310 EMBRYOLOGY AND CLASSIFICATION. 
the Gasteropods, described above. I have satis- 
fied myself of this fact while studying the trans- 
formation of the young of our common Squid 
(Loligo illecebrosa). Nor are the young of our 
common Bivalves, and even those of our Ascidians 
(the so-called soft-shelled Clams) or of the Bryo- 
zoa, essentially different ; while a closer compari- 
son of the condition of the young of the two latter 
groups, during the stage when they swarm in the 
water as free floating animals, will readily show 
that they bear a similar relation to the young 
Gasteropods and the young Cephalopods as the 
Echinoderm larve bear to the young swarming 
Acalephs or the young Polyps. In the later 
phases of their advance toward a mature con- 
dition they constantly recall the appearance and 
form of other representatives of their respective 
classes, so that even an imperfect acquaintance 
with this subject leads to the inference that there 
probably exists between the successive stages of 
growth of the Mollusk the same correspondence 
with the different members of their respective 
classes, in their natural gradation, as has already 
been observed in the other types. A comparison 
of the young of many Gasteropods, which, like 
Natica, Pyrula, Buccinum, and Purpura, under- 
go their early development in solid egg-cases, 
has already furnished very interesting results ; 
and collectors cannot be too careful in gather- 
