76 THE GASTROPODA 
topographically on the left side, and vice versa. (2) The visceral 
commissure, while maintaining its position in respect to the 
digestive canal, becomes twisted (Fig. 52) in such a manner that 
its right moiety with its ganglion passes over to the dorsal side of 
oesophagus (Fig. 57), and is therefore called supra-intestinal, while 
the left moiety passes under the oesophagus towards the right side, 
which explains the name infra-intestinal given to this portion of 
the commissure and the nerve-centre borne on it. (3) The 
original symmetry of the organisation disappears. The anus 
does not remain in the centre of the pallial cavity, but is dis- 
placed towards the right side. The organs situated on the topo- 
graphically right— but morphologically and originally left—side 
atrophy (Pleurotomaria, Scissurella, etc.), and eventually disappear. 
An essential feature of the asymmetry of Gastropods is the atrophy 
or disappearance of the topographically right (morphologically left) 
Fria. 55. 
Trochus cinerarius, heart and 
kidneys, dorsal aspect (some- 
what schematic). I, left renal 
pore; II, right renal pore; III, 
right kidney; IV, papilla with 
the common opening of the peri- 
cardium and of the gonad into 
the right kidney ; V, right reno- 
pericardial duct; VI, anterior 
part of the gonad; VII, right 
auricle; VIII, ventricle; IX, 
pericardium ; X, left auricle; 
XI, left reno-pericardial orifice ; 
XII, branchial efferent vessel; 
XIII, left kidney ; XIV, rectum ; 
XV, gill. 
Vill Vil 
half of the circumanal complex, involving the ctenidium, auricle, 
osphradium, hypobranchial gland, and kidney. In forms with 
situs inversus, or, as they are generally called, sinistral forms, the 
phenomenon is reversed: the organs of the left side are preserved, 
those of the right side atrophy or disappear. In dextral Gastropods 
the only structure found on the topographically right side of the 
rectum is the genital orifice. But this is not an original organ. 
It is wanting in forms which, like Plewrotomaria, Haliotis, etc., have 
preserved the maximum of symmetry. Moreover, in the most 
primitive stage of organisation, the gonads opened into. the kidneys. 
As soon as the asymmetry makes its appearance, even while there 
are yet two kidneys, the genital products are conducted only into 
the right kidney (Patellidae, Trochidae, Fig. 55, Fissurellidae). 
Consequently the right kidney cannot disappear altogether, but 
persists in-part as the gonaduct. The latter structure, therefore, 
is the remains of the topographically right kidney, a view which 
