DISTRIBUTION OF ORGANISMS. 



177 



We now come to the MOLLUSCA, a portion of the animal 

 kingdom, the importance of which, in point of numbers, and 

 the part they play in creation, none but students of zoology 

 could fully appreciate. The infinite variety of bivalve and 

 univalve shells presented upon our own coasts, and brought from 

 all parts of the world, will convey some idea of the multitude 

 of forms comprehended under this sub-kingdom. The whole 

 mass is, after all, resolvable into three divisions ; one of them 

 comprising headless moilusks in bivalve shells ; the other two, 

 headed moilusks in univalve shells (some, however, of all the 

 three divisions being naked). The whole sub-kingdom appears 

 to have a very brief genesis in the radiata, the only preceding 

 forms in embryo being the infusorial and polypian. Here, too, 

 as in the Articulata, we find that we must start at a point very 

 near the fountain-head of organic existence. 



In the headless division, naturalists place three sub-divi- 

 sions, called by them classes, in the following rank, according 

 to ascending grade of organization Tunicata, Brachiopoda, 

 and Lamellibranchiata. The two latter are the shell-fish of 

 popular observation, headless, and mostly sessile, or destined 



c 



Various forms of Tunicated Mollusca:A., Ascidia ; B, Boltenia; 



C, Cynthia. 



to spend their lives in fixed positions. The Tunicata are 

 similar in all essential respects, except in being of humbler 

 organization, and inclosed, not in shells, but in a cartilaginous 



N 



