256 On the Development of Tentacles in Hydra. 



Thus the appearance of the tentacles in Hydra occurs in the 

 following order : — The first two tentacles appear at the same 

 time and are arranged opposite to each other; the other 

 tentacles also appear in pairs, and are also arranged opposite 

 one another: but the second tentacle of each pair always 

 appears later than the first ; and this retardation is greater in 

 the third pair than in the second, and still greater in the fourth 

 pair. 



Taking into consideration all that has been said, I hope 

 it will be seen that the genus Hydra^ and especially Hydra 

 oligactis, is derived from the type which is represented by the 

 formula 2 x 4, as I have indicated it in the genealogical table 

 given in my article on MonobracMum *. 



A bud does not require a very long time for its develop- 

 ment. The first tubercle of the individual increases in size 

 very considerably in half an hour, and already develops two 

 little tentacles. In about 20 hours this bud had four well- 

 developed tentacles ; and in 20 hours more there were already 

 five tentacles. 



In all this we may easily see that the facts are subjected to 

 a general law, although, owing to the great complexity of the 

 facts, the law does not strike one at once, and can only be 

 ascertained by carefully studying the genesis of the animals. 

 As a general conclusion we may admit that in Hydra each 

 jpair of opposite tentacles forms a system^ and that these two 

 tentacles are singularly connected in such a manner that the 

 appearance of one tentacle is followed by that of another oppo- 

 site to it. In this we observe a sort of polarity between the 

 two metameres — a fact the frequent occurrence of which 

 among the Hydroids I hope to prove some other time. 



EXPLANATION OF PLATE XII. 



Fig. 1. Hydra vulgaris. An adult individual with seven tentacles in a 

 fully extended state ; a little below the middle of the body it 

 bears four buds or young individuals, namely: — i., a bud without 

 tentacles; ii.,a larger bud, furnished with two tentacles {a and b) 

 in the form of tubercles, which have appeared simultaneously 

 and are arranged opposite one another; iii.^a still more advanced 

 bud, the tentacles a and h being considerably enlarged, and the 

 third tentacle (r) only presenting the form of a tubercle, without 

 yet having the fourth tentacle opposite to it ; iv., a young indi- 

 vidual with four tentacles, two longer (a and 6) and two others 

 shorter, also arranged opposite each other (c. and d), the four 

 tentacles arranged so as to form a cross. The tentacle c, which 

 appeared earlier (see m.), is a little longer than d. 



Fig. 2. A young individual of the same species, representing the same 

 stage as fig. 1, ii., but in a moi*e advanced state. First pair of 



* Ann. & Mag. Nat. Flist. ser. 4, vol. xx. p. 227. 



