‘ 
376 Agassiz on the Relations between Animals 
of their relations to the media in which they dwell, allowing in 
this revision, a due influence to embryology as far as it can influ- 
e 
cea, we have five classes of Mollusca left, if we follow Cuvier’s, 
arrangement of these animals, as he distinguishes Cephalopoda, 
Foraminifera bear to a still earlier period of their embryonic 
growth, when the yolk is undergoing its process of gradual succes- 
sive division, which seems to me to be exemplified in a perma- 
nent form in the numerous cells into which the body of Polythal- 
amia or Foraminifera is naturally divided. If this view be cor- 
rect, the class of Gasteropoda would therefore consist of the three 
types of Foraminifera, Pteropoda and true Gasteropoda, among 
which we would place the Heteropoda, lowest, and the Pulmonata 
highest, both on account of their structure, and on the ground of 
the peculiar mode of development of Pulmonata. 
eee a paper upon the homologies of Gasteropoda and Acephala ihe Biyoee 
0 systematic position of Pteropoda, Foraminifi Brachio and oz0a, 
