EMBRYOLOGY AND CLASSIFICATION. 309 



furnished hereafter. In many marine slugs ana 

 univalve shells the development of the young has 

 been traced again and again ; and their great re- 

 semblance among themselves, during the earlier 

 phases of their growth, has already attracted the 

 attention of all zoologists. This is the more re- 

 markable when taken in connection with the 

 extraordinary external difference in the appear- 

 ance of the adult. The young resemble a some- 

 what compressed oblong bag, supporting a broad 

 crescent-shaped veil, stretching evenly in every 

 direction on one side of the bag, and provided 

 around its edge with powerful vibratile cilia, by 

 the agency of which these small animals rotate in 

 the water with great activity. In this condition 

 the bag is protected by a very thin transparent 

 shell, existing even in those which are destitute 

 of shell both in the earlier and later stages of 

 their existence, being unprovided with any such 

 covering at first, and dropping it before they com- 

 plete their growth. The young of the Sea-Slugs, 

 which, with a large number of our marine Gas- 

 teropods and Pteropods, have been very carefully 

 observed, may give an idea of the younger stages 

 of, all Mollusks ; for, different as may be the 

 appearance of the young Cephalopod at some 

 periods of its life, it is not difficult, nevertheless, 

 to trace their homology, and even their close 

 resemblance, at certain periods, to the young of 



