324 E. RAY LANKESTER. 



the ground that in the former the siphonal tube is externally 

 single, wliilst in the latter it is bifid. In the accompanying 

 plate (Plate XXIV, figs. 1 and 2) I have given a drawing 

 of two embryos of Cyclas cornea (two out of many observed 

 by me at the end of April this year) which is a true copy 

 of the appearances seen with a power of 400 diameters, 

 minute histological details being omitted. From this it 

 will be seen that both ' shell-gland ' and ' hyssus-gland ' are 

 present in the young Cyclas ; both are perfectly defined and 

 obvious. I am led to infer from this what is probable enougb 

 from analogy, namely, that Pisidmm pusillum does not 

 develop a byssus-gland at all, or if at all at a late period, 

 whilst Cyclas cornea develops a byssus-gland at an early 

 period, soon after the shell-gland has made its first appear- 

 ance.] 



Mr. Carl K.abl has a paper also in the ninth volume of 

 the ' Jenaische Zeitschrift' entitled " Ueber die Ontogenie der 

 Siisswasser Pulmonaten." Mr. Rabl at the conclusion of his 

 memoir compares his observations with ray own on Limnseus 

 and whilst expressing his disagreement with some of my con- 

 clusions does so in a simple and straightforward way which 

 appears to me perfectly legitimate and rational. He has 

 adopted in his paper Lereboullet's view as to the anal nature 

 of the cone-like shell-gland and maintains this view in spite 

 of my demonstration that it is the shell-gland. Probably by 

 this time he would admit that he has been in error on this 

 matter. He also maintains that there is a condition of the 

 Limnseus embryo when it consists of a number of equiformal 

 cells arranged in the form of a hollow sac (blasto-sphaera). 

 This, he says, I have overlooked. On the other hand I 

 maintain that such a phase does not occur, and that Rabl 

 has made a mistake in following Lereboullet as to this matter 

 as he has in the matter of the ' Anal-kegel/ Mr. Rabl ex- 

 presses regret that I had not used the method of transverse 

 sections in my study of Limnseus development and states 

 that I should then have arrived at other conclusions than 

 those which I have adopted, concerning the origin of the 

 hypoblast. I am not aware, however, that Mr. Rabl or any 

 other embryologist has succeeded in cutting sections of em- 

 bryos so small as are the earlier stages of Limnaeus. The 

 figures which he himself gives are diagrams based upon 

 ' optical sections.' Mr. Rabl also considers that I have over- 

 looked the otocysts and the canals of Stiebel, and the pedal 

 ganglion. To this I must reply that in the stages most care- 

 fully examined by me (stages equivalent to those preceding 

 his figure 25) these structures with the exception of Stiebel's 



