342 M. Souleyet on the Gasteropod Mollusca. prothorax very short : scutum of the mesothorax broad ; sutures of the parapsides indistinct ; axillae large, triangular ; scutellum rhom-boidal : metathorax very short : propodeon short, decumbent : po-deon very short : abdomen circular, smooth, shining, concave, a little broader and much shorter than the thorax, having a fulvous spot towards the base ; metapodeon large ; octoon and the following segments short : legs yellow ; coxae green : wings having a slight fulvous tinge ; nervures fulvous ; humerus much less than half the length of the wing ; ulna more than half the length of the humerus; radius not longer than the ulna; cubitus hardly more than half the length of the ulna; stigma small, emitting a very short branch. Alten. XLIV. — Observations on the Gasteropod Mollusca, designated by the name of Phlebenterata by M. de Quatrefages. By M. Souleyet*. M. de Quatrefages has communicated to the Academy of Sci-ences a series of researches upon a group of Gasteropod Mollusca, whose organization differs widely, according to that naturalist, from that of the animals of the same type, and presents very sin-gular peculiarities. M. de Quatrefages says, in fact, that he has observed in these Mollusca the partial or complete disappearance of the organs of circulation, whence would result a corresponding degradation in the respiratory organs ; and he is moreover very much inclined to the opinion that the posterior aperture of the digestive tube disappears even in some of these Gasteropods, which would consequently bring them much nearer to animals of the most simple structure — the Medusae for example. The strangeness of these results, which appear to me contrary not only to all known facts on the organization of Mollusca, but also to all admitted and recognised principles in zoology, led me to extend the researches which I made some years ago on the anatomy and physiology of these animals, to those which form the subject of the memoirs published by M. de Quatrefages. Having already had occasion to observe some of these Gasteropods during the expedition round the world of the Bonite, I endeavoured to complete this investigation by that of species which are found on our coasts ; but, as I cannot yet submit to the Academy the work which I have written on this subject, and which is retarded by the execution of the drawings accompanying it, I meantime re-quest permission to state succinctly some of the results at which I have arrived, and which appear to me to contradict almost en-tirely those announced by M. de Quatrefages. It is known that among the Mollusca in question there are first * From the Comptes Rendus for August 12th, 1844.
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