160 COMPARATIVE ANATOMY CHAP. 



distinctly in this condition, being the symmetrical primitive Gastropod adapted to 

 life in mud, and provided with a turret-like shell and posterior pallial complex. 

 The perforation at the upper end of the shell, which freely projects from the 

 mud, is of great morphological importance, corresponding physiologically with the 

 siphons of the limicolous Lamellibranchia. A comparison between Dentalium and 

 a Fissurella with its pallial complex twisted back, and with a long and turret- 

 like shell, is, from our point of view, very appropriate. A Fissurella^ so transformed, 

 would almost exactly resemble the hypothetical symmetrical racial form of the Gas- 

 tropoda, in which, however, we should have to assume a mantle- and shell-cleft 

 reaching to their edges (cf. Fig. 136, I). 



The anatomy of the Protobrancltio, which has recently been more closely studied, 

 and especially the posterior position of the two gills, the flat sole for creeping, and 

 the presence of the pleural ganglia, justify us in deriving the Lamellibranch la also 

 from the racial form of the Gastropoda, in which the cleft edge of the mantle would 

 correspond with the posterior or siphonal edge of the mantle in the former. This 

 edge of the mantle, having a similar physiological function, often possesses tentacles, 

 papillae, etc., in both groups. 



Dentalium further fits in with our theory, for the forward curve and the position 

 of the columellar muscle on the anterior side of the visceral dome which would be 

 disadvantageous to a freely reptnnt, is not so to a limicolous, animal. 



16. 

 The Dextral and Sinistral Twists. 



Most Gastropods have the visceral dome and shell twisted dextrally. The direction 

 of the twist has been determined by the fact that the visceral dome and shell origin- 

 ally inclined to the left, and then more and more backward, thus pushing the 

 pallial complex along the right mantle furrow. It cannot be determined why the 

 incline to the left was originally chosen. The shell might just as well have inclined to 

 the right at first, and then more and more backward, pushing the pallial complex along 

 the left mantle furrow. The consequent asymmetry would then have been exactly 

 reversed. To take a concrete example : in a Monotocardimi, with visceral dome and 

 shell twisted sinistrally, the original left parietal ganglion would become the supra- 

 intestinal ganglion on the right. The original right half of the pallial complex 

 would disappear, and the left half which persisted would lie to the right of the anus 

 or rectum, which would take up its position to the left of the median line. 



Gastropoda with sinistrally twisted shells are actually known, many of them 

 having the asymmetrical organs in the inverse position which corresponds with this 

 twist. Such are. among the Prosobranchia, Ncptunca contraria, Triforis, and occa- 

 sional specimens of Buccinum ; among the Pulmonala, Physa, Clausilia, Helicter, 

 Amphidromus, and occasional specimens of Helix and Limmaea. In Bulimus per- 

 versus, individual specimens with either sort of shell are found, with the special 

 asymmetry of the organs belonging to it. 



17. 



There are, however, snails whose shells are dextrally twisted, but which possess 

 the organisation of animals with sinistrally twisted shells. This is the case among 

 the Prosobranchia in the sinistrally twisted sub-genus Lanistcs of the genus Ampul- 

 laria ; among the Pulmonata, in Choanomphalus Maacki and Pompholyx solida ; 

 among the OpisthobrancMa, in those Pteropoda which, whether as adults (Lima- 

 cinidce) or larvae (Cymbuliidcc), have a twisted shell. This fact is entirely against 

 our theory in explanation of the asymmetry of the Gastropoda, for this theory 



