230 HYDROIDiE. Part IV. 



proboscis, inasmuch as it has a mouth {Figs. 2% 2", and 2", m) which at times opens 

 as widely {Fig. 2", m), and is as changeable in shape, as that of the sterile forms 

 {Figs. V', V, \^, V, and 2"^, m) ; and, in fact, considering the mobility and activity 

 of particular parts of the mouth and proboscis, independently of other parts of the 

 body, we have no hesitation in saying that it is as truly an organ for the prehen- 

 sion and reception of food, as is the proboscis of the sterile forms. Sometimes the 

 tentacles separate, and simply disclose the mouth {Fig. 4", m), without protruding the 

 proboscis. In such cases, the tentacles are usually arranged in two rows, those 

 of one row alternating with those of the other, and forming, together, a depressed, 

 turban-shaped mass {Figs. 1, A h, 1^, t, 3, 4, h, and 4*). The tentacles, as a usual 

 thing, are globular, but now and then, during the dilatations of the head and pro- 

 boscis, they stretch to a slight extent at the base, so as to stand out {Fig. 2^ t) 

 from the head on a short, thick pedicel {Fig. 2\ a). It can hardly be said that 

 this pedicel belongs to the tentacle, but is rather a lateral hernia of the walls of 

 the head, with a hollow interior {Fig. 2', d), such as is never found in the tentacles, 

 either of the sterile forms of Hydractinia, or among any of the marine Hydroids. 

 They vary in size and number without any apparent reference to the age or size 

 of the hydroid to which they respectively belong. Of two fully-grown hydroids, 

 one {Fig. 1, A), for instance, may have numerous and small tentacles {h), and the 

 other {Fig. 1, B) only a few large ones {h). The highest number of tentacles that 

 we have ever been able to count on any one head is sixty {Fig. 2, K), and the 

 lowest, only four {Fig. 1, F). In the latter case, they were larger than any we 

 have ever observed upon older hydroids of the same species. When seen in an 

 extended state {Fig. T), it becomes evident that the tentacles are composed of two 

 walls, the outer one of which {a) is continuous with the outer wall {Fig. 3, a) of 

 the body, and almost entirely composed of densely packed lasso-cells {Fig. 11, a h), 

 while the inner wall {Fig. 2\ h) is continuous with the inner wall {Fig. 3, b) 

 of the body. 



The body of the hydroid, or that portion which is above the horizontal uniform 

 layer, is composed of two walls. The outer one {Fig. 3, a) is so thin, when com- 

 pared to the inner one {b), that it appears like an epidermis to the latter. It 

 commences at the tip of the proboscis, and, including the thick, lasso-cell layer of 

 the tentacles, extends to the base of the hydroid, where it becomes continuous 

 with the uniform, horizontal layer {Figs. 5, a, 5% d, 5^ a, 5", a, and G, d), which forms 

 the common basis of all the hydroids. When the hydroid is full of medusae-buds, 

 it is an easy matter to see that this outer wall {Fig. 3, a) is continuous with 

 the outer Avall {a}) of the medusoids. (See, also. Figs. 7 and 8, a b.) There are 

 but few lasso-cells in the outer wall, below the head, the great mass of them being 

 congregated on the tentacles. 



