294 INVEBTEBRATE MORPHOLOGY. 



plainly secondary inasmuch as the primitive asymmetry is 

 indicated in certain of the organs in all cases. In order to 

 understand the exact nature of this asymmetry it will be nec- 

 essary to consider what may have been the original form of 

 the Gasteropoda. Judging from what is known of the Amphi- 

 neura, it may be supposed that in the primitive Gasteropod 

 (Fig. 131, A) the anus (a) was terminal and opened into a 

 mantle-cavity, the mantle being, except posteriorly, only a 

 slight fold. In this mantle-cavity there was present also a 

 single pair of branchial plumes (ct), and into it the two 

 nephridia opened (?i), passing from the posteriorly-situated 

 pericardium which contained the heart provided with two 

 auricles. 



It may be imagined now that in such a form the visceral 

 hump enclosed by a dome-like shell became elevated to such 

 an extent that it could no longer be retained in an erect posi- 

 tion, but fell over to one side — it may be supposed the left 

 side. The result of this would be an interference with the 

 development of the mantle-cavity towards the left side, and 

 a prevention of the perfect growth of the left branehia and of 

 the proper functioning of the left nephridium. There would 

 be a tendency then for the mantle-cavity, and with it the anus 

 and indeed the entire posterior region of the body with the 

 heart and nephridia, to be pushed over towards the right side 

 (Fig. 131, B), and this process might in some cases be con- 

 tinued until the mantle-cavity and the organs associated with 

 it had been pushed round through 180° (Fig. 131, 0, D) and 

 had come to lie apparently in front of the visceral hump [D). 

 The anus in such a case would open into the mantle-cavity in 

 the mid line, dorsal to the mouth, and what was originally the 

 right branehia would lie upon the left side of the body ; the 

 digestive tube, which may -originally have been practically a 

 straight tube, would now be bent upon itself, and furthermore 

 the original right parietal nerve-ganglion would have passed 

 over to the left side of the body and the original left ganglion 

 to the right side, a crossing of the pleuro-parietal connectives 

 (■uc) being thus brought about. 



The original pressure of the shell upon the left half 

 of the mantle-cavity would, however, as pointed out, have 



