198 • ■ HYDROID^. Part IV. 



Fig. 14, and wood-cut 11, a b) of the disk gradually grows smaller and smaller by 

 constriction, till finally it is cut through, and the medusa drojis from the parent 

 stem, and swims away. Shortly before dropping from the hj'dra, the medusa becomes 

 very restless; it contracts and expands in rapid succession, by jerks which throw 

 it to and fro about the stem of the hydra. The hydra itself contributes also 

 to the liberation of the medusa, by coiling itself around the jieduncle, which still 

 holds the medusa fixed by its abactinal summit to the place from which it has 

 been budding. In thus coiling itself around the base of the medusa, the hydra 

 gradually pushes the medusa off, and the next jerk sets it altogether free. At 

 the time of its birth, the medusa is about one sixteenth of an inch in diameter (PI. 

 XVIII. Fiff. 15). For a while the outer and inner walls cling to each other at 

 the point where the peduncle was divided (PI. XVIII. Fir/. 15", a ; PI. XIX. Fig. 16, 

 n), so that the summit of the medusa exhibits a funnel-shaped depression. As 

 soon as the disk is freed from the restraint of the horny film, the whole animal 

 expands, and the outer Avail separates for a considerable distance from the middle 

 one, except where they form the transverse septum, and at the i:)oint where they 

 were attached to the parent. At this last-mentioned j^liice, the outer (PI. XIX. 

 Fig. 16, 0^) and inner walls (/«*) are drawn toward each other by their mutual 

 efforts to separate, and the outer one (o') being drawn in, forms a depression (o) 

 very often noticed in young Meduste, whilst the inner one, being drawn out, becomes 

 conical. As they retreat from each other, the depression (PI. XIX. Fig. 15, o) 

 becomes deeper, the cone (h*) more pointed and higher, and the point of adherence 

 (n) less and less, till finally the two walls suddenly separate. The outer one 

 retires till it comes nearly to a level with the surrounding portion, still remaining 

 slightly depressed (wood-cut 25, «', p. 202) and the inner one sinking, the hollow 

 cone disappears. The widely separated outer and middle walls of the medusa just 

 born (PI. XVIII. Fig. 15"), form a very remarkable feature when contrasted with their 

 relations at a period just before birth (PL XVIII. Fig. 14), where the outer (wood- 

 cut 17, a), and middle {b) ones press very closely against each other. It is not 

 possible to say, precisely, at Avhat time the mouth of the proboscis is formed, but 

 it is certainly open (PI. XVIII, Fig. 15", e) by the time the medusa becomes free. 

 The radiating tubes (PI. XVIII. Fig. 15", c; PI. XIX. Figs. 16, e^ and 17, l>) are, 

 proportionally, a great deal larger than in the full-grown animal, and have very 

 irregular walls ; a peculiarity not noticed in earlier stages, nor in later ones. At 

 the junction of the radiating and circular tubes (PI. XIX. Figs. 17, b% 18, b, and 

 19, b), and also where the four radiating tubes mutually empty into the proboscis 

 (PL XVIII. Figs. 16, d, and 17, d; PL XIX. Figs. 16, i\ and 20, a), their walls are 

 lined with dense accumulations of dark-brown granules, which are constantly loosening, 

 and circulating with the chymiferous fluid, and finally cast out from the mouth. 



