254 M. C. Mereschkowsky on the Mode of 



dual of Hydra vulgaris with four tentacles. It will be seen 

 that the tentacle c is a little longer than the tentacle d, which 

 is explained by the fact that c appeared earlier than d, and 

 consequently has had a longer time to grow. This difference 

 can only be easily observed at first ; later on, when the four 

 tentacles have grown (fig. 1, IV.), one can scarcely distinguish 

 the tentacle c from d. But even much later one can still 

 easily distinguish the first pair of tentacles from the second 

 pair, which are shorter. It is true that generally each ten- 

 tacle contracts independently of the other, so that it may 

 happen that the tentacle a is more strongly contracted than c, 

 and thus may even be tlie shorter of the two ; but in this case 

 it is easy to recognize the nature of the tentacle by its greater 

 thickness. The individual iv. of fig. 1 had two long tenta- 

 cles {a and b), which measured in the uncontracted state 

 1*25 millim. ; the other two were scarcely more than half this 

 length, one being 0'65 and the other 0*68 millim. long. 



I have observed the mode of appearance of the tentacles 

 many times in the two species ; and I have no longer any 

 doubt, 1, that they do not appear together, but one immedi- 

 ately after the other, and, 2, that they are arranged opposite 

 each otlier and in the middle of the spaces between the first 

 pair*. All these conclusions are not merely the results of the 

 observation of different forms which I have had before me, 

 but I have been able to follow them uninterruptedly upon the 

 same individual. 



The fifth tentacle (PL XII. fig. 4, e) appears, like the third, 

 in the form of a tubercle placed between two other tentacles, 

 and not followed by the sixth — which does not make its appear- 

 ance for some time, and, like all the others, appears on the side 

 opposite and exactly facing the fifth. But what is especially 

 remarkable is, that the appearance of the sixth tentacle is 

 delayed much longer than that of the fourth (d) after the third. 

 yVe have seen, in fact, that the fourth tentacle {d) appeared when 

 the third (c) was still only a short cylindrical protuberance. 

 The fifth tentacle (e), on the contrary, has time to become 

 tolerably long and filiform before the sixth (_/) makes its 

 appearance. From all this it follows : — 1, that the appear- 

 ance of the sixth tentacle is much longer delayed than that 

 of the fourth ; and, 2, that the third pair is composed of two 

 opposite tentacles, like the first two pairs. 



This curious mode of appearance of the tentacles in the 

 genus Hydra is, so far as I know, peculiar to it, and does not 

 occur elsewhere among the Hydroida, in Avhich we observe 



* The two pairs being arranged so as to form a cross. 



