SHELL-GLAND OF CYCLAS AND PLANULA Or LIMNJSUS. 323 



overlook these structures, but I looked carefully for them and 

 determined their absence. Mr. Jhering appears not to have 

 reflected on the enormous improbability of an observer dis- 

 covering a new organ, such as the shell-gland (subsequently- 

 observed and recognised by other observers) through mistak- 

 ing another organ for it. 



In his second paper Mr. Jhering explains more at length his 

 interpretation of my published drawings of Pisidium develop- 

 ment. He declares them to be " sehr skizzenhaft,^' and 

 accordingly assumes that they are grossly inaccurate. I am 

 ready to admit that some of them may be found wanting in 

 the finer histological details, but they are not sketches : they 

 are careful drawings made from specimens under the micro- 

 scope, and some of them by aid of the camera lucida. They 

 were made five years ago, but I cannot entertain the suppo- 

 sition that they are erroneous in such points as Mr. Jhering 

 suggests. Mr. Jhering declares that what I have shown (as 

 I think) to be the primitive invagination to form the hypo- 

 blast is the real '' shell-gland," that my shell-gland is the 

 byssus-gland, and that what I have called the rectal pedicle 

 or pedicle of invagination is the oesophagus. This looks to 

 me very much like wanton contradiction. There are some 

 limits to the probable not to say possible errors of a practised 

 observer. When such sweeping condemnation is attempted 

 as that which Mr. Jhering has ventured upon, one looks for 

 evidence in the form of new observations illustrated by 

 careful drawings. This Mr. Jhering does not think it neces- 

 sary to offer; and in the absence of such "material for dis- 

 cussion," I can only say that I do not think that he has 

 brought forward any fact which affects the accuracy of my 

 statements. Mr. Jhering cites the observations of Professor 

 Ganin, of Warsaw, which were published some months 

 subsequently to my first statements with regard to Pisidium. 

 At present I have not heard that Professor Ganin has pub- 

 lished any figures in illustration of his observations. As far 

 as I can gather from the abstract in Schwalbe andNitsche's 

 * Jahresbericht ' Professor Ganin's observations coincide in 

 some points with my own, whilst in others we differ con- 

 siderably. But I am unable to fully apprehend his views 

 as to either Cyclas or Limneeus in the absence of illustra- 

 tive drawings. 



[Since the above Avas written I have taken occasion to 

 examine the embryos of Cyclas, at Oxford, and I am able to 

 confirm most emphatically, from the examination of that 

 form, my previous statements as to Pisidium. The genus 

 Pisidium is separated by systematists from Cyclas mainly on 



