GASTEROPODA. 419 
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UTR 
Fic. 228.—Anatomy of Chzton. 
A, ventral surface (after Cuvier), B, dorsal view of alimentary 
canal (after Lankester). C, genital and excretory organs from 
dorsal surface (after Lang and Haller, diagrammatic). 7., 
mouth ;a@., anus ; 6v., numerous simple gills ; 7, foot ; 4., buccal 
mass ; 4., liver 3 z., intestine ; ao., aorta; v., ventricle of heart ; 
ya and Za., right and left auricles ; ov., ovary ; od., oviduct ; 
od'., opening of oviduct ; ., part of nephridium, represented in 
black throughout ; ~o., external opening of nephridium; Z., 
outline of pericardium. 
Sub-class II. GasTEROPODA ANISOPLEURA, 6.2. 
Snail, Whelk, Limpet 
In these more or less asymmetrical Gasteropods, the head 
region, which is well developed, remains symmetrical, and so 
does the foot, which ts typically a flat creeping organ. But 
the visceral mass_or hump, with its mantle fold, is more or less 
twisted forwards and to the right. Thus the pallial, anal, 
nephridial, and genttal apertures usually lie on the right side, 
more or less anteriorly. A further asymmetry is shown by 
the twisting of the morphologically right gill to the left side, 
while the original left gill ts usually lost. Similarly, one of 
the nephridia, probably that which is morphologically the left, 
tends to disappear, and in most cases only one persists— 
topographically on the left side. The main torsion must be 
distinguished from the spiral twisting which the visceral 
hump often exhibits, and from the frequently associated spiral 
coiling of the untvalve shell, Moreover, a superficial secondary 
bilateral symmetry tends to be acquired by free-swimming 
Jorms, e.g. Heteropods. “There are never more than two gills 
