﻿74 



PROCEEDINGS OF THE 



[Dec, 1856. 



is unanswerable proof of its being, what he considers it, a foot- 

 stalked Medusa, representing to a great degree, in the embryonic 

 history of the hooded-eyed section of the order, that structure 

 which is permanent among the naked-eyed genera. It is not sur- 

 prising, therefore, since it has the circular and radiate tubes, the 

 vail, bell-cavity and proboscis of a naked-eyed Acaleph, that it 

 should also exhibit a mode of budding which Prof. Forbes has 

 shown to take place in the genus Sarsia, (S. prolifera.) 



The strange mode of budding known to occur among the Cya~ 

 ihophyllidse in the class of true Polypi, by which one, two or three 

 polyp-buds are developed on the tentacular disk, is the indubitable 

 homologue of this mode of budding among hydroids. And so we 

 find that the two modes of generation which occur among the true 

 Polypi, occur also in their embryonic representatives, the hydroids, 

 save only that among the latter the basal or, as it has been called, 

 the lateral mode of budding is that on which the multiplication of 

 hydroids by gemmation is wholly dependent, while the tentacular 

 mode is entirely confined to the production of Medusa-buds, ex- 

 hibiting thus a further specialization in the gemmiparous economy 

 of these hydroids corresponding to their higher position in the 

 scale of being, as being not true Polypi, but polypoid stages of 

 Medusan larvas. 



The consideration of this tentacular mode of budding leads us 

 directly, and naturally, to the next stage in the development of 

 Turritopsis. It is precisely in this region, between the tentacula 

 and mouth of the hydroid, in which Medusa-buds only are pro- 

 duced, that, in the present larva, a series of changes now occur, tend- 

 ing towards the metamorphosis of the whole larva into a free 

 swimming Medusoid. Before, however, entering upon the delinea- 

 tion of their changes it would be, perhaps, best to assume some 

 position as the normal one, by which we will be able to make de- 

 finite reference in using the words above and below. Most hydroids 

 in nature assume all positions, and individuals of the same species 

 are found with mouth indifferently directed either upwards or down- 

 ward, or with the oro-basal axis of the animal horizontal, or in- 

 clined at any intermediate angle. This, also, is essentially true of 

 the larvae of Turritopsis, but if we assume the natural position of 

 the parent to be that of a swinging' bell, mouth downwards, we 

 find that the position of the larva is, on the whole, rather a con- 

 stant one, having the reverted tentacula clinging to the upper part of 

 the parent's bell-concave, and the mouth hanging downwards. I 



