io6 



THE GASTROPODA 



admitted by a very extensible pallial siphon. In the puhnonate 

 Streptoneura the pulmonary chamber retains the whole of the 

 primitive opening of the pallial cavity ; in the Euthyneura, on the 

 contrary, the opening of the lung or pneumostome is much 

 reduced by the fusion of a large extent of the mantle border with 

 the neck of the animal, a fusion that leaves only a minimal but 

 extensible posterior aperture (Fig. 177, V) in the neighbourhood of 

 the anus. This disposition allows of the blood, on its arrival at the 

 lung, being carried round a more or less annular circumpulmonary 

 venous sinus. In the Oncidiidae the lung is somewhat rudimentary, 

 being reduced to arborisations ramifying among the lobes of the 

 kidney. In other Pulmonates such as Ancylus and the Vaginulidae 

 (Fig. 87) the reduction of the lung is carried to the point of 

 complete disappearance. Finally, there is a family of Pulmonates 

 in which, instead of a vascularised lung, there is a pulmonary 



HTM, Van 



Fro. 87. 



I'ayinula occidentalis, right-side view, with the mantle partially removed on this side, an, 

 anus ; aw, auricle ; o.f, female orifice ; o.r, renal opening in the rectum ; o.r.p, reno-pericardial 

 pore ; o.r.u, orifice of the kidney in the ureter ; p, foot ; pa, mantle ; pe, pericardium ; r, kidney ; 

 re, rectum (the dotted line shows the direction of the intestine) ; ten, tentacles ; ur", ur", 

 primary and secondary ureters ; ren, ventricle. 



chamber continued into numerous tubules which penetrate into 

 the surrounding blood sinuses : these tracheate Pulmonates are the 

 Janellidae (Fig. 90, tr). A large number of Pulmonate Gastro- 

 pods, while preserving their aerial respiration, have returned to an 

 aquatic life ; such are the Basommatophora (Limnaeidae, etc.). 

 Among these the marine genera Amphibola, Siphonaria, and Gadinia ; 

 Limnaea abyssicola, an inhabitant of deep lakes; and Planorbis 

 nautilus, have a pallial pulmonary cavity which, instead of being 

 filled with air, may temporarily or continuously be filled with 

 water, as in the larvae of aquatic Pulmonates. Here we see a 

 return and readaptation to aquatic respiration, but for all that 

 the ctenidium does not reappear, a fact which illustrates the 

 irreversibility of evolution. But in these cases respiratory pallial 

 outgrowths or secondary branchiae may be formed near the 

 opening of the pulmonary cavity or even in its interior. Such 

 is the contractile extrapulmonary tegumentary appendage at the 

 base of which the anus opens in Planorbis (as this is a sinistral 



