1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 881 



Neritida? appear, and if so, to what extent, in the more primitive 

 Tsenioglossa. If they do not i-eappear, it cannot be maintained 

 that the latter group is descended from the former. Secondly, 

 whether such resemblances as there may be between the two 

 groups may be attributed to inheritance from a common ancestor 

 or to convergence, or whether possibly both these factors have 

 taken a share in producing these resemblances, and, if so, what 

 «hare. 



Taken by itself, the persistence of the left post-torsional kidney 

 as the functional excretory organ in the Neritidse and Pectini- 

 branchia would be strong evidence of their relationship, but 

 relationship does not imply that one group is descended from the 

 other. It may be remote and may only indicate that both 

 groups are descended from a common ancestor, and this is clearly 

 the true conclusion in the matter. Among the special characters 

 of the Keritidse those of the nervous system are the most 

 important. If the Pectinibranchs were descended from a Neritid 

 stock, we should expect to find in the more primitive members of 

 the suborder traces of the special featui-es of the Neritid nervous 

 system. But we find nothing of the sort. The generalized 

 Pectinibranchs such as Pahtdina, Cyclophorus^ Littorina, or 

 Cyclostoma are typically dialyneurous. In none of them is the 

 subintestinal ganglion approximated to the left pleiu^al. There is 

 not a zygoneurous connection between the right pleural and the 

 subintestinal ganglia. There is no trace of a direct commissural 

 connection between the right and left pleural ganglia. The 

 supra-intestinal nerve shows no sign of reduction or disappearance. 

 Paludina, as Bouvier has shown, is quite rhipidoglossan in 

 respect of its nervous system. The evidence is clear that the 

 archaic Tgenioglossa cannot have descended from the Neritidfe. 



On the other hand, the persistence of the left kidney as the 

 functional excretory oi'gan, and the fact that the permanent 

 relations of the reduced right kidney (gonaduct) of female 

 Neritidse to the coelom almost exactly represent an embryonic 

 phase in Pcdudina, are coincidences which must almost certainly 

 be attributed to inheritance fi-om a com pa on ancestor. At some 

 remote age the Prorhipidoglossan stem must have divided into 

 two branches. In one the left kidney underwent reduction, and 

 this branch gave rise to the Pleurotomariidas, Haliotida?, Trochida\ 

 Pissurellid?e, and probably also to the Docoglossa. In the other 

 branch the left kidney retained its size, and with the reduction 

 of the right kidney became the only excretory organ. Fi'om 

 this branch all the Gastropoda which retain the left kidney only 

 — the Neritidfe, the Pectinibranchia, and the Euthyneura — are 

 descended. The last tAvo must have branched off at a very early 

 period, while the ancestral form still retained all the primitive 

 characters of the nervous system, as these are preserved in 

 Paludina and in Actceon. The Neiitacea remain as the much 

 modified representatives of the primitive stock. Their special 

 characters are peculiar to themselves and are not to be explained 



56* 



