290 HYDROID^. Part IV. 



marked, sometimes very strongly {Fig. 2, a l e) and at others rather faintly {c d), 

 by rings, varying from five to ten in number. There are instances where the 

 whole primary branch is at least wavy, if not comjjletely ringed thi'oughout. Each 

 pedicel has an average length of one eighth of an inch, and is terminated by a 

 single head. The horny tube, as it passes on to the head, thins out to a mere 

 film, which disajjpears entirely at the base of the crown of tentacles. The head 

 bears only a single row of slender, tapering tentacles {Fig. 3, t f), varying from 

 fifteen to twenty in number. They have the appearance of being ringed, on account 

 of the transverse rows of lasso-cells, which project from the surface Yika short 

 bristles. This arrangement reminds one of the tentacles of the Campanularians. 

 The proboscis (m) is very short, forming a mere conical papilla; in fact it cannot 

 be said to have a greater prominence than the mouth region of the genus 

 Hydra. Just below the head, during the summer months, the medusge-buds {Fig. 2, 

 A B) may be observed scattered along the pedicel in an irregular manner; each 

 bud arising singly from the hydra walls, and protected by a filmy capsule {Figs. 5, 

 6, and 7, i), prolonged from the horny sheath of the stem. The outer wall 

 {Fig. 3, a a^) of the stem and head is moderately thick upon the proboscis {in) and 

 over the head {d) ; but at the junction (c^) of the pedicel it becomes quite thick, 

 and then again grows thinner as it passes down the stem, until it is about one 

 sixth as thick as the diameter of the two Avails and the included chymiferous canal. 

 Upon the tentacles (a^) it has about the same relative thickness as in the last place 

 mentioned ; and the inner wall (i^) occupies the remaining four sixths of the 

 diameter of these organs. In the proboscis and head the inner wall (/y b^) varies 

 from twice to three times the thickness of the outer one, but, lower down the 

 stem, it decreases in this respect, until it is about as thick, on the average, as 

 the outer wall. The digestive cavity {d) is densely lined l)y a layer of deep, 

 purplish-red pigment-cells, which extend, in diminished quantity, throughout the whole 

 length of the stem and branches. 



Proles medusoidea. — I have not traced the embryology of the medusa through 

 all its stages to the fully-developed state, but only just far enough to recognize 

 the identity of its mode of- evolution, at least in the earlier stages, with that of 

 Coryne, and to ascertain the identity of these medusEe-buds with the free medusse 

 described many years ago by me, under the name of Hippocrene superciliaris. 

 Professor Leidy has also observed its development, and forwarded to me an exquis- 

 ite drawing of a Hydroid stock, bearing a number of well-advanced medusa3- 

 buds. If we compare PI. XXVII. Figs. 5, 6, and 7, respectively, with PI. XVIII. 

 Figs. 7, 6, and 10, it Avill readily appear, that the development of one of these 

 Hydroids, the Coryne mirabilis, may serve to illustrate that of the other, Bou- 

 gainvillia superciliaris; and that the principal value of the figures given on 



