344 M. Souleyet on the Gasteropod Mollusca. 



liquid swell the auricle and at length penetrate into the thickness 

 of the tissues of the external envelope, forming currents which it 

 is possible to follow as far as the branchial appendices ; I have 

 never seen the injected liquid spread itself into the visceral ca- 

 vity. It is also possible, by a very attentive examination, to re- 

 cognise the small venous vessels which, from the viscera and 

 especially from the ovary, proceed into the external envelope. 

 But I must also recall the fact, that in most of the Mollusca, the 

 venous system is much less apparent than the arterial system, 

 and that it often happens, as M. de Blainville has pointed out in 

 his ' Traite de Malacologie,^ that the sides of the venous vessels, 

 already extremely thin, are moreover so blended with the tissue 

 of the parts, that it becomes very difficult to recognise them : 

 most commonly then these venous vessels only assume a very di- 

 stinct vascular appearance in the large trunks which go to the 

 respiratory organs, when the latter are ver^y circumscribed ; but if 

 these organs do not present this last character, as is the case evi- 

 dently in the Bolides, the venous system will necessarily present 

 an analogous diffusion. Facts therefore appear to me to agree 

 with reasoning and with analogy, to prove that the venous system 

 really exists in the Bolides and in all the other mollusca of the 

 same group. 



The details into which I have entered, and those in addition 

 which it will be possible for me to give on the structure of the 

 external appendices of these mollusca, will also show, I hope, that 

 these appendices perform really the respiratory functions. 



M. de Quatrefages thinks he has discovered the reason of the 

 degradation of the organs of circulation and respiration in the 

 Mollusca Pldebenterata in an anatomical peculiarity first observed 

 by MM. Milne Edwards and Lowen in Calliopaia and Bolis, 

 and which consists in a prolongation of the digestive cavity in 

 the appendices of the branchire. That naturalist is of opinion 

 that this disposition of the digestive tube has the object of 

 supplying the absence of the organs of respiration, by permit- 

 ting the direct action of the air on the nutritive matters. 



The following facts and arguments appear to me again wholly 

 to contradict this theory : — 



1. If such were really the object assigned by nature to that 

 organic disposition, there ought evidently to be a relation between 

 the progressive degradation of the organs of respiration and cir- 

 culation and the development of those ramifications of the di- 

 gestive cavity which should supply their functions : now precisely 

 the contrary takes place. Thus the Bolides, which, according to 

 M. de Quatrefages himself, still possess a circulation and nume- 

 rous branchial appendices, have also a vej-y ramified digestive 

 tube ; and the last genera of his order which he designates under 



