40 



ELEMENTS OF BIOLOGY. 



Division II. Mollusca Proper. — Nervous system consisting of three 

 j)rincipal pairs of ganglia. Ueart well developed, of at least two 

 chambers. 



Fig. II. — Mollusca. Diagram of a Cuttle-fish. (Altered from Huxley.) 



Class D. Lamellibranchiata. — No distinct head or teeth. Body- 

 enclosed in a bivalve shell. One or two leaf-like gills on each side of 

 the body. £x. Oyster, Mussel, Cockle. 



Class E. Gasteropoda. — A distinct head and toothed tongue. 

 Shell, when present, univalve or multivalve, never bivalve. Locomo- 

 tion effected by creeping about on the flattened under-surface of the 

 body (" foot"), or by swimming by means of a fin-like modification of 

 the same. Ex. Whelk, Periwinkle, Snail. 



Class F. Pteropoda. — Animal oceanic, swimming bv means of 

 two wing-like appendages, one on each side of the head. Size minute. 

 £x. Cleodora. 



Class G. Cephalopoda. — Animal with eight or more processes or 

 " arms" placed round the mouth. Mouth armed with jaws and a 



