EMBRYOLOGY AND CLASSIFICATION. 309 
furnished hereafter. In many marine slugs and 
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 zodlogists. 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 
