(r 



8.V 





V( 





EnoliA 



•t, 



, < 



It 



mi 



\- 





--i • 



*. IlCil; 



1 



I 



' V 



1 • 



D^. 



hi 



■* 1 





Q 



lci">- 



, I: 



* 



= in It 



*1 



^.* 



It V 



t . 



llllv 



1. ■'' 



,'. t. 





Liii'- 





Ch. XL.] 



MIGEATION OF TESTACEA. 



375 



verbially slow motion of snails and mollusks in general^ and 

 altliongli many aqnatic species adhere constantly to the same 

 rock for their whole lives, they are by no means destitute of 



area. 



disseminatin 

 3 MoUusca,' £ 



metamorph 



either in the egg or ont of the egg. The Gasteropoda com- 

 mence life nnder the form of a small spiral shell, and an 

 animal furnished with ciliated wings, or lobes, like a pteropod, 

 by means of which it can swim freely, and in this form can 

 mio-rate with ease throug^h the sea.'"^ 



We are accustomed to associate in our minds the idea of 



mature 



Fig. 135. 



A 



The youiig fry of a cockle (Cardium pygmseum), from Loven's Koiigl. "N^etenskaps 



Akadem. Handling, 1848. 



A. The young just hatched, magnified 100 

 diameters. 



B. The same farther advanced. 



a. The ciliated organ of locomotion with 



its filamentous appendage b. 



c. The rudimentary intestine. 



d. The rudimentary shell. 



perfect state of each species of invertebrate animal^ especially 

 when they undergo a series of transformations ; but in all 

 the MoUusca the reverse is true. The young of the cockle^ 

 for example [Cardium) ^ possess^ when young or in the larva 

 state, an apparatus which enables them both to swim and to 

 be carried along easily by a marine current. (See fig. 135.) 



w 



These small bodies here represented, which bear a con- 

 siderable resemblance to the fry of univalve, or gasteropodous 

 shells, above mentioned, are so minute at first as to be just 

 visible to the naked eye. They begin to move about from the 

 moment they are hatched, by means of the long cilia, a, a, 

 placed on the edges of the locomotive disk or velum. This 



"^ Edin. New Phil. Journ. April, 1844. 



