378 E. IIAY LANKESTER. 



Dr. Bobvetzky has been led to conclude from his obser- 

 vations that the mode of development observed by him in 

 Nassa, Natica, and Fusus holds good for all the Mollusca, and 

 since on an important point the facts observed by him differ 

 from certain of those which I have observed and published 

 in regard to other Mollusca, he does not hesitate to declare 

 those particular observations of mine to be erroneous. His 

 attempted generalization would have carried more conviction 

 with it if he had been in a position to correct my state- 

 ments by a study of the identical species — namely, Paludina 

 vivipara, Limnaus stagnalis, and Pisidium pusillum, to which 

 they had reference. I frankly admit that I have fallen into 

 a similar error in suggesting, though I did this with some 

 caution, that the facts observed by me in the above Molluscs 

 have a wide significance and may be generalised so as to 

 include not only the Mollusca but other animal groups in 

 which the planula or gastrula develops by means of a blas- 

 topore. 



The point as to which Dr. Bobretzky and I are at issue is 

 this : Does the blastopore or gastrula's orifice of invagination 

 in no case coincide with the mouth as I have maintained ? 

 Does it in no case among the Mollusca become the anus as 

 Bobretzky has maintained ? The truth appears to be that, 

 as Bobretzky has shown, the blastopore or orifice of in- 

 vagination of the gastrula does in certain cases among Mol- 

 lusca coincide with the mouth, and on the other hand that 

 it does in other cases coincide with the anus, as I have 

 stated in Paludina and in Pisidium and as Agassiz estab- 

 lished in certain Echinoderms ; or it may coincide with 

 neither mouth nor anus ; or again, as I have pointed out 

 in the case of Limnseus,^ it may have an elongated form cor- 

 responding (in position though not in continuity) with the 

 anus at one end and the mouth at the other. 



Dr. Bobretzky having rejected my observations on the 

 development of Paludina to which, as he rightly says, I 

 attach most weight, and having expressed incredulity with 

 reference to the two diagrammatic woodcuts which accom- 

 panied my note on the subject,^ I give on the present occa- 

 sion a more complete series of drawings illustrating the early 

 stages of development of the Mollusc in question. Some of 

 these drawings and notes I have had by me for some time, 

 whilst others are due to observations made since Dr. Bob- 

 retzky 's criticisms. I have to thank my friend Professor 

 Lawson, of Oxford, for a copioussupply of fine Paludinse 



1 See this Journal, July, 1876. 

 3 This Journal, April, 1875. 



