GENERAL CHARACTERS & CLASSIFICATION OF MOLLUSCA
Introduction: Mollusca includes soft bodied animals. It was first applied by Ariistotle to the cuttle-fish of the Aegean Se. The Moiluscans are soft bodied, triploblastic coelomate & bilaterally symmetrical animals. There are about 10,000 living species. It is the second biggest phylum of the animal kingdom.
- They are mostly aquatic ani mals, mostly marine and a few are fresh water. Some are terrestrial.
- The body is soft and unsegmented.
- The body shows 4 parts. 1) visceral mass 2) the head 3) the foot and 4) mantle. Head is absent in Pelecypoda.
- They are bilaterally symmetrical, but some gastropod are assymmetrical.
- Head bears a terminal mouth, eyes, tentacles and other sensory organs. It is absent in the Pelecypoda and the Scaphopoda.
- The foot is ventral, muscular and locomotory organ. It is absent in the oysters.
- The visceral mass, includes the vital organs of the body.
- The mantle or pallium is muscular fold of the body wall which covers the visceral mass dorsally and laterally.
- The space present between the mantle and the visceral mass is called pallial cavity.
- In the pallial cavity gills and the openings of the digestive, and reproductive systems are present.
- The mantle secretes a hard shell, which covers the body. It may be absent in some molluscs Ex: Doris.
- The body cavity is haemecoel.
- The true coelomic cavities are restricted.
- Open type of circulatory system is seen except cephalopoda. The ciorsai hear t Having one to fcuf auricles and one ventricle, is enclosed within the pen ardial cavity.
- Tht olood is colourless. Respiratory pigment, if present is haemocyanin.
- The alimentary canal is straight, or U-shapedor coiled.
- The buccal cavity contains a raduia, except in bivalve.
- A large digestive gland or hepatopancreas and salivary glands are present.
- The espiratory organs are ctenidia. Pulmonary chamber is developed for their respiration in the terrestrial forms.
- The excretory system contains one to six pairs of kidney.
- The nervous system shows paired cerebral, pleural pedal and visceral ganglia.
- The sexes are usuaUy separate. Few are hermaphrodite.
- Cleavage is spiral, and holoblastic. The development is either direct or indirect. Glochidium larva is seen in Unio. Veliger larva is seen in Gastropods.
The classification followed here is based on Morton and Yonge (1964). The Phylum is divided into six classes.
Class 1: MONOPLACOPHORA: Body is bilaterally symmetncal. with internal segmentation. Shell shows single piece. Foot is ventral and flat. Heart contains 2 ventricles. Most of the species were extinct. Ex: Neopilina galathea is recently described species.
Class 2: AMPHINEURA: Body is cylindrical. They show bilateral symmetry. Body is covered by a shell consisting of a row of eight transverse plates cn the dorsal side. In some animals shell is absent. Mouth and anus are terminal. Heart contains one ventricle. One pair of nephridia will be present. It is divided into 2 orders.
Order 1: Aplacophora ( Solenogastrea): These are worm like amphineurans. Shell is absent. Foot is absent or vestigeai. Mantle is thick, and show calcareous spicules. Sexes are separate or bisexual. Ex: Neomania, Chaetoderma.
Order 2: Pclyplacophora: Shell is made of eight plates present on the dorsal side. Foot is large, muscular and ventral. In between the mantle and body many gills are present. Sexes are separate. They are marine animais. Ex: Chiton.
Class 3: SCAPHOPODA: These are burrowing molluscs called tusk shells. Body is bilaterally symmetrical. Distinct head is absent. Eyes and gills are absent. They are marine. Shell has single valve and open at both ends. Foot is conical and used for burrowing, Sexes are separate. Ex: Dentalium.
Class 4: GASTROPODA: These are asymmetrical molluscans. Head is distinct with one or two pairs of tentacles and eyes. Buccal cavity has radula. Shell has single valve. Foot is large and ventral. One or two nephridia are present. Sexes are separate. Some are bisexual.
Sub Class I: Prosobranchia: Mantle cavity opens anteriorly and contains gills. Head shows a single pair of tentacles. It undergoes 180° torison. Nervous system becomes "8" shaped. Sexes are separate.
Order 1: Archacogastro poda: Primitive forms show bilateral symmetry They contain 2 ctenidia. Ex : Patella, Haliotes.
Order 2: Mesogastropoda: They show single monopectinate ctenidium. The nervous system is more concentrated. Ex: Pila, Strombus.
Order 3: Neogastropoda: Single monopectinate ctenidium is seen Ex: Murex, Buccinum.
Sub Class II: Opisthobranchia: Mantle cavity opens posteriorly and contains one ctenidium and one nephridium . Shell is reduced or absent. Bisexual animals.
Order 1: Tectibranchia: The shell is thin and has a large head Ex: Aplysia.
Order 2: Nudibranchia: Shell, mantle cavity and ctenidia are absent. Anal gills are present. Ex: Doris, Eolis.
Sub Class III : Pulmonata: Anterior part of mantle becomes pulmonary cavity. Gills are absent. Shell is spiral or absent. Head has 1 or 2 pairs of tentacles, and one pair of eyes. Hermaphrodite animals are included.
Order 1: Stylommatophora: They live in moist places. They show 2 pairs of tentacles. Ex: Helix, Umax.
Order 2: Basommatophora: They are fresh Water forms. Head bears one pair of tentacles. Ex: Lymnaea, Planorbis.
Class 5: PELECYPODA: Body is bilaterally symmetrical. Radula and tentacles are absent. Shell is bivalved & wedge shaped. Gills are 1 or 2 pairs in number. They are marine and fresh water arms.
Order 1: Protobranchia: Primitive members. Gills are posteriorly placed. Foot is flat. 2 adductor muscles are seen. Ex: Nucula.
Order 2: Filibranchia: Gill filaments are long, folded with ciliary connections between lamellae. Adductor muscles are two. Anterior muscles may be reduced. Ex: Mytilus, Ostraea, Pecten.
Order 3: Eulamellibranchla: Gill filaments folded and lamellae are united by tissue junctions, 2 equal size adductor muscles are present. Ex: Unio, Anodonta.
Order 4: Septibranchia: Gills become horizontal muscular partitions in between inhalent chamber and suprabranchial cavity. Ex: Poromya.
Class 6: CEPHALOPODA: Body is elongated. Bilaterally symmetrical animals. Distinct head bears large eyes, radula and jaws. Shell is external, or internal or absent. Foot is modified into arm around the head and into siphon. All are marine and free-swimming.
Sub Class I: Nautioidea (Tetrabranchia): External shell is present. Tentacles are many suckerless Nephridia are two pairs. Two pairs of gills will be seen. Ex : Nautilus.
Sub Class II: Coleoidea (Dibranchia): Shell is internal or absent. 8 to 10 arms are present with suckers. Nephridia are one pair of gills will be seen.
Order 1: Decapoda: Body shows 2 long tentacles and 8 short arms. Ex: Sepia, Loligo
Order 2: Octopoda: Body shows 8 equal arms. Ex: Octopus. Argonauta, Loligo.