BLASTOPORE AND ANUS IN FALUDINA VIVIPARA. 385 



Conchtsions. — The facts in the development of Paludina 

 which I have now placed before the reader by word and by 

 carefully executed drawings do not leave the possibility of 

 a doubt that in this particular Gastropod the blastopore coin- 

 cides with the anus and that the mouth is a remote and 

 entirely independent formation. Such being the case the 

 observations of Bobretzky to the effect that, in three allied 

 Gastropods whose eggs contain copious food-material, the 

 blastopore coincides with the mouth — cannot be held to have 

 a general import. The fact must be admitted, improbable 

 as it had seemed both to Bobretzky and to myself, that there 

 is not only one possible fate for the blastopore of the Molluscs 

 (or Ave may suppose of other Metazoa) in cases where it does 

 not simply close and leave no trace. It appears that the anal 

 invagination or proctodseum is sometimes formed at the very 

 spot where the blastopore closes or even before it closes, and 

 on the other hand the oral invagination or stomodaeum is 

 sometimes formed at that spot. In fact, there is no necessary 

 connection between the blastopore and either the mouth or 

 the anus ; sometimes they are all three independent, some- 

 times the former coincides with one of the two latter, and 

 sometimes, being elongated in shape, coincides with both. 



Dr. Bobretzky's hazardous process of reasoning, by which 

 he was led to deny the correctness of my account of the 

 " pedicle of invagination" and development of the mouth and 

 anus in Pisidium ('Phil. Trans. ,^ 1875), as well as in Limnceus 

 and Paludina, consisted in the inference oi generality for the 

 phenomena observed by him in the particular cases of Nassa, 

 Natica, and Fusus. On bringing this inference to the test of 

 facts, it is found to be devoid of justification, since in in- 

 volves the contradiction of published observations beyond 

 Dr. Bobretzky's experience — which observations are and can 

 readily be shown to be, correct. Though I am thus brought 

 into collision with the Russian author in matters of inference, 

 I wish to express my high appreciation of the value of his 

 work, the difficulty of accomplishing which and the conse- 

 quent skill and courage of the author none can estimate so 

 well as those who have occupied themselves with the same 

 material of research. 



