10 The Shell-Collector’s Handbook. 
they live—the respiratory organs consist of pectinated — 
branchiz enclosed in a branchial chamber formed by 
a folding-in of the mantle, the water being admitted, 
in many species, by a tubular prolongation of the 
mantle edge (anterior siphon), and expelled through a 
similarly constructed posterior siphon. But in the 
Pulmogasteropoda—which inspire atmospheric air— 
inspiration and expiration are effected through the 
pulmonary aperture. This pulmonary aperture leads 
into the mantle-cavity (lung), the upper and lateral 
walls of which are formed by a thin fold of the body- 
skin (mantle), and the floor by the convex, muscular 
dorsal wall of the body. The mantle is extremely 
vascular in character, the vascularity being due to the 
reticulated arrangement of the afferent and efferent 
pulmonary vessels which converge to form the pul- 
monary vein. There the blood is arterialised, by the 
absorption of oxygen from the air, and the excretion 
of carbonic dioxide as a waste tissue product.* 
Inspiration is effected by the contraction of the 
muscular bulging on the floor of the mantle-cavity. 
This produces a flattening of the floor, and, conse- 
quently, an enlargement of the cavity, and air rushes 
in through the pulmonary orifice to fill up the void. 
The return of the muscles to their former state of 
relaxation results in an expiration. An analogous con- 
dition may be found in the contraction and relaxation 
of the mammalian diaphragm. 
THE Kipney, Foot-GLanp, AND TatL-GLanp.—The 
* Paludina has both branchiz and lung, while Limneza 
possesses no branchize, and uses its air receptacle as a hydrostatic 
organ. 
