Classification of Mollusca


Phylum MOLLUSCA

Class MONOPLACOPHORA, Ex. Neopilina galatheae

It was discovered in 1952 from the Pacific Ocean at a depth of 3,500 metres on the coast of Costa Rica. Shell is dome-like and foot flat and circular. Gills are 5 pairs and monopectinate. Visceral mass is 5-segmented. Nephridia are 6 pairs. Radula is well developed and nervous system is ladder-like. It is a connecting link between Annelida and Mollusca.

Class AMPHINEURA

Nervous system with double nerve cord with cross connections giving it a ladder-like look. Head is reduced and larva is trochophore.

Subclass Aplacophora, Ex. Neomenia; Chaetoderma

They are worm-like molluscs without shell and foot but with calcareous spicules all over the body. Mouth located on the anterior end and cloaca on the posterior end. A pair of cloacal gills is used for respiration. Locomotion is affected by a ciliated groove on the ventral side of body.

Subclass Polyplacophora, Ex. Chiton, Chaetopleura, Lepidopleurus

Marine molluscs found on the hard bottom feeding on algae. They have small head, long radula and 8 dorsal shell plates over the body. Mantle cavity has numerous lateral gill lamellae. Locomotion affected with a single ventral oval foot.

Class SCAPHOPODA, Ex. Dentalium

It is commonly known as elephant tusk shell that burrows in sandy bottom using a boat-like foot. Head, gills and eyes are absent and radula is simple. Excretion is done by nephridia and feeding by adhesive tentacles called captacula.

Class GASTROPODA

They possess well-developed head, eyes and tentacles and a large foot attached on belly. Shell is present or absent.

Subclass Prosobranchia (=Streptoneura), Ex. Pila, Cypraea, Patella

One or two monopectinate ctenidia located anterior to the heart. Nervous system is twisted to a figure of 8 due to torsion. Sexes are separate. Osphradium and operculum are present.

Subclass Opisthobranchia (=Euthyneura), Ex. Aplysia, Clione, Bulla, Doris

Gills are located posterior to the heart. Shell and operculum are absent or with a small internal shell. Nervous system is symmetrical and not twisted. They are hermaphrodite and detorted animals that may be free swimming or bottom dwellers.

Subclass Pulmonata

They are air breathing animals with a pulmonary sac, which has an opening on the dorsal side of body. Operculum is absent but shell may be present or absent. Hermaphrodite molluscs that feed on succulents and some are carnivorous on worms and snails.

Class PELECYPODA (=BIVALVIA)

Body is enclosed in a two valve shell and bears no head, eyes, tentacles and jaws. Foot is hatchet-like, used for burrowing in sand by digging. Respiration is by gills and food capture by filter feeding with the help of gills. Sexes are separate.

Subclass Protobranchia, Ex. Malletia, Sokemya, Acila

Deep water clams with two pairs of simple bipectinate gills used only for respiration. Large labial palps are used for feeding. Foot is divided into papillate margins.

Subclass Lamellibranchia, Ex. Oysters, Anodonta

Gills are paired, filibranchs and used for respiration as well as filter feeding. They are Eulamellibranch molluscs with asymmetrical valves.

Subclass Anomalodesmata, Ex. Cuspidaria, Poromya

Both shells are equal in size. Hinge margins are thickened and with reduced dentition. Mantle lobes are fused ventrally. They are active or ambush predators.

Class CEPHALOPODA (=SIPHONOPODA)

Foot is modified as head appendages with suckers. Radula is reduced and there is a pair of powerful jaws for feeding. A siphon is attached to mantle cavity for breathing and jet propulsion. There is no larval stage. Development is direct.

Subclass Nautiloidea (=Tetrabranchia), Ex. Nautilus

Body enclosed in a spirally coiled shell. Tentacles are 60-90 in number and without suckers. Gills as well as kidneys are two pairs. Ink sac is absent. Shell is filled with gas to help the animal float in water.

Subclass Coeloidea (=Dibranchia)

Shell is absent or internal. Tentacles are provided with suckers. Gills are only one pair. Ink sac is present attached to rectum. One of the arms in male is hectocotylised to transfer spermathecae into the mantle cavity of female.

Order Decapoda, Ex. Sepia, Loligo

Ten head appendages, i.e. eight oral arms and two tentacles are present on the head for capturing prey. They swim by ejecting a jet through the funnel on the ventral side of neck. Fins are present on the lateral side of the body. Left 4th arm is hectocotylised. Shell is internal.

Order Octopoda, Ex. Octopus, Argonauta

Oral arms are eight and tentacles are absent. Suckers are in two rows. Body is without shell or with a fragile shell as in Argonauta female. Fins are absent. Third right arm of male is hectocotylised. Arms are generally webbed at base. Octopus is a benthic hunter with poisonous saliva.

CLASSIFICATION OF MOLLUSCA (After Ruppert & Barnes)

Class APLACOPHORA

Subclass CHAETODERMOMORPHA, Ex. Chaetoderma

Subclass NEOMENIOMORPHA, Ex. Neomenia

Class POLYPLACOPHORA (2 orders), Ex. Chiton

Class MONOPLACOPHORA, Ex. Neopilina, Vema, Monoplacophorus

Class SCAPHOPODA, Ex. Dentalium

Class GASTROPODA

Subclass Prosobranchia (3 orders)

Subclass Opisthobranchia (8 orders)

Subclass Pulmonata (3 orders)

Class BIVALVIA

Subclass Protobranchia, ex. Nucula; Malletia

Subclass Pteriomorphia, ex. Mytilus; Pecten

Subclass Paleoheterodonta, Anodonta; Mutela

Subclass Heterodonta, Lucina; Teredo

Subclass Anomalodesmata, Clavegella; Pandora

Class SCAPHOPODA,Dentalium

Class CEPHALOPODA

Subclass Nautiloidea, Nautilus

Subclass Coeloidea (5 orders)