﻿Dec, 1856.] 



ELLIOTT SOCIETY. 



73 



cavity or interspace between its internal walls and the proboscis, 

 in which the young planulse are first seen. Here, in both instances, 

 they remain and develope tentacula, in the Turritopsis larva only 

 four, in that of Tubularia sometimes four, but usually six. Up to 

 this point, also, neither larva is fixed, and each uses its tentacula 

 as means of locomotion. But here the history of the Tubularian 

 larva diverges from that of Turritopsis. Its first locomotive act is 

 to free itself from the bell-concave, of the parent Medusa, and after 

 this is accomplished, it for a time moves about very actively by 

 means of the enlarged tips of its tentacula, and then fixes itself, 

 for the first time, by the now somewhat elongated stem-end of the 

 body, and thence forward is a fixed hydroid. Another identity of 

 phenomenon, with difference in the relative time of its manifesta- 

 tion remains to be noticed. It is that the multiplication of hydroids 

 by gemmation which, in Turritopsis, takes place in a free larva, in 

 Tubularia never occurs, that I am aware of, until the larva becomes 

 fixed. But the gemmation of the hydroid larva of Turritopsis cor- 

 responds exactly with what is already known of the corresponding 

 mode of multiplication in the fresh-water Hydra. In proportion of 

 parts, this, the typical hydra, is exactly the reverse of our larva. 

 The proboscis in hydra is null, in our larva it is enormously de- 

 veloped, the stem-end or pedicle is of considerable length in the 

 former, but is null in the latter, and the tentacula which in hydra 

 are often prodigiously elongated in the larva of Turritopsis are of 

 medium length, and are never seen to vary to any considerable 

 degree. Both, however, are free, neither being rooted to a particu- 

 lar spot, and the typical hydra, as well as the present larva, uses 

 its tentacula as at least accessory means of locomotion. And re- 

 membering that the footstalk of Hydra is the homologue of the 

 posterior flat region between the tentacula, (fig. 20, PI. 6,) we see 

 that the modes of budding in the two animals are precisely homo- 

 logous. And, indeed, there are no rooted hydroids known to me, 

 and I believe none known to Science, which produce h}^dra buds 

 from any other part of the body than this, which is, of course, ho- 

 mologous with the whole ramified stem of compound genera. Thus, 

 we never find any other than Medusa-buds produced among the 

 tentacula or in the clear surface between the principal circle of 

 tentacula and the extremity of the proboscis. Of course, in the 

 view of those who consider Strobila an hydroid polyp, its tentacu- 

 lar gemmation would be considered as overthrowing this view. But 

 the structure of that larval Discophore as given by Prof. Steenstrup, 



