Mollusca IB biology Hana

Mollusca Mollusc and mollusc are both derived from the French mollusque which originated from the Latin molluscus, from mollis (soft)

What is it Mollusca is the second largest phylum of invertebrate animals Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. They are highly diverse in Size Anatomical structure Behaviour habitat

class Mollusca is classified into seven classes Gastropoda (single shelled cowries, cones, etc.) Bivalvia (two shelled like clams, mussels, etc.) Aplacophora (solenogasters) Monoplacophora (segmented limpets) Polyplacophora (or amphineura as it was earlier called – chitons) Scaphopoda (tusk shells) Cephalopoda (nautilus, squids, cuttlefish, octopus, etc.)

Features Molluscs have developed such a varied range of body structures, It is difficult to find defining characteristics to apply to all modern groups.

Features Mantle mantle with a significant cavity used for breathing and excretion Anus and genitals open into mantle cavity Dorsal body wall which covers the visceral mass (usually protrudes in the form of flaps) For gastropods, the mantle is usually fully or partially hidden inside the gastropod shell

FEATURES Radula A hard rasping radula is used for feeding It is sometimes compared to a tongue. It is like teeth, a minutely toothed, chitinous ribbon typically used for scraping or cutting food before the food enters the oesophagus. The radula is unique to molluscs, and with the exception of the bivalves, every class of mollusc has it.

Features Bilateral symmetry Most molluscs have one pair of gills/singular gills Two pairs of nerve cords, the visceral cords serving the internal organs and the pedal serving the foot. All produce eggs Some rely on external fertilization, some on internal fertilization Calcareous Shells Mostly made of CaCO3  Containing lime or being chalky

examples “head-footed” “stomach-footed”

(“bearing many plates”) Examples Aplacophora (“without plates”) Small, cylindrical, worm-like creatures (sea cucumber) Monoplacophopora (“with one plate”) Small and have a single cap like shells that make them look like limpets. Head is reduced with no tentacles or eyes; they have a rounded foot. (neopilina) Scaphopoda (“boat-footed”) Usually remain buried in sand. Worm-like animal is found within tusk shaped shells that are open at both ends. (Tusk shells) They do not have eyes, tentacles or gills. Polyplacophopora (“bearing many plates”) Eight plates called valves that are overlapping and attached to a part of the mantle called girdle. These plates are distributed around and underneath the edges.  (Chiton)