LEPTOMEDUS.^C — OBELIA. 253 



Obelia gracilis Calkins. 



Obelia gracilis, Calkins, 1899, Proc. Boston Soc. Nat. Hist., vol. 28, p. 353, plate 3, figs. 13-13C, plate 6, fig. 13D. 



The hydrorhiza is creeping and slightly branched. Stems of hydroid erect, simple or 

 slightly branched, 6 to 20 mm. high. The main stem has 4 to 6 rings at base and 2 or 3 above 

 each branch. The side branches have 4 or more annulations at base and are occasionally 

 provided with tendril-like processes. The hydrothecae alternate at angles formed by the very 

 slight zigzag of the main stem; usually there are 2 hydrothecae at each angle, but in the older 

 angles, near the base of the main stem, one hydrotheca is at the end of a long stalk having 

 ^ to 4 annulations at base and 3 below the hydrotheca; the other hydrotheca is on a short stalk 

 borne in the axil of the former. The cup of the short-stalked hydrotheca is about 0.36 mm. 

 long and 0.32 mm. wide, while that on the long stalk is smaller and only about 0.32 mm. long 

 and 0.19 mm. wide. The margins are plain and the diaphragms double. The hydranths have 

 about 28 tentacles. The gonothecae are club-shaped, widest at outer end, 0.75 mm. long, and 

 0.26 mm. wide at end. The large medusa-buds have about 28 tentacles. Medusa; develop 

 from all sides of the blastostyle. 



Found on grasses at Port Townsend, Puget Sound, Pacific coast of North America. 



The stems of this hydroid resemble "Eucope" diaphana A. Agassiz, but the stalks of the 

 hydrothecae are only partially ringed instead of being ringed throughout, as in Agassiz's form. 

 The gonothecae are, however, quite different from those of Obelia diaphana. 



Obelia corona Torrey. 



Obelia corona, Torrey, 1904, Univ. California Publications, Zool., vol. 2, p. 14, figs. 5, 6. 



Colonies very low; stems simple, short, slightly flexuous, arising from a creeping stolon, 

 with 3 to 6 annulae distal to each pedicel. Hydrothecae long, narrow, tapering. Margin with 

 8 to 10 teeth, each with 2 sharp cusps, pedicel short, completely annulated with 2 to 4 annulae. 

 Hydranth with about 24 tentacles. Gonangia club-shaped with wide aperture, 3 times as long 

 as broad. Pedicel short, completely annulated with 2 to 4 rings. Numerous medusae, largest 

 with 24 tentacles. The hydrotheca ranges from 0.43 to 0.44 mm. long and 0.18 to 0.20 mm. 

 wide. Gonangia, including pedicel, 0.76 mm. long and 0.2 mm. wide. San Diego Bay, Cal- 

 ifornia, under wharves and creeping over sponges. 



Obelia helgolandica Hartlaub. 

 Obelia helgolandica, Hartlaub, 1884, Zeit. fur wissen. Zool., Bd. 41, p. 165, text-fig. II, taf. 11, fign. 3, 4, etc.— 1894, Wissen. 

 Meeresuntersuch. Kom. Kiel, Helgoland, ser. 2, Bd. 1, p. 173. 



Medusa flat, shield-shaped, 0.3 mm. wide. When mature there are about 80 tentacles, 

 but when set free from hydroid there are but 24. Tentacles not half as long as bell-radius. 

 Manubrium short and thick, with 4 wide lips with their radial corners turned upward. The 

 4 gonads lie upon the outer thirds of the 4 radial-canals, not touching ring-canal. Hartlaub 

 shows that in this medusa, as in Obelia adelungi, the germ-cells originate upon the upper, 

 interradial, ectodermal walls of the manubrium. They begin to appear upon the second day 

 after the medusa is set free from the hydroid, and only later do they migrate outward to their 

 position in the gonads on the radial-canals. The young eggs all originate in the ectoderm of 

 the manubrium, but later some of them migrate into the entoderm of the manubrium. The 

 young eggs of the ovaries on the radial-canals are commonly found in the entoderm, but a 

 number of these migrate into the ectoderm to ripen. The sperm-cells originate in the ectoder- 

 mal, interradial walls of the manubrium and later migrate into the ectoderm of the gonads on 

 the radial-canals. As Hartlaub says, this process would indicate that primitively the gonads 

 were upon the manubrium as in the Anthomedusae and that their position upon the radial- 

 canals is secondarily acquired. 



Hydroid.— The hydroid of Obelia helgolandica forms thin zigzag stems about 100 to no 

 mm. high. These stems are brown; the side branches are short and arise alternately from the 

 sides of the main stem. These side branches also give rise to several alternate ramuli. The 

 hydrothecae are large, about 0.5 mm. long, smooth-lipped, and mounted on pedicels which 

 are about 8 mm. long and ringed throughout. Gonangia about I mm. long, often curved, 

 mounted upon short-ringed pedicels, which usually arise from lower parts of the hydrocauh. 



