Royal Society. 455 Before I finish I may allow myself the observation, that, till now, the whole family of Muscicapidce 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 Mollusca, 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 Mollusca, 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 ^l post-abdomen. 2. In the Heteropoda, the "foot," in its most perfect condition, consists of three portions, a. propodium, mesopodium and metapodium. 3. ThQ 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 Mollusca, to ascertain (a) the primary form of all cephalous Mollusca, 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 Mollusca
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