332 HYDROIDiE. Part IV. 



but occasionally they arise at a point nearer to the main axis [Fig. 14), but 

 always on that side toward which the twin hydra3 calycles converge. At first, 

 one of these reproductive bodies {Figs. 5, 5", and 14, a) resembles the incipient 

 stage of a pair of hydrte {Fig. 14"), and, in fact, the process of the development 

 of the walls is the same in both. Primarily the organ in question is turbinate, 

 and strictly circular in a transverse section {Figs. 5, 5% and 14) ; the outer wall 

 {Fig. 5", a a^) is very thick, and composed of very distinct, columnar cells; whereas 

 the inner wall {d c), although it has a very well-marked contour, exteriorly 

 appears like a confused mass of dark brown cells. The cavity contains granules, 

 a2)parently detached from the inner wall, which are in a constant state of circulation 

 around its sides, and backwards and forwards through the pedicel, in direct com- 

 munication with the common canal of the main stem. Before the axis of the 

 reproductive body has completed its terminal growth {Fig. 10, g), the outer wall 

 {(i^) detaches itself from the now oval calycle {c), and Ijegins the development of 

 its medusa {h I?). At this time, the outer wall («') of the axis is no thicker 

 than in the main stem ; but the inner one {c) has a much greater depth, and 

 the cellular structure is quite recognizable. The medusa commences, as a mere 

 lateral hernia {b l>^) of the outer and inner walls of the axis, and, eventually, by 

 a process similar to the mode of development in the medusa of Hydractinia 

 (PI. XVI. Figs. 7, 8, and 9), Thamnocnidia (PL XXII. Figs. 1-7), and Parypha (PI. 

 XXIII. Figs. 3-8), becomes a double-walled sac (PI. XXXII. Fig. 8, b, b'), the disk 

 proper, in the axis of which a probosciform, single-walled, actinostome [I) projects, 

 as we have described more fidly on a previous page (p. 329). In the mean 

 time, the axis («^) increases in length, by the further development of the solid 

 mass {g), which fills the mouth of the calycle, and, when mature {Fig. 8, «^), is 

 attached terminally to the thin edge of the calycle and its operculum {g). 



