MiaceUnneous. 411 



of the Turri(((i\ from wliich we must at tlie same time remove the 

 genus Cori/deuilriuiii, whicli Allman had included in it on account 

 of the uncertainty which still exists to its reproductive organs. 



In regard to the habitat of this form, I may observe that my 

 Bpecimens were found in a very different condition from those of 

 the first specimen described. Weismaun had a colony which had 

 been taken from a depth of 70 to 80 metres. On this account ho 

 refers to Dendrochiva Dohrnii as a form living at great depths. 

 My specimens on the contrary had been taken off the coasts of 

 Nisida, and it is only necessary to glance at a bathymetric map of 

 the Gulf of I'ozzuoli to see that the island of Xisida is surrounded 

 by waters of very inconsiderable depth. — BollHino Sclcnlijico, N. 3 

 e 4, Anno ISDI." 



On the Develo^itncnt o/Bythinia tentaculata. By Dr. 11. v. 

 Erlakgkr, of the Heidelberg Zoological Institute. 



Having been occupied for a long time with the embryology of 

 Gastropods, I thought it desirable to test upon another Prosobranch 

 the observations which I had made upon Paludina viv'qmra. For 

 this purpose Ihjthinia teatacidata appeared to me to be most advan- 

 tageous, since all the stages of development are to be had in any 

 quantity, and the youngest are very suitable for sections on account 

 of their relative size. Another circumstance strengthened me in 

 my intention. Bijthinia has already been the subject of a lengthy 

 paper by P. Sarasin *, whose results were by no means to be recon- 

 ciled with those which I had attained in the case of Paludina. The 

 sequel will show that in almost all important points I have arrived 

 at precisely opposite views to Sarasiu, and that the development of 

 Biithinia possesses a great similarity to that oi Paludina. 



After the expulsion of the directive vesicles the segmentation 

 proceeds in the manner which is typical for the majority of Gastro- 

 pods, and conforms closely to that of PJanorhis and Xeritina. 

 Immediately after the division into two it becomes evident that the 

 cells of the germ do not all divide simultaneously, but that the 

 macromeres which are first formed gradually give rise to a large 

 number of micromercs. I traced the segmentation as far as the 

 stage with forty-eight cells : I did not succeed in following it 

 further, on account of the excessive number of segments. By the 

 time this stage is reached a segmentation-cavity of considerable size 

 has been developed, which soon afterwards acquires its greatest 

 dimensions. At the vegetative pole the four macromeres only are 

 present, while the micromeres, which give rise to exclusively ecto- 

 dermal elements, gradually diminish in size from the vegetative to 

 the animal pole. The macromeres exhibit precisely the same 

 arrangement as the corresponding cells in Planorbis. The anterior 

 and posterior are in contact with one another, forming a sharply 



* P. Sarasiu, ' Entwicklungsgeschichte der Bythinia tentaculata,' In- 

 augural-Dissertation. Wiesbaden, 1882. 



