236 Miscellaneous. 



Finally, in the adult the pulmonary artery conveys almost pure 

 venous blood, and thus commences to realize the conditions exhibited 

 by the circulatory system of vertebrates with separate ventricles. — 

 Comptes Benclus, t. cxix. no. 1 (July 2, 1894), pp. 98-100. 



Branchiate Pulmonates. By M. Paul Pelseneer. 



I. Among the aquatic pulmonate Mollusca of Madagascar there 

 is ftrand a sinistral form which normally exhibits, below the pul- 

 monary aperture and to the left of the anus, a well constituted gill. 

 This gill is plicated, and not pectinated (that is to say, that it is 

 formed like that of the Opisthobranchs), and is attached merely by 

 its base. But it is not homologous with the gill or ctenidium of 

 the rest of the Gastropoda : it is, as a matter of fact, situated entirely 

 outside the pallial chamber, while in the latter it is contained within 

 it. It is therefore a new formation. 



II. The appearance of this organ upon a Pulmonate is explained 

 by the study of our indigenous forms, certain of which already 

 possess this gill, but in a less developed condition : Planorbis and 

 Ancylus may be taken as instances. 



Planorbis corneus exhibits, outside the pallial or pulmonary 

 chamber and to the left of the anus, a flattened, smooth, and exten- 

 sile tegumentary lobe, the structure of which reveals its respiratory 

 function ; the same lobe, proportionately smaller, exists in Planorbis 

 marginatus. 



Ancylus also possesses this lobe (on the right side in A. lacustris), 

 which in this case has for a long time already been designated the 

 gill, and which performs the functions of such an organ iu a con- 

 tinuous manner, for in this genus there is no longer any trace of a 

 pallial chamber (or lung). Now we know that Planorbis is of a 

 much less aerial habit than Limncea, and we are also aware that in 

 pure water Ancylus remains almost entirely immersed (which 

 explains the disappearance of its lung). 



These Pulmonates, having lost the original molluscan gill (or 

 ctenidium), but having subsequently reverted to an aquatic life, 

 there is nothing astonishing in the fact that they have developed a 

 fresh gill, morphologically different from the former, although in 

 the case of the Pulmonate from Madagascar it bas a similar con- 

 formation ; we have here merely a remarkable example of homo- 

 plasy and of the irreversibleness of evolution, that is to say of the 

 powerlessness of an organ which has been lost to reappear. 



III. The mollusk from Madagascar in question is only known 

 conchologically, and bears the name Physa lamellata. But its whole 

 organization shows that it does not belong to the genus Physa ; I 

 confine myself here to pointing out the absence in the latter (as in 

 Limncea) of the para-anal branchial apparatus. 



Physa lamellata constitutes the type of a genus very closely allied 

 to Planorbis, which I propose to term Pulmobranchia. — Comptes 

 Bendus, t. cxix. no. 5 (July 30, 1894), pp. 354, 355. 



