146 Descriptive Zoology. 



while the scallop swims actively ; clams and oysters feed 

 on microscopic forms swept in by currents of water, while 

 the cephalopods prey upon the most active fishes ; there is 

 strong contrast between the monotonous existence of the 

 headless clam, burrowing in the mud, and the free life of 

 the cuttlefish, with its distinct head and highly developed 

 eyes; the oyster is fixed to his spot, almost as passive as a 

 sponge, while the squid darts so swiftly that it is called 

 the arrow fish. 



Nevertheless the following characteristics belong in 

 common to the various classes of mollusks : 



1. Aside from the shell the body is soft; hence the 

 name "mollusk," soft. 



2. The body is unsegmented, in distinction from the 

 arthropods, the vertebrates, and many worms. 



3. There is an extension of the skin called the " mantle," 

 which usually produces a shell, univalve, bivalve, or rarely 

 multivalve. 



4. There is usually a ventral muscular extension, the 

 foot, which, in most forms, serves in locomotion. 



5. They are mostly bilaterally symmetrical, but some 

 are much distorted. 



6. The nervous system consists of about three pairs of 

 ganglions, connected by nerve cords. 



CLASSIFICATION OF THE MOLLUSCA. 



As all earlier classifications are based on superficial 

 characteristics, it was to be expected that the first classifi- 

 cations of mollusks would be by their shells. Hence the 

 science of Conchology. But now we class the mollusks, 

 as other groups, by their general plan of structure, 

 mainly of the soft parts, for these parts make the shell, 

 and the shell does not mould them. 



