorted into a figure-eight pattern. Because of the space made available by torsion in the mantle cavity, the animal’s sensitive head end can now be withdrawn into the protection of the shell, and the tougher foot (and sometimes the protective covering or operculum ) forms a barrier to the outside. The strange arrangement that results from torsion poses a serious sanitation problem by creating the possibility of wastes being washed back over the gills, causing fouling. There is actually no really perfect explanation for the embryonic development of torsion, and some groups that formerly exhibited torsion in their ancestral groups are now known to have reversed the process.

Gastropods also have a foot that is modified for crawling. Most gastropods have a well-defined head with tentacles and eyes. A complex radula is used to scrape up food particles. In aquatic gastropods, the mantle cavity encloses the gills (ctenidia), but in land gastropods, the mantle itself is the major respiratory structure, acting as a kind of lung. Nephridia (“kidneys”) are also found in the mantle cavity.

Everyday Connection

Can Snail Venom Be Used as a Pharmacological Painkiller?

Marine snails of the genus Conus (Figure 28.26) attack prey with a venomous stinger, modified from the radula. The toxin released, known as conotoxin, is a peptide with internal disulfide linkages. Conotoxins can bring about paralysis in humans, indicating that this toxin attacks neurological targets. Some conotoxins have been shown to block neuronal ion channels. These findings have led researchers to study conotoxins for possible medical applications.

Conotoxins are an exciting area of potential pharmacological development, since these peptides may be possibly modified and used in specific medical conditions to inhibit the activity of specific neurons. For example, conotoxins or modifications of them may be used to induce paralysis in muscles in specific health applications, similar to the use of botulinum toxin. Since the entire spectrum of conotoxins, as well as their mechanisms of action, is not completely known, the study of their potential applications is still in its infancy. Most research to date has focused on their use to treat neurological diseases. They have also shown some efficacy in relieving chronic pain, and the pain associated with conditions like sciatica and shingles. The study and use of biotoxins—toxins derived from living organisms—are an excellent example of the application of biological science to modern medicine.

Class Cephalopoda (“head foot” animals), includes octopuses, squids, cuttlefish, and nautiluses. Cephalopods include both animals with shells as well as animals in which the shell is reduced or absent. In the shell-bearing Nautilus, the spiral shell is multi-chambered. These chambers are filled with gas or water to regulate buoyancy. A siphuncle runs through the chambers, and it is this tube that regulates the amount of water and gases (nitrogen, carbon dioxide, and oxygen mixture) present in the chambers. Ammonites and other nautiloid shells are commonly seen in the fossil record. The shell structure in squids and cuttlefish is reduced and is present internally in the form of a squid pen and cuttlefish bone, respectively. Cuttle bone is sold in pet stores to help smooth the beaks of birds and also to provide birds such as egg-laying chickens and quail with an inexpensive natural source of calcium carbonate. Examples of cephalopods are shown in Figure 28.27.

Cephalopods can display vivid and rapidly changing coloration, almost like flashing neon signs. Typically these flashing displays are seen in squids and octopuses, where they may be used for camouflage and possibly as signals for mating displays. We should note, however, that researchers are not entirely sure if squid can actually see color, or see color in the same way as we do. We know that pigments in the skin are contained in special pigment cells ( chromatophores ), which can expand or contract to produce different color patterns. But chromatophores can only make yellow, red, brown, and black pigmentation however, underneath them is a whole different set of elements called iridophores and leucophores that reflect light and can make blue, green, and white. It is possible that squid skin might actually be able to detect some light on its own, without even needing its eyes!

All animals in this class are carnivorous predators and have beak-like jaws in addition to the radula. Cephalopods include the most intelligent of the mollusks, and have a well-developed nervous system along with image-forming eyes. Unlike other mollusks, they have a closed circulatory system, in which the blood is entirely contained in vessels rather than in a hemocoel.

The foot is lobed and subdivided into arms and tentacles. Suckers with chitinized rings are present on the arms and tentacles of octopuses and squid. Siphons are well developed and the expulsion of water is used as their primary mode of locomotion, which resembles jet propulsion. Gills (ctenidia) are attached to the wall of the mantle cavity and are serviced by large blood vessels, each with its own heart. A pair of nephridia is present within the mantle cavity for water balance and excretion of nitrogenous wastes. Cephalopods such as squids and octopuses also produce sepia or a dark ink, which contains melanin. The ink gland is located between the gills and can be released into the excurrent water stream. Ink clouds can be used either as a “smoke screen” to hide the animal from predators during a quick attempt at escape, or to create a fake image to distract predators.

Cephalopods are dioecious. Members of a species mate, and the female then lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. While most other aquatic mollusks produce trochophore larvae, cephalopod eggs develop directly into a juvenile without an intervening larval stage.

Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident when examining Dentalium, one of the few remaining scaphopod genera (Figure 28.28). Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals have a single conical shell, which is open on both ends. The head is not well developed, but the mouth, containing a radula, opens among a group of tentacles that terminate in ciliated bulbs used to catch and manipulate prey. Scaphopods also have a foot similar to that seen in bivalves. Ctenidia are absent in these animals the mantle cavity forms a tube open at both ends and serves as the respiratory structure in these animals.

