192 HYDROID^. Part IV. 



blind sac (PI. XX. Figs. 3, c\ and 6, c^), following more or less closely the surface 

 of the head. As the Hydroid grows older, this sheath seems to cling more closely 

 to the surface of the head, and, as we have mentioned above (p. 189), forms an 

 almost imperceptible film over the tentacles, to their very tips. 



If, by accident, the head of a hydroid is destroyed, a new one is reproduced 

 at the end of the stem of the old stock. In this process the injured end of 

 the stem (PI. XX. Fig. 1, b) spreads laterally {I?), till it touches the horny sheath 

 (ffl), to which it becomes attached, while at the same time the open end is closed 

 over. When it reaches the end of the horny tube, it forms, in connection with 

 the old one (PL XX. Fig. 6, c), a new sheath (PI. XX. Fig. c^), which covers it 

 like a hood. From this new head the tentacles develop, as we have described 

 above. 



The budding of medusa;. Hitherto the medusoid generation of Coryne has been 

 referred to the genus Sarsia, of Lesson, and the species, here described, called Sarsia 

 mirabilis ; ^ but inasmuch as, long before these medusae were known, their hydroid 

 form had been referred to the genus Coryne of Gaertner, the name Sarsia, as the 

 generic appellation of this type, must yield to a prior claim. The meduste-buds 

 appear at two different seasons of the year, one lasting from January to April, 

 the other in November. Each medusoid originates directly from the head, either 

 just below (PL XVII. Figs. 2, m, 3, a a}, 5, a, 9, md, 11, 12, and 13), or, now and 

 then, intermixed with the tentacles (PL XVII. Fig. 2, md). Usually, however, they 

 develop below the tentacles, and, being not more than five or six in number, at 

 the utmost, do not cluster like those of some other genera. There may be seen 

 on the same head all stages of development, from those just beginning to bud 

 (PL XVII. Fig. 2, m), to such as are about ready to drop (PL XVII. Fig. 2, md). 

 It is worth while here to recur to the fact that the hydroid form buds only 

 from the stem, below the head and neck, in order to contrast it with another 

 fact, namely, that the medusoid form buds only from the head, or at the junction 

 of the latter with the neck. 



The earliest indication of the formation of a medusa-bud, is a thickening of 

 the exterior wall of the head of the Hydra (PL XVIII. Fig. 1, d), which produces 

 a papillate elevation (PL XVII. Fig. 3, a^) on the outer surface. This is soon 

 followed by a corresponding thickening of the inner wall (PL XVIII. Fig. 1, c), 

 at its exterior surface, and directly under the thickening of the outer wall. This 

 advance is made without tending to form a true diverticulum of the conjoined 

 walls. But soon both walls protrude, perpendicularly, from the surface of the head, 

 in the form of a blind sac (PL XVIII. Fig. 2, c d), into which the digestive canal 



' See Mem. Amer. Acad, of Arts and Sciences, Vol. IV. 



