106 THE GASTROPODA 
admitted by a very extensible pallial siphon. In the pulmonate 
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 cireumpulmonary 
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 
/ | of ze leap abs 
Fic. 87. 
Vaginula occidentalis, right-side view, with the mantle partially removed on this side. an, 
anus; aur, auricle ; o.f, female orifice ; 0.7, renal opening in the rectum ; 0.7.p, reno-pericardial 
pore ; 0.7.u, orifice of the kidney in'the ureter ; p, foot ; pa, mantle ; pe, pericardium ; r, kidney ; 
ve, rectum (the dotted line shows the direction of the intestine); ten, tentacles; wr’, wr”, 
primary and secondary ureters ; ven, ventricle. 
chamber continued into numerous tubules which penetrate into 
the surrounding blood sinuses: these fracheate 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, ete.). 
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 
