MORPHOLOGY OF THE HELlCINIDiE, 



805 



the Palfeozoic age came to be examined, all these marks were 

 found, some of them in a more pronounced degree than in any 

 other known Gastropod, and in no system of organs were these 

 marks more conspicuous than in the nervous system, the im- 

 portance of which Dr. Simroth seeks to minimize. Among these 

 marks may be enumerated— a cerebral commissure situated far 

 forward on the pharyngeal bulb ; a distinct labial commissure ; 

 elongated and scalariform pedal nerve-centres ; a long crossed 

 visceral commissure; two auricles to the heart: the ventricle 

 lapped round the rectum; a rhipidoglossate dentition. Other 

 characters might be enumerated, but these suffice for the present 

 purpose. All these characters are absent in the Pulmonata : all 

 of them are present in the Neritidte. Moreover, by discovering 

 the oviduco-coelomic funnel, I was able to demonstrate, beyond all 

 reasonable doubt, the homology of a part of the gonaducts tothe 

 right kidney of Pleurotomao-ia and other rhipidoglossate Aspido- 

 branchs, a homology which Thiele had already asserted on other 

 grounds. Now it is quite clear that, if structural resemblance is 

 of any value as a guide to affinity, we have a choice between two 

 alternatives. Either the Neiitidse, to which Ave must add the 

 Helicinidfe, are descended from Aspidobranch ancestors, which 

 they resemble in all the points enumerated above, and have in- 

 dependently acquired genital ducts superficially similar to those 

 of Pulmonata ; or, as Dr. Simroth will have it, they have descended 

 from stylommatophorous Pulmonata, have preserved the characters 

 of the genital ducts of the latter group, but have independently 

 acquired all the other characters enumerated above, characters 

 possessed by no Pulmonate, but invariably present in those 

 Aspidobranchs from which, on Dr. Simroth's showing, the 

 Neritidaj are not descended. I am not quite sure whether he 

 would go so far as to assert that the remaining Aspidobranchs 

 possess those characters because they are descended from the 

 Neritida;. To make such an assertion would, indeed, be flying in 

 the face of all reasoned opinion on this subject, and would amount 

 to a declaration tliat the geologically more recent Pulmonates are 

 the parents of their predecessors of Cambrian age ! 



I submit the alternative to the judgment of my readers, and in 

 doing so beg leave to entei' a protest against the growing tendency 

 to throw over long-established and carefully reasoned conclusions 

 founded upon morphological evidence, because of their uncon- 

 formity with some new and as yet insufficiently tested hypothesis, 

 or because they do not help in the solution of certain limited 

 problems. I was quite aware, when I discussed the subject, that 

 the geographical distribution of the Neritidt\3 was a puzzle, and 

 that'l had failed to find a solution to it. The distribution of the 

 Helicinidre is scarcely less puzzling and awaits a final solution. 

 But with all respect 'for Dr. Simroth's authority aiid deserved 

 reputation as a zoologist, I submit that the solution that he offers 

 is improbable, raises a crop of other puzzles, and throws 

 morphology into confusion. 



