146 MOLLUSKS 



Mussels and snails are the most common representatives 

 of the great Branch, Mollusks. 



Summary. — What special preparation is made by the 

 snail for its life in the water? Can a snail remain for an 

 indefinite period under water? What change in structure 

 would be necessary to enable the snail to breathe in water? 

 The gastropods that live in the sea have that specialization. 



Land snails are used extensively for food, but water snails 

 are not generally considered edible. 



Which has a stronger shell, a land or a pond snail ? Can 

 you give a reason for this? 



The cephalopods constitute another class of Mollusks. 

 They have arms, or tentacles, surrounding the head, and are 

 usually destitute of external shell except the Nautilus which 

 has a coiled, chambered shell. 



Are the cephalapods which have no shell as able to pro- 

 tect themselves as shelled Mollusks? Is passive protec- 

 tion among animals as successful as active, aggressive pro- 

 tection? Give examples of each. 



Branch mollusca. — Adapted from McMureich. 



I. Class Amphineura. — Visceral hump not developed ; bilaterally 

 symmetrical ; shell represented by scattered spicules or by a series 

 of calcareous plates ; anus terminal. 



1. Order Solenogastres. — Shell represented by scattered calcir- 

 eous spicules Neomenia. 



2. Order PoLYPLAcopnoEA.- — Shell formed by eight plates on dor- 

 sal surface of body Chiton. 



II. Class Gasteropoda. — Visceral hump usually well developed ; 

 body asymmetrical ; shell univalved and usually spirally coiled, 

 sometimes absent; anus not terminal. 



I. Order Prosobranchia. — Ctenidia present, situated in front of 

 the heart ; auricle in front of ventricle ; mantle edge not fused 

 with body. 



