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 s ; des 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. 



