MARINE MOLLUSCS 



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because a knowledge of them is essential to the student of the 

 mollusca, we shall briefly note their principal characteristics. 



The pteropods are extremely abundant in some seas, occurring 

 in such vast numbers that they discolour the water for miles. 

 They swim about by flapping the pair of wings already referred 

 to. They are known to form an important article of the diet of 

 the whale, and are also devoured in enormous numbers by various 

 sea birds ; and they are themselves carnivorous, feeding on various 

 smaller creatures that inhabit the open waters. 



In appearance they much resemble the young of higher species 

 of molluscs. The nervous system consists of a single ganglion 

 situated below the gullet, and the eyes and tentacles are either 

 rudimentary or absent. The digestive system includes a muscular 

 gizzard provided with teeth for the mastication of food, and 

 a digestive gland or liver for the preparation of a digestive fluid. 



FIG. 160. PTEROPODS 



The heart has two cavities, and respiration is effected by a surface 

 covered with minute cilia. This surface is either quite external 

 or is enclosed in a chamber through which water freely circulates. 



The shell is very different from that of a typical head-bearing 

 mollusc, for it generally consists of two glassy, semitransparent 

 plates, situated dorsally and ventrally respectively on the body of 

 the animal, with an opening for the protrusion of the body, and 

 others at the sides for processes of the mantle ; and it terminates 

 behind in one or three pointed processes. Sometimes, however, 

 its form is conical or spiral, with or without an operculum. We 

 append illustrations of a few pteropods, selecting for our purpose 

 species that have been found in the Atlantic. 



It will have been noticed from the above short description 

 that the pteropod is very unlike the typical Cephalophore as 

 outlined in our general remarks on the group, especially in the 



