254 HYDROID^. Part IV. 



in the medusoids ; but, when full-grown (PI. XXIIT. Figs. 12, 1.3, and 14), they have 

 a simple single-walled proboscis {d), a double-walled disk (a), and from six to ten 

 tentacles, very much compressed, laterally, crest-shaped, and hollow {f P)- When 

 full of young, ready to come out, they have a globular, or broad oval shape 

 {Figs. 12 and 13), and the tentacles (/) are in the form of a low crest, which 

 is hardly half so high as long; but when the young are nearly, or altogether 

 set free, then the medusoids have an elongate, ovate shape {Fig. 14), and the cristate 

 tentacles {f P) have almost twice the height of their centripetal diameter. In this 

 condition they are very conspicuous {Fig. V', b) among the other globular medu- 

 soids. The male medusoids {Figs. 2, 2% and 15) never have any tentacles, nor do 

 they deviate from an almost perfectly spherical shape. As the details of the struc- 

 ture of the medusoids will necessarily be given in the history of their develop- 

 ment, to avoid repetition, we only refer here to the next pages, relating to the 

 embrj'ology of this genus. 



Einhryology. Proles hydroidea. — We have never been able to find the least trace 

 of eggs in the medusoids, though we have searched diligently for them. It is 

 barely possible that they should have escaped our notice, when all stages of the 

 growth of the medusoids have been closely scrutinized. 



At first the medusoid is a simple, double-walled blind sac {Fig. 4, a b), which is 

 in direct continuation with the walls (a^ b^) of the branch to which it is attached ; 

 afterwards the two walls separate and leave a space {Fig. 5% e), which, from the 

 moment of its formation, is filled by a very faintly granular, but excessively trans- 

 parent substance. As the space between the walls increases in size, the transparent 

 mass grows also, and at the same time becomes more coarsely and distinctly granu- 

 lar {Fig. 9, e), and deep yellow in color, but otherwise there is no internal change 

 to be noticed. When the medusoid has attained to about one half of its adult 

 diameter, the granular mass {Figs. 9, c, and 10, e) clings more clo.sely to the pro- 

 boscis {d), and has retracted from the region around the aperture of the disk, or, 

 more pi-operly speaking, has ceased to grow as fast as the still increasing cavity of 

 the disk. Soon after this, the mass begins to be subdivided,^ and, from time to 

 time, throws off large spherical portions {Fig. 11, e). That which adheres to the 

 proboscis still continues to grow, notwithstanding the process of self-division. The 

 separated portions of the mass lose their yellow color, and, becoming semi-trans- 

 parent, soon undergo a change, which, very early, indicates their destiny ; each 

 becomes flattened and angular {F^. 21), with from six to eight sides, forming a 

 polygonal disk. At first the angles are rounded, and not always equidistant {Fig. 



^ The details of the process of subdivision will Tubularia Couthouyi, in which this phenomenon 

 be found more fully illustrated in the section on was first traced. 



