366 



MEDUSA OF THE WORLD. 



more probable from the fact that C. sowerbii has hitherto been seen only in relatively small 

 artificial waterlily tanks, and its native habitat remains unknown. It may theretore tail to 

 develop as fully in these restricted, artificial surroundings as in a state of nature. The 

 medusa may have been introduced into the Yang-tse-kiang through the well-known religious 

 interest of the Chinese in the cultivation of waterlilies. 



Genus MICROHYDRA Potts, 1885. 



Mkrohydra, Potts, 1885, Science, vol. 5, No. 125, cover sheets, p. v; 1897, American Naturalist, vol. 31, p. 1032. 

 Potlsia, Ryder, 1885, Science, vol. 5, p. 1236. 



The type species is Mkrohydra ryderi, of a fresh-water stream called Tacony Creek, near 

 Philadelphia. 



Potts established this genus for a small medusa with 8 tentacles, 4 radial-canals, 4 lips, and 

 neither lithocysts nor ocelli. It is set free from a minute clavate hydroid which lacks tentacles 

 and grows singly or in small clusters upon stones in the bottom of the creek. The medusa-bud 

 is developed upon the side of the hydroid and arises singly, being mounted upon a well- 

 developed pedicel. The hydroid also reproduces asexually. The mature medusa is unknown 

 and it is, therefore, impossible to define any generic characters. 



Mkrohydra ryderi Potts. 



Mkrohydra ryderi, Potts, 1885, Science, vol. 5, No. 123, cover sheets, p. v. (hydroid); 1897, American Naturalist, vol. 31, p. 

 1032, Annals and Mag. Nat. Hist., ser. 7, vol. I, p. 130 (medusa and hydroid); 1908, Proc. Delaware County Institute of 

 Sci., vol. 3, p. 89, plates 1-4; 1906, Quart. Journal Microscop. Sci., vol. 50, p. 623, plate 36, figs. 13-26 (hydroid and 

 medusa). — Browne, 1906, Quart. Journal Microscop. Sci., vol. 50, p. 635, plate 37, figs. I, 2, 4 (medusa). Ryder, 1885, 

 Amer. Naturalist, vol. 19, p. 1232. 



Pottsia ryderi, Ryder, 1885, Ibid., p. 1236 (medusa). 



Just after separation from the hydroid the medusa is about 0.4 mm. in diameter. Bell 

 thin-walled, dome-shaped, 0.3 mm. high. 8 equally developed tentacles, about half as long 

 as bell-diameter. No marginal lithocysts. Velum wide, its aperture only one-third to one-fourth 

 of diameter of bell. Manubrium simple, conical at base, and four-cornered below, about 

 one-half to two-thirds as long as bell-height. 4 straight, slender radial-canals, no gonads. 



Fig. 208. — Microhydra ryderi, from camera-lucida drawings by Prof. J. Percy Moore, kindly lent to author by Mr. Edward 

 Potts. The medusae were drawn from specimens 48 to 50 hours old. 



These medusae bud off singly from the side of a small hydroid which is devoid of tentacles 

 and lives in the rocky bed of Tacony Creek, a rapidly flowing, fresh-water tributary of the 

 Delaware River, near Philadelphia, Pennsylvania. Potts believes that this form may event- 

 ually prove identical with Craspedacusta sowerbii of the Regents Park, London. 



The hydroid is clavate, almost cylindrical, with a rounded, upper mouth-end, devoid of 

 tentacles. It usually grows singly, although occasionally two are seen connected at their 

 common base. The hydroid gives rise to sausage-shaped buds which arise from the side of 

 the parent hydroid. These buds develop in such a manner that the final separation of the 

 bud occurs at one side instead of being terminal, as in Craspedacusta. When set free the bud 

 falls to the bottom and moves about to some slight extent by amoeboid or vermiform con- 



