PHYLUM MOLLUSC A 



197 



aquatic forms depend upon periodic visits to the surface of 

 the water for renewing the oxygen supply in the lung, though 

 in some instances the lung has become adapted secondarily for 

 water respiration. Physa, Lymnaea, Planorbis, and the limpet- 

 shaped Ancylus are fresh-water pulmonates. The slugs (Limax, 

 Arion, and Agriolimax) and many land snails (Polygra, Helix, 

 Pupa, Bulimulus) are also pulmonate Euthyneura. 



Class Cephalopoda 



The cephalopods are the most highly organized molluscs. 

 In habits, they are exclusively marine. Among present-day 

 forms, they include the squids (Fig. 93 A), cuttlefishes, devilfishes 



Fig. 93. — Cephalopods. .4, ventral view of a squid, Loligo opalescens; B, dorsal 

 view of Polypus bimaculatus. (After Berry). 



(Fig. 93 B), and Nautilus. When a shell is present it is almost 

 always internal, for only Nautilus of our recent cephalopods 

 fives within a shell. In most instances (except in Nautilus), a 

 head is well defined. A pair of strong jaws, each in shape 

 resembfing the beak of a parrot, are located within the mouth 

 cavity. The eight or ten characteristic arms which surround 

 the mouth and the funnel-shaped siphon are modifications of 



