284 



MOLLUSC A. 



freshwater. Some, the sea-slugs or Nudibranchs, lose their 

 shells and have an external approximation to piano- 

 symmetry. Others are adapted for a pelagic life, they are 

 usually transparent, and the shells if present are thin 

 and pellucid. The foot is usually reduced but may form 

 a swimming organ. The great majority of the sub-class, 

 however, creep on the sea-floor and may be carnivorous 

 scavengers, e.g., whelks, or herbivorous^ e.g., periwinkles. 

 The shells of such types as the 



rStoTeo-IaLTS™ »™P<^t« -d --hells {Haliotis) are 

 not spirally twisted. 



Class II. — Lamellibranchiata. 



The bivalve Mollusca are usually 

 completely enveloped in the paired 

 shells. The ctenidia have been 

 enormously developed and serve to 

 feed the animal. They are mostly 

 burrowing types, all aquatic, and 

 most are marine. They illustrate 

 degrees in degeneration, the oyster 

 entirely losing its foot. The scallop 

 {Pecten) moves actively through the 

 water by snapping its shells together. 



Teredo is a worm-like form with 

 very small shells which bores i 

 way through wood. Cockles an 

 mussels are other common species. 



Class III. — Cephalopoda. 



In these the molluscan plan 

 reaches its highest level. 



Sepia is a very fair type of the 



class. They are all active free- 



g; Eight hooked tentacles (the swimming forms, with the fore part 



t:i^Zl)^TVre:v^L^^ of the /oot produced into tentacles, 



part containing the shell ; b, the hind part iuto a siphon, and 



the organs are piano-symmetric. 



shell 



phragmacone ; </, ink-sac 

 siphon (or funnel). 



