343 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 : raetathorax ver}- 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 ; coxse 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 Plilebenterata 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. JM. de Quatrefages says, in fact, that he has 

 observed in these Mollusca the partial or complete disappearance 

 of the organs of circulation, w hence 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 Medusce for example. 



The strangeness of these results, which appear to me contrary 

 not only to all known facts on the organization of IMollusca, 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 ]\I. de Quatrefages. 



It is known that among the Mollusca in question there are first 



* From the Covipies Rendtis for August 12th, 1844. 



