Royal Society. 455 



Before I finish I may allow myself the observation, that, till now, 

 the whole family of Mkscicapidce has been in a condition of the great- 

 est confusion, and that the greatest number of genera must go down, 

 or must be considered as subgenera of some larger genera. 



ROYAL, SOCIETY. 



May 27. — " Upon the Morphology of the Cephalous MoUusca, as 

 illustrated by the Anatomy of certain Heteropoda and Pteropoda." 

 By Thomas Huxley, Esq., F.R.S. 



In the present memoir the author endeavours to determine, upon 

 anatomical and embryological grounds, the true homologies of the 

 different organs of the Cephalous MoUusca, and thence to arrive at 

 some idea of the archetypal form, as definite modifications of which 

 the existing molluscous forms may be considered to have arisen. 



The Pelagic Heteropoda and Pteropoda, from their small size and 

 extreme transparency, are peculiarly favourable subjects for the ana- 

 tomical part of this investigation, and it is from a detailed examina- 

 tion of those systems of organs which are of importance for the pur- 

 pose that the author deduces the following conclusions : — 



1 . In the Heteropoda the intestine is bent towards the dorsal or 

 hcemal side in consequence of the development behind the anus of 

 the visceral "hernia," which is therefore called k post-abdomen. 



2. In the Heteropoda, the " foot," in its most perfect condition, 

 consists of three portions, a. propodium, mesopodium and metapodium, 



3. The Heteropoda are more or less prosobranchiate, the degree 

 depending upon the amount of development of the post- abdomen. 



4. In the Pteropoda the intestine is bent towards the ventral or 

 neural side, in consequence of the development of the visceral " her- 

 nia" in front of the anus. It is therefore called an abdomen. 



5. In the Pteropoda, the foot, besides the parts mentioned above, 

 possesses an additional appendage, the epipodium, which forms the 

 expanded wing characteristic of the group. 



6. The Pteropoda are opisthobranchiate, prosobranchiate, or in- 

 termediate in character, according to the degree of development of 

 the abdomen. 



The Heteropoda and Pteropoda, then, may be considered to repre- 

 sent two opposite phases of the modification of the molluscous 

 archetype. 



In the second part of the paper, the author endeavours, by care- 

 fully collating the known facts of the development of the MoUusca, 

 to ascertain (a) the primary form of all cephalous MoUusca, and (6) 

 the mode in which, in the course of development, this embryonic 

 form becomes metamorphosed into the adult form ; in order, if pos- 

 sible, to account, on the safe basis of ascertained morphological laws, 

 for the peculiar modifications of structure which have been found, 

 anatomically, to obtain among the Heteropoda and Pteropoda. 



He finds that it is possible not only to deduce the structure of the 

 Heteropoda and Pteropoda from a simple and symmetrical archetype 

 by such morphological laws, but that all the cephalous MoUusca 



