BRITISH NUDIBRANCHIATE MOLLUSCA. 7 



anus was considered to be absent. The alimentary system of these Mollusks was thus 

 reduced to a single opening for the reception of food and the rejection of excrementitious 

 matters, a condition hitherto only known to exist among the inferior divisions of the. 

 JRadiata* 



" The Mollusks whose history I have now given," says M. de Qnatrefages, " appear to 

 me to merit the especial attention of zoologists. In the vicinity of animals which all 

 naturalists place in the class Gasteropoda, we see them preserve the general aspect and 

 external characters from which this large group derives its name, but, at the same time, we 

 see their organisation depart in such a manner from the primitive type, that the principal 

 systems of vital organs are modified profoundly, and that two of them, which are generally 

 considered essential, disappear." "Holts, Calliopcsa, Zepliyrina, &c," he adds, "are so 

 evidently gasteropodous Mollusks, by their external form, that all naturalists have referred 

 them to this extensive group. To Pelta and Chalidis the same place would certainly be 

 assigned, yet the anatomical characters of these animals exclude them not only from the class 

 Gasteropoda, but even from the department of the Mollusca."f 



This Memoir was presented to the French Academy of Sciences, and a commission of 

 that learned body was appointed to report upon it. The report, drawn up by M. Milne 

 Edwards, spoke very favorably of M. de Quatrefages' researches, which were described as 

 leading to results highly important in the history of the Mollusca ; and it further expressed an 

 opinion that among the works by which zoology had been enriched for many years, there 

 was, perhaps, not one which embraced so great a number of new and curious facts. This 

 report was adopted by the Academy, as was also a resolution expressing the importance of 

 making similar researches on the Phlebenterata of the Mediterranean. 



In consequence of this recommendation, M. de Quatrefages was sent out by the French 

 government, in the summer of that year, on a scientific expedition to the coast of Sicily, in 

 company with M. Milne Edwards and M. Blanchard. Many valuable essays and monographs, 

 resulting from this expedition, have appeared from time to time in the 'Annales des Sciences 

 Naturelles,' and have since been published in a collected form. Among these no account of 

 the researches of M. de Q-uatrefages on the Phlebenterate Mollusks have yet been given to the 

 public. We learn, however, from a letter addressed to the Academy of Sciences by that 

 gentleman, that his researches had tended to confirm the results he had already arrived at. 

 In June, 1 844, he writes that he had had the good fortune to collect twenty-one new species 

 of these animals, a small number only of which could be included in known genera, and that 

 he had studied the anatomy of them in great detail. He states that the circulatory apparatus 

 did not exist, even in a rudimentary state, in the greatest number of the Phlebenterata. That 

 in the whole of their external characters they resembled the Nudibranchs, but that they were 

 distinguished from them by the tendency to a bilateral symmetry of the external organs, and 

 by the repetition of the same organs in longitudinal series ; and that in all the function of 

 digestion was confounded with those of respiration and circulation. % 



* It may be necessary to state, that though M. de Quatrefages adopts this opinion, and founds his 

 generalisations upon it, he afterwards, in the same essay, makes a reservation of the possibility of his 

 having overlooked the anal opening in some of the species. 



f Loc. cit., p. 168. 



\ c Comptes Rendus/ v. 19, p. 190. 



