Phylum Molusca

Kingdom Mollusca

Characteristics

·         Multicellular animals

·         Bilaterally symmetrical

·         Reached an organ system level of organization

·         Triploblastic

·         Protosmial

·         Scizocoelomates

·         Head with eyes usually on tentacles

·         Have a complete digestive system with extracellular digestion

·         Largest phylum of living organisms outside of the Athropoda

·         Primarily aquatic organisms found in almost all marine and freshwater habitats

·         Also live in terrestrial environments although they avoid direct sunlight areas

·         Open circulatory system that has a heart, several major arteries, blood sinuses and respiratory blood pigments

·         Gas exchange is across gills, lungs, or the mantle wall

·         Excretion through the use of metanephridia

·         Nervous system of most consists of 3 or 4 pair of ganglia, interconnecting fibres and sensory cells

·         The muscles pull against the shell or a hydrostatic skeleton

·         Dioecious and reproduce only sexually

Classification of Mollusca

                     I.        Class Gastropoda (snails, sea slugs)

                   II.        Class Bivalvia (clams, mussels, oysters, scallops)

                 III.        Class Cephalopoda (squids, octopus, nautilus, cuttlefish)

Class Gastropoda

Characteristics

·         Have shells

·         Asymmetrical

·         Have a single, often spirally coiled shell (univalve), undergo a developmental process called torsion, possess a mantle and a muscular foot used for locomotion

·         Use radula to scrape food

Torsion (twisting) in Gastropoda

·         Twisted nearly 180 in a counterclockwise direction

·         Gills, anus and reproductive glands, which lie directly over the head

Advantages and disadvantages of tortion

·         Increased water currents

·         Allowing the animal to withdraw more deeply into the shell

·         Disadvantages is the excrete from the anus comes out just above the head

·         The mantle cavity is designed so the poisonous wastes are quickly dispersed into the water and are quickly washed away from the Mollusca

Class Chephalopoda

·         Cephalopoda means “head foot”

·         Completely merged head and foot, with a ring of arm and/or tentacles surrounding the head. The arms, tentacle and funnel are all derivatives of the foot

·         The mantle surrounds the visceral sac and possesses strong muscles required for contraction of the cavity and respiration

Class Bivalvia

·         Includes clams, oysters, mussels, and scallops

·         Have a shell consisting of two rounded plates called valves joined at one edge by a flexible ligament called the hinge

·         Have no head, very little cephalization and no radula

·         A foot is present but laterally compressed

·         Generally have a large mantle cavity with ciliated gills that hang down on either side of the visceral mass

·         All bivalves are filter feeders