Phylum Annelida

Phylum Annelida comprises the true, segmented worms. These animals are found in marine, terrestrial, and freshwater habitats, but the presence of water or humidity is a critical factor for their survival in terrestrial habitats. The annelids are often called “segmented worms” due to their key characteristic of metamerism , or true segmentation. Approximately 22,000 species have been described in phylum Annelida, which includes polychaete worms (marine annelids with multiple appendages), and oligochaetes (earthworms and leeches). Some animals in this phylum show parasitic and commensal symbioses with other species in their habitat.


Annelids display bilateral symmetry and are worm-like in overall morphology. The name of the phylum is derived from the Latin word annullus, which means a small ring, an apt description of the ring-like segmentation of the body. Annelids have a body plan with metameric segmentation, in which several internal and external morphological features are repeated in each body segment. Metamerism allows animals to become bigger by adding “compartments,” while making their movement more efficient. The overall body can be divided into head, body, and pygidium (or tail). During development, the segments behind the head arise sequentially from a growth region anterior to the pygidium, a pattern called teloblastic growth . In the Oligochaetes, the clitellum is a reproductive structure that generates mucus to aid sperm transfer and also produces a “cocoon,” within which fertilization occurs it appears as a permanent, fused band located on the anterior third of the animal (Figure 28.29).


The epidermis is protected by a collagenous, external cuticle, which is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Chitinous bristles called setae (or chaetae) are anchored in the epidermis, each with its own muscle. In the polychaetes, the setae are borne on paired appendages called parapodia .

Most annelids have a well-developed and complete digestive system. Feeding mechanisms vary widely across the phylum. Some polychaetes are filter-feeders that use feather-like appendages to collect small organisms. Others have tentacles, jaws, or an eversible pharynx to capture prey. Earthworms collect small organisms from soil as they burrow through it, and most leeches are blood-feeders armed with teeth or a muscular proboscis. In earthworms, the digestive tract includes a mouth, muscular pharynx, esophagus, crop, and muscular gizzard. The gizzard leads to the intestine, which ends in an anal opening in the terminal segment. A cross-sectional view of a body segment of an earthworm is shown in Figure 28.30 each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments.

Most annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. In addition, the dorsal and ventral vessels are connected by transverse loops in every segment. Some polychaetes and leeches have an open system in which the major blood vessels open into a hemocoel. In many species, the blood contains hemoglobin, but not contained in cells. Annelids lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. In the polychaetes, the parapodia are highly vascular and serve as respiratory structures. Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney” that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment toward the ventral side. Annelids show well-developed nervous systems with a ring of fused ganglia present around the pharynx. The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment.

Annelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads (as in polychaetes). However, cross-fertilization is preferred even in hermaphroditic animals. Earthworms may show simultaneous mutual fertilization when they are aligned for copulation. Some leeches change their sex over their reproductive lifetimes. In most polychaetes, fertilization is external and development includes a trochophore larva, which then metamorphoizes to the adult form. In oligochaetes, fertilization is typically internal and the fertilized eggs develop in a cocoon produced by the clitellum development is direct. Polychaetes are excellent regenerators and some even reproduce asexually by budding or fragmentation.

Link to Learning

This combination video and animation provides a close-up look at annelid anatomy.

Classification of Phylum Annelida

Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms, leeches, and their relatives). The earthworms and the leeches form a monophyletic clade within the polychaetes, which are therefore paraphyletic as a group.

There are more than 22,000 different species of annelids, and more than half of these are marine polychaetes ("many bristles"). In the polychaetes, bristles are arranged in clusters on their parapodia—fleshy, flat, paired appendages that protrude from each segment. Many polychaetes use their parapodia to crawl along the sea floor, but others are adapted for swimming or floating. Some are sessile, living in tubes. Some polychaetes live near hydrothermal vents. These deepwater tubeworms have no digestive tract, but have a symbiotic relationship with bacteria living in their bodies.

Earthworms are the most abundant members of the class Oligochaeta ("few bristles"), distinguished by the presence of a permanent clitellum as well as the small number of reduced chaetae on each segment. (Recall that oligochaetes do not have parapodia.) The oligochaete subclass Hirudinea, includes leeches such as the medicinal leech, Hirudo medicinalis, which is effective at increasing blood circulation and breaking up blood clots, and thus can be used to treat some circulatory disorders and cardiovascular diseases. Their use goes back thousands of years. These animals produce a seasonal clitellum, unlike the permanent clitellum of other oligochaetes. A significant difference between leeches and other annelids is the lack of setae and the development of suckers at the anterior and posterior ends, which are used to attach to the host animal. Additionally, in leeches, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates, a condition in which they are said to be “engorged.” The subclass Brachiobdella includes tiny leechlike worms that attach themselves to the gills or body surface of crayfish.