7 6 



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 (Plewotomaria, Scissurella, etc.), and eventually disappear. 

 An essential feature of the asymmetry of Gastropods is the atrophy 

 or disappearance of the topographically right (morphologically left) 



FIG. 55. 



Trochus cinerarius, heart and 

 kidneys, dorsal aspect (some- 

 what schematic). I, left renal 

 pore ; II, eight 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 dnct ; 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. 



VIM VII 



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 Pleurotomaria, 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 Avhich 



