CHARACTERISTICS OF THE PHYLUM MOLLUSCA

The name mollusca comes from the Latin word mollusc meaning soft. They are invertebrates with about 110,000 living species. Most are marine but there are few freshwater and terrestrial mollusk. They possess a visceral mass which contains the internal organs, a mantle which is a covering that lies to either side of but does not completely enclose the visceral mass. It also possess a foot which is a muscular organ that may be adapted for locomotion, attachment, food capture or a combination of functions. Another  feature is the radula, an organ that bears many rows of teeth and is used to obtain food. There is a  nervous system consisted of ganglia connected by nerve cords. The coelom is reduced and is largely limited to the region around the heart. Most mollusks have an open circulatory system. Some mollusks are slow moving and have no head, many others undergo marked cephalizatoion.

PROPERTIES OF EACH OF THE CLASS

CLASS GASTROPODA

    1.      Distinct head, bearing prominent tentacles and eyes

    2.      Visceral hump mostly twisted

    3.      Flattened muscular foot for crawling and radula for  feeding

    4.      Single shell which is coiled to accommodate twisted body.

CLASS BIVALVIA

1.      Shell develops as two valves held together by a hinge

    2.      They have reduced or no head

    3.      Eyes, tentacles and radula absent

    4.      There is very little cephalization

 CLASS CEPHALOPODA

    1.      Well developed head, surrounded by a ring of tentacles which bear suckers.

    2.      They have well developed sense organs

    3.      Shell is internal (if present)

DISTINCTIVE FEATURES

                                         BETWEEN BIVALVIA AND CEPHALOPODA

	CLASS BIVALVIA	CLASS    CEPHALOPODA
FOOD INTAKE	Filter feeder	Active predator
SKELETON	Heavy shell for protection	No external skeleton
CIRCULATION	Open	Closed
CEPHALIZATION	None	Marked
LOCOMOTION	Hatchet foot	Jet propulsion
NERVOUS SYSTEM	Three separate ganglia	Brain and nerves

                  BETWEEN BOTH (BIVALVIA AND CEPHALOPODA ­­ AND GASTROPODA

	BIVALVES AND CEPHALOPODS	GASTROPODS
SKELETON	Protection in bivalves	Protection and prevention of desiccation
LOCOMOTION	Suitable to beach and water respectively	Suitable to dry surface
RESPIRATION	Gills are kept moist by external water	Mantle serves as lungs
EXCRETION	Ammonia diluted in water	Uric acid as a solid
REPRODUCTION	No penis, separate sexes, larval stage in clam	Penis, hermaproditism, no larval stage