Branch Mollusca 85 



and forcibly expel it through the siphon tube, and 

 this enables it to dart through the water with great 

 rapidity. 

 VII. — Make a drawing of the ventral view of the squid. 



THE CLASSIFICATION OF THE MOLLUSCA 



The Mollusca is the great branch, or phylum, to which 

 belong the mussels, snails, limpets, and cuttlefishes. Many 

 of its members are believed to be closely allied to worms, 

 though the members of the two branches differ widely in 

 external appearance. In the development of the Mollusca 

 the segments have been fused together, while the skin has 

 secreted a calcareous shell ; the relative length of the body 

 has decreased, and in the lower groups there is no distinct 

 head. The alimentary system is greatly developed, espe- 

 cially the liver. The phylum includes terrestrial, fresh- 

 water, and marine forms. There are three principal 

 classes : — 



I. — Pelecypoda. Oysters and Clams. 



This class includes " laterally compressed Mollusca 

 without separated head, with bilobed mantle and bi- 

 valve shell, composed of a right and left half and con- 

 nected by a dorsally placed hgament." The sexes are 

 generally separate, and there are large gill-plates on 

 account of which the class has commonly been called 

 Lamellibranchiata. The locomotive organ consists of 

 a single median muscular foot, and the parts of the 

 body are arranged with bilateral symmetry. Both 

 salt and fresh water forms are included, all of which 

 breathe by means of gills. 



II. — Gastropoda. Snails, Slugs, and Limpets. 



The members of this class usually have a shell in a 

 single piece which commonly has the shape of a spiral 



