4O ELEMENTS OF BIOLOGY. 



DIVISION II. MOLLUSCA PROPER. Nervous system consisting of three 

 principal pairs of ganglia. Heart -well developed, of at least two 

 chambers. . '. 



Fig. ir. 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. Ex. 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 by means of 

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

 Ex. Cleodora. 



CLASS G. CEPHALOPODA. Animal with eight or more processes or 

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



