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.
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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.
Distribution—Pacific coast.
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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.
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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.
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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.
5. Gills absent.
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.
6. Jaws and radula absent.
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.
5. Ink gland absent.
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.
3. Tubular siphon.
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.
6. Eyes with lens.
7. Ink gland present.
Examples: Loligo, Sepia, Octopus, Argonauta, etc..