Animals and the Elements in which they Live, 203 



combined with Polypi. As to Pteropoda and Gasteropoda, 

 though they are still generally considered as two classes, we 

 shall, for reasons explained elsewhere,* and from embryolo- 

 gical evidence, place Pteropoda below Gasteropoda proper ; 

 not as an intermediate type between Gasteropoda and Cepha- 

 lopoda ; for Pteropoda, or rather an embryonic type exem- 

 plifying, in a permanent form, that stage of development of 

 common Gasteropoda "^;yhen they are provided with large 

 vibracula, and a thin symmetrical shell deciduous in so many 

 of them ; bearing to that state of development of the common 

 Gasteropoda the same relation which Foraminifera bear to a 

 still earlier period of their embryonic growth, when the yolk 

 is undergoing its process of gradual successive division, which 

 seems to me to be exemplified in a permanent form in the 

 numerous cells into which the body of Polythalmia or Fora- 

 minifera is not usually divided. If this view be correct, 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 Pul- 

 monata. 



The third class is that of Cephalopoda, which has always 

 been circumscribed within natural limits, since Foraminifera 

 have been removed from it. The position which I ascribe 

 here to Foraminifera will appear very natural to those who 

 are equally conversant with the succession of fossil types in 

 geological periods, and with embryology, and who know, as 

 we have seen it to be the case also among Radiata, that the 

 higher classes reproduce in their lower forms, types analo- 

 gous to the lower ones. For the great number of fossil- 

 chambered shells, existing in earlier geological periods, is 

 very striking when we compare those old representatives of 

 the class Cephalopoda with their condition in the present 

 period of the creation, and the natural gradation and analogy 

 between Bryozoa, as the lowest type of Acephala, with the 



* See a paper upon the Homologies of Gasteropoda and Acephala, with re- 

 ference to the systematic position of Pteropoda, Foraminifera, Brachiopoda, and 

 Bryozoa, read before the American Association, &c. 



