150 COMPARATIVE ANATOMY CHAP. 



The assumption that the pallial complex originally lay posteriorly is also well 

 founded. In all symmetrical Molluscs, the anus lies as the centre of the complex 

 posteriorly in the middle line, and further, in all symmetrical Molluscs, the nephiidial 

 and genital apertures lie posteriorly at the sides of the anus. When the ctenidia 

 and osphradia have been retained in symmetrical Molluscs, they lie symmetrically 

 on the posterior side of the visceral dome. This is the case in the Cephalopoda, and 

 in the most primitive Lamellibranchia, the Protobranchia (Nucula, Leda, Solenomya), 

 and even in some Chitonidce, and those Solenogastrcs which still have rudiments of 

 gills. 



In keeping with the posterior position of the pallial complex, the mantle fold 

 which hangs down round the base of the visceral dome is, in symmetrical Molluscs, 

 widest posteriorly where it has to cover the complex ; at this part the mantle 

 furrow deepens into a mantle cavity. 



In connection with the second assumption, it still remains unexplained why in 

 the Amphineura the commissure between the pleuro-visceral cords runs over the 

 intestine ; whereas on the other hand, in all other symmetrical Molluscs, the 

 visceral ganglion lies, as in the Gastropoda, below the intestine. 



The third assumption, that the pallial complex has shifted forward, requires 

 separate discussion. 



If the pallial complex did thus shift forward, chiastoneury must necessarily 

 have taken place ; the original left half of the complex must necessarily have become 

 the present right half, and vice versa. Further, the right pleuro-visceral connective 

 would have to become the supra-intestinal connective and the left the infra-intestinal 

 connective ; the original right parietal ganglion the supra-intestinal ganglion, and 

 the original left parietal the infra-intestinal ganglion. But why did such a shifting 

 take place ? We shall here attempt to answer this question. 



Cause of the shifting forward of the pallial complex. We have assumed the 

 symmetrical racial form of the Gastropoda (with posterior mantle cavity and sym- 

 metrical pallial complex) to be a dorso-ventrally 

 flattened animal with a broad creeping sole, a 

 snout-like head with tentacles and eyes, and a 

 somewhat flat cup - shaped shell covering the 

 dorsal side of the body (Fig. 129). It therefore 

 resembled in outward appearance a Fissurella, a 

 Patella, or a Chiton, if we assume the imbricated 

 "p 1 **" shell of the last to be replaced by a single shell. 



The body of such a racial form was only pro- 



FIG. 129. Hypothetical primitive , . J , , 



Gastropod, from the side, o, Mouth ; tected doi ' sall y b Y the shell. The hard surface 

 fc, head ; sm, shell muscle ; oso, apical along which the animal slowly crept served to 

 shell aperture ; a, anus ; n, renal aper- protect its lower side, the dorsal shell being 

 * u o ; t ^' mantlecavity; c *> ctenidium ; presse d firmly against the substratum, when 



necessary, by the contraction of a powerful shell 



muscle (cf. Fig. 106, p. 122). When the shell was thus pressed down, communica- 

 tion between the pallial cavity and the exterior (for the purpose of inhaling and 

 exhaling the respiratory water, and ejecting the excreta, excrement, and genital 

 products) was rendered possible by means of a cleft in the posterior edges of the 

 mantle and shell. 



Unlike their racial form, all known Gastropoda (except those whose body form 

 has been secondarily modified, generally in connection with the rudimentation of the 

 shell) are distinguished by the fact that the viscera with their dorsal integumental 



