BOUGAINVILLTA RAMOSA. 313 



by ^vllich tlicy l)econic converted into the mature Bougaiwcillia form. -Indging, however, from 

 the observations of Wriglit and Agassiz, and from the condition of the Jiouf/ainvillia Brilamiicu 

 medusa, which is not improbably the adult planoblast of this species, the change consists mainly 

 in three successive bifurcations of the labial tentacles, in the multiplication of the marginal 

 tentacles, by which each basal bulb, instead of carrjdng only two tentacles, as at first, carries a 

 fasciculus composed of many, and in the greatly increased thickness of the umbrella, whose cavity 

 now occupies but a small portion of the entire volume. In this state the medusa has assumed a 

 nearly gol)uIar form. 



The labial tentacles of the medusa (Hate IX, fig. 8) to which the name of Hoti(/uinvUlia Brifan- 

 iiica has been assigned are not placed on a level with the month, but spring from a point some 

 distance above it. Every ultimate branch of these tentacles terminates in a little cluster of 

 thread-cells, which is not imbedded in the substance of the tentacle, but is elevated on the 

 summit of a very short peduncle. The entire tentacle is solid and is composed of an external 

 thin ectodermal layer, enclosing a distinctly cellular endodermal pith. 



Each of the four fasciculi of marginal tentacles springs from a crescentic cushion-like projec- 

 tion of the umbrella margin. Every tentacle carries its ocellus on the iimer side of the thickened 

 base. 



The generative elements are contained in four oval projections of the manubrium walls, which 

 extend symmetrically from the base of the manubrium to within a short distance of the labial 

 tentacles. These generative lobes, the labial tentacles, and the radiating canals with the marginal 

 tentaculiferous cushions, do not alternate with one another, but are situated, each set in one and 

 the same meridian plane. 



When the medusa is floating on the water the marginal tentacles are usually extended and 

 thro^^•u back upon the umbrella, their slender extremities at the same time bending outwards in a 

 graceful curve. In this position the basal cushions become everted, and the ocelli, now directed 

 outwards, are fully exposed. 



The manubrium, with its generative lobes, and the basal bulbs of the marginal tentacles, where 

 they spring from their common cushion-like support, are of a fine golden-yellow colour. 



The retractility of the hydranth in BoiiyainviUla ramosa is very great. In complete retraction 

 the whole hydranth, to within a short distance of the tips of the tentacles, is withdrawn within the 

 perisfirc, which here forms a cup-ldce expansion. This cup, however, is nothing more than a thin 

 extension of the perisarc over the body of the hydranth, to which it is more or less closely 

 adherent, and during whose retraction it is thrown into transverse rugae. It is thus entirely 

 different from the hycbotheca of the campanularian and sertularian hydroids. 



After the death and disappearance of the hydranth the ramulus which had supported 

 it continues to bear the gonophores, and these may now be often seen to form umbel-like groups, 

 terminating the branchlet. 



In Van Beneden's original memoir on the present species a specimen _ is figured and 

 described in which some of the branches carry on their sides groups of piriform bodies, which 

 ajjjjcar to bud one from the other in such a way as to acquire the form of fan-shaped clusters. 

 I cannot offer any opinion as to the nature of these bodies. In a subsequent memoir,' 

 M. Van Beneden suggests the probabihty of their being male gonophores, the female gono- 



' ' Ilcch. sur la Faune lit. do Belgique,' p. 1 13. 



