252 M. C. Mereschkowsky on the Mode of 



I expressed the opinion that the fundamental number in the 

 Hydroids (that is to say, the number which enters into the 

 composition of all the other numbers) was not 4, but 2. I 

 arrived at this opinion not only because numbers such as 

 6, 10, 14, 22, &c., which are not formed by the number 4, are 

 to be met with often enough among the Hydroida, but also 

 because many facts which I had come across in literature or 

 observed myself have shown me tlwit the appearance and some- 

 times the disappearance of organs in the Hydroida takes place 

 in such a manner that they appear or disappear simultaneously 

 two at a time. 



Thus in the Medusse belonging to Monohrachmm j^drasttum 

 I observed in a very young stage only four germinative sacs, 

 which afterwards increased to eight sacs. The division of the 

 four sacs into eight takes place by the simultaneous division, 

 first, of only two sacs opposite one another, subsequently the 

 other two sacs alone also beginning to divide longitudinally*. 



In the following note it is my intention to give a description 

 of my observations on the mode of production of the tentacles 

 in Hydra vnlgaris and //. oligactis. These observations were 

 made in the spring of 1877 and 1878, and will serve to confirm 

 my opinion as to the fundamental number in the Hydroida, 

 and to establish a general law which governs the formation 

 and the order of appearance of every organ in this class. 



In the ponds of the neighbourhood of St. Petersburg Hydra 

 vxdgaris is met with very frequently during tlie whole of the 

 summer season, and more rarely Hydra oligactis — distin- 

 guished from the former by the form of the body, which is 

 distinctly divided into a cylindrical body and a peduncle of 

 much greater tenuity. 



In the month of Slay of the year 1877 I observed an ex- 

 ample of the former species more than a centimetre in length 

 in its normal uncontracted state, and furnished with seven 

 long tentacles. Nearly in the middle of the body, but a little 

 towards the base, this individual bore a whole colony of small 

 Hydrce, buds in different stages of development, from indivi- 

 duals almost completely developed to others which were only 

 in the form of short protuberances or monticules. I have 

 represented the individual in question in PL XII. fig. 1. There 

 will be seen first a small protuberance (i.) of a cylindrical and 

 slightly conical form, entirely destitute of all traces of tenta- 

 cles. The next stage here seen is represented by the bud (ii.), 

 which is a little larger and shows slight traces of tentacles, 

 but still only as two slight elevations (a and h) of the superior 



• Loc. cit. p. 223, pi. vi. tigs. 12-14. 



