MALACOZOA. 19 



here adopted, they form six classes, of which three contain 

 animals having a prominent or distinct head, and the 

 other three, animals in which the head is not apparent. 



MALACOZOA CEPHALATA. 



I. CEPHALOPODA. The body enclosed in a sac, open 

 anteriorly, from which emerges the head, surrounded by 

 filaments, which are at once organs of prehension and 

 locomotion. 



II. PTEROPODA. The body not saccate, nor the head 

 surrounded by filaments ; but the sides of the neck fur- 

 nished with membranous expansions. 



III. GASTEROPODA. None of the characters of the pre- 

 ceding classes ; but a fleshy foot occupying the lower 

 surface of the body, enabling the animal to crawl or swim. 



MALACOZOA ACEPHALATA. 



IV. TROPIOPODA. Four lamellar branchiae, and a com- 

 pressed fleshy foot. 



V. BRACHIOPODA. Branchiae not distinct from the 

 mantle ; two ciliated filaments in place of a foot. 



VI. APODA. The body enclosed in a cartilaginous sac ; 

 branchiae not distinct ; no special organs of motion. 



