300 HYDROID^. Part IV. 



to its calyx, on account of the peculiar form and proportions of the latter. The 

 reproductive hydra? (PI. XXYIII. Fig. 2, b) bring forth planulte {Figs. 17 and 17"), 

 which are developed within a very low form of medusa. Each calycle contains 

 but one medusa, either male (PI. XXVIII. Figs. 13, 13% /, 14, /, and 19, i; PI. XXIX. 

 Figs. 2, e f g, 3, i, 4, i, and 5, h i) or female (PI. XXVIII. Figs. 15 and 16, i), 

 and in each colony the medusae are either all males or all females. The relations 

 of the medusa to the axis or axes of the reproductive calycle, can be better 

 understood in connection with the process of development of this part of the 

 hydromedusarium ; and, therefore, we will merely state here that there are more 

 frequently two, three, four, or five axes, than one, and that the medusa develops 

 either from one side of the axis, whether single (PI. XXVIII. Fig. 14, c) or multiple 

 {Fig. 13, c), or arises from within the circle of axes, at their point of branching 

 {Figs. 15, 16, and 19; PI. XXIX. Figs. 2, 3, 4, and 5). 



Embryology. Proles medusoidea. — The young reproductive calycles (PI. XXVIII. 

 Figs. 2, c c\ 11, and 12) have a broad, pyriform contour, being mere hernia-like 

 expansions of the stem, with double walls {Fig. 11, (■} y). In the very earliest 

 stage they seem to be perfectly identical with the young sterile hydra-buds {Fig. 4); 

 and have very thick outer {a) and inner {b) walls, which press closely against 

 the horny covering (c). As development goes on, the terminal portion {Fig. 11, 

 d) always presents a broad outline, and imiformly adheres to the horny sheath {k), 

 which, by the way, it constantly secretes from its exterior surface. At a certain 

 period, however, that part of the axis which is already well developed, retracts 

 from the sheath, and occupies a central position ; and as fast as this occurs, the 

 cellular structure, which is so conspicuous in the terminal portion {d), becomes 

 obscure. Almost immediately after this, the medusa (/) begins to bud from the 

 axis, and usually, near its base. In its incipient condition it is a slight lateral 

 divergence of the double walls of the axis; but it soon increases to much larger 

 dimensions, and assumes, by degrees, a broad, cylindrical form {Fig. 12, i), with a 

 rounded end. At this stage, the terminal growth of the axis {d) is considerably 

 broader than in the previous phase, and the free portion of the axis immediately 

 below it is bent to one side ; but what is, perhaps, most noteworthy here, is that 

 the outer wall (/?) has increased to an enormous thickness, and fills the entire 

 space of the calycle not occupied by the medusa. At a later period {Fig. 13) 

 we find the medusa ( i) possessing four radiating chymiferous tubes (/), which 

 appear to be excavated within the thickness of the inner wall, after the manner of 

 the earlier stages in the medusa of Tubularia Couthouyi (PI. XXIV. Fig. 11, c). 

 In the specimen which we have represented (PI. XXVIII. Fig. 13) the axis is 

 quadruple {c c^ c^), the original and single axis {a) having diverged in four direc- 

 tions during the process of development. At the actinal end, where the axis is 



