VII 



MOLLUSC A RESPIRATORY ORGANS 



89 



inner wall of the mantle, becomes attached to the mantle by its other edge 

 also (viz. that along which the branchial artery runs), somewhat to the right 

 of the first line of concrescence. In this manner, which is illustrated by the 

 accompanying diagrammatic sections (Fig. 86), the mantle cavity is divided 

 by the branchial septum into two unequal parts, which open into one another 

 anteriorly. 



Into the much smaller upper division the one row of smaller branchial leaflets 

 projects, while the opposite row of larger leaflets hangs down into the lower and 

 larger chamber. The anterior end of the 

 gill, however, is still free, its point pro- 

 jecting anteriorly (Trochidce, Turbinidce, 

 Nentidas). 



In the Docoglossa (Patellidce} the ar- 

 rangement of the gills is very varied. 

 While the Lepctidce have no gills whatever, 

 we find in Patella a single row of numerous 

 small branchial leaflets right round the 

 body, on the inner or under side of the 

 short encircling mantle fold, between it and 

 the foot. This row is broken only in one 

 place anteriorly on the left. It is, how- 

 ever, evident that these gills, which some- 

 what resemble those of the Chitonidce, are 

 no true ctenidia, from the fact that there 

 are Docoglossa (e.g. some forms of Tectura 

 and Scurria) which possess, in addition 

 to this marginal row of leaflets, a typical 

 ctenidium corresponding in every way with 

 that of the Turbinidce, Trochidce, etc. 

 Other forms, such as Acmcea, have only the 

 true ctenidium and no marginal branchial 

 leaflets. 



In the large second division of the 

 Prosobranchia the; Monotocardia the 

 arrangement of the gills is, on the whole, 

 remarkably uniform. There is only a 

 single gill feathered on one side (Fig. 71, 

 p. 73), united to the mantle along almost 

 its whole length ; this gill corresponds 

 with the left gill in Fissurella and Haliotis, and the single gill in Turbo and 

 Trochus. It generally lies quite to the left in the mantle cavity. 



The rise of this gill can best be explained by recalling the arrangements already 

 described in Turbo and Trochus. We have only to assume that the row of small 

 leaflets turned towards the mantle in Turbo disappears, and that the branchial 

 septum unites with the mantle across its whole width (Fig. 86, C, D). 



A few anomalous forms alone require special mention. 



1. In a series of terrestrial Monotocardia, aerial respiration has taken the place of 

 aquatic respiration, and the ctenidium has disappeared (Acicula, Cyclostoma, Cyclo- 

 phorus, etc.). 



2. The Ampullaria are amphibian Prosobranchia. A doubling of the mantle 

 'gives rise to a very spacious pulmonary sac, on the inner surface of which the respira- 

 tory vascular network spreads out. The lower wall of this pulmonary sac, which 

 forms at the same time the roof of the mantle cavity, is perforated by an aperture 



FIG. 85. Subemarginula after removal of the 

 shell (after Fischer). A, from above ; B, from 

 right side. The mantle cavity is exposed by 

 bending back the mantle fold 4. 1, Snout ; 2, 

 tentacle, with the eye on its short stalk behind 

 it ; 3, right ctenidium ; 4, mantle fold ; 5, shell 

 muscle ; 6, edge of the mantle encircling the 

 body ; 7, epipodium ; 8, foot. 



