236 HYDROID^. Part IV. 



the latter for further details. The young medusoid buds of the genus Hydractinia 

 always arise singly, and directly from the ujjright stem of the parent (PI. XVI. 

 Figs. 1, A, e, B, e, F, e, 2, A, e, B, e, C, e, 3, 4, a h, A", a h c d e f g h i, and 4''), 

 in the form of a double-walled, lateral protrusion {Fig. 7). The growth of these 

 medusoids may be traced on the same parent stem, inasmuch as all stages of 

 growth are to be oftentimes seen at one time {Fig. A", a-i). At a very early age 

 the female medusoid contains eggs {Fig. 3, e), which always lie loose in its cavity 

 around the proboscis (^j). We have never seen any thing but eggs in the female 

 medusoids, even at the time the male medusoids were discharging their spermatic 

 particles, and, on this account, cannot doubt that the segmentation of the yolk and 

 the subsequent growth of the hydroid take place outside of the medusoid, in the 

 open sea. The male medusoid {Figs. 4, a h, 4% a-i, 4**, 8, and 9) does not retain 

 the universally rounded form of the female, but varies from an elongate cylindrical 

 {Fig. 14", «) to a perfectly globular form {Figs. 4", c, and 9). As fast as the 

 spermatic particles are discharged, the walls of the medusoid shrink and become 

 wrinkled {Fig. 4, a b), and at the same time the proboscis shrivels also, and the 

 peduncular attachment constricts, till eventually the Avhole medusoid becomes a 

 shapeless mass, with a very slight hold on the parent stem. 



In this half-resolved state they fall from the parent hydroid and die. Till 

 within a short time before the spermatic particles are discharged, their Avhole mass 

 has a yellow tinge, but when they are fully developed, they have, altogether, a 

 dead-white color. Neither the male nor female medusoids have any tentacles. The 

 number of eggs which a medusoid may contain amounts to at least a dozen, and, 

 perhaps, to sixteen or eighteen, since, sometimes, as many as eight or nine may be 

 counted in one half of the parent, as it stands out in profile {Fig. 1, C). Often- 

 times we have seen a young medusoid {Fig. 8) pretty thickly covered by lasso- 

 cells (/), which gave it a bristling appearance, while at other times there are very 

 few of these cells present {Fig. 3). 



SECTION III. 



HISTOLOGY OF HYDRACTINIA. 



The Hydroid. — The outer wall of the young hydroid (PI. 16, Fig. S*", a}) is com- 

 posed of very irregularly columnar, transparent cells, each one of which occupies its 

 whole thickness. These cells are identical with those which enter into the composition 

 of the outer wall (a) of the stolonic part of the colony. In the latter, they are 

 not so conspicuous, but stand out isolately, as if they were imbedded in a homo- 



i 



