Development of Tentacles in the Genus Hydra. 255 



three tjpes of development, viz. : — 1, appearance in pairs ; 

 2, appearance by four at a time ; and, 3, appearance of all the 

 tentacles at once, as, for example, in Tubularia. This excep- 

 tional case would serve very well to explain the fact (which 

 is also exceptional) that in Hydra we very often observe the 

 number 7, which does not accord with the formula 2 x w, 

 that in general characterizes all the Coelenterata. In fact, 

 if the sixth tentacle does not appear until long after the fifth, 

 we may expect that in the following (fourth) pair of tentacles 

 the seventh will appear earlier than the eighth, and that this 

 last will be delayed much more than even the sixth. It is in 

 this way that we find a variable number of tentacles in the 

 different species of Hydra, sometimes six, sometimes seven, 

 sometimes eight, or even more. It may well be supposed that 

 the individual sometimes dies before having had time to 

 acquire an eighth tentacle, and that then, having only seven, 

 its formula would be (2 x n) — 1 or (2 x 4) — 1 ; but from 

 what we have seen I do not see why we should think that the 

 number of tentacles in Hydra is subject to such variations 

 that it cannot be governed by any law. On the contrary, we 

 see that chance has nothing to do with it, that a very vigorous 

 and constant law governs the appearance of the tentacles, and 

 that the mode of appearance belongs to the first type that we 

 have just established (appearance by pairs), although here it 

 may be more or less modified. 



From the diagrams below we may easily see the mode of 

 appearance of the tentacles. 



