86 



BIOLOGY AND HUMAN LIFE 



Fig. 55. The snail, a belly-footed 

 mollusk 



Fig. 56. The scallop 



Class 2 — Pelecypoda ("hatchet-footed"). Bivalve (having shells 

 of two valves). {Examples. Oysters, clams, piddock, 

 scallop, mussel, shipworm.) 



Class 3 — Cephalopoda ("head-footed"). The foot partly sur- 

 rounds the head and has a number of arms, or tentacles. 

 {Examples. Octopus, cuttlefish, squid, nautilus.) 



BRANCH XI — CORDATA. Animals having a notochord, or internal axial 

 basis for a skeleton. It is from this structure that the vertebral 

 column develops. There are a number of small animals which 

 never develop a true backbone, but which nevertheless have a 

 structure that suggests the beginning of such a column. These are 



Fig. 57. The octopus, a cephalopod 

 These animals have eyes that resemble in many ways those of backboned animals 



