246 MEDUSAE OF THE WORLD. 



terminates in a short neck at the upper end of which there is an opening through which the 

 medusae escape. From 10 to 16 medusae are often found at one time in each calycle. When set 

 free the young medusa usually has 16 tentacles and no trace of gonads. The tentacles are 

 slender and about as long as the diameter of the bell. 



This species extends from Charleston, South Carolina, to Eastport, Maine. It is possible 

 that it may prove to be identical with "Campanularia geniculate" of Van Beneden, 1843, 

 described from the coast of Ostende. Torrey, 1902, has discovered 0. commis surahs in San 

 Francisco Bay, California, between tide limits. 



The distinctive characters of 0. commissuralis are that the pedicels are distinctly annulated 

 and arise simply, not from distinct, swollen, shoulder-like enlargements of the stem. The 

 hydrothecae are goblet-shaped and deeper than those of 0. fiabcllata Hincks. Their margins 

 are simple and entire. 



Obelia dichotoma Hincks. 

 Plate 30, figs. 1-4. 



Sertularia dichotoma, Lmrf, 1758, Systema Naturae, Ed. 10, reformata, p. 812; 1767, Systema Natura-, tome 1, Pars 2, p. 1312. 



Campanularia dichotoma, Meyen, 1834, Acta Acad. Caes. Leop. Carol., Nat. Cur., tome 16, Suppl. p. 317, plates 40, 41. 



Laomedea dichotoma, var. a, Johnston, 1847, Hist. British Zoophytes, Ed. 2, p. 102, plate 26, figs. 1, 2. 



Eucope articulata, A.GASSIZ, A., 1865, North American Acalephae, figs. 130, 131. 



Obelia dichotoma, Hincks, 1868, British Hydroid Zoophytes, p. 156, plate 28, fig. i.-Nutting, 1901, U. S Fish Commission 

 Bulletin for 1899, pp. 350, 380, fig. 37; Proc. Washington Acad. Sri, vol. 3, p. ^.-Torrey, 1902, University of Ca - 

 ifornia Publications, Zool., vol. 1, p. 57; I9°4. University of California Publications, Zool, vol. 2, p. 15 (coast of Cal- 

 ifornia) -Browne, 1905, Proc. Royal Soc. Edinburgh, vol. 25, p. 769 (medusa reared from hydroid to maturity).— Billard, 

 inoc, Compt. Rend. Soc. Biol., Paris, tome 57, p. 1048 (differing rates of regeneration in still and in running water). 



Obelia sp Lrulina, Haeckel, 1879, Syst. der Medusen, p. 173. -van Rees, .884, Tijdschrift Nederland, Dierk, \er. Suppl., 



Deel. 1, p. C87. . , j. 



(?) Obelia australis, von Lendenfeld, 1884, Proc. Linnean Soc. New South Wales, vol. 9, pp. 604, 920, plate 43, figs. 19-22. 

 {Obelia dichotoma=0. australis), Hartlaub, 1905, Zoolog Jahrbiichern, Suppl. 6, p. 580. 

 Obelia rhunicola, Billard, 1901, Compt. Rend. Acad. Sci., Paris, tome 133, 523. 



Hydroid.— The main stems are not more than 75 to 150 mm. high and are erect, filiform, 

 and give rise irregularly to side branches in a tree-like manner. The side branches also branch 

 irregularly. The main stems and basal parts of the branches are of a deep horny-brown color. 

 The pedicels of the hydrothecae are quite long and often ringed throughout with 4 to 16 annula- 

 tions. 2 pedicels often arise from the main stem at the base of a reproductive calycle. The 

 hydrothecae are long and nearly conical, with slightly bulging sides. The margin is very slightly 

 sinuated, the sinuosities being so minute that they are readily overlooked excepting with 

 high powers of the microscope. The polypites have about 24 tentacles and the mouth is at 

 the extremity of a flaring proboscis. (See text-figures 125 to 127.) 



The reproductive calycles are mounted upon long pedicels having 4 to 10 rings. They are 

 long, cone-shaped, with a smooth outer surface, and with a tubular opening at outer end. 

 About 15 medusa-buds in various stages of development are to be seen within each calycle. 

 When set free the young medusae are about 1 mm. wide and usually have 16 (occasionally 

 24) tentacles. The 4 gonads are adjacent to the manubrium on the 4 radial-canals, but at the 

 end of 3 days they are seen to have migrated outwardly to the middle of the 4 radial-canals. 

 The hydroid is very abundant on stones, shells, and seaweeds. It may be recognized by 

 its small size, tree-like appearance, dark horny-brown color, and conical reproductive calycles. 

 This species is common along the Atlantic coasts of Europe, and in America from Maine 

 to the Carolinas. It is also found on the Pacific coast at San Diego, California (Torrey). 

 Obelia dubia Nutting, from Alaska, is closely related to, if not identical with, O. dichot- 

 oma. Obelia australis of von Lendenfeld and Bale, from Australia and New Zealand, may be 

 a variety of 0. dichotoma. n , 



Medusa— Browne succeeded, by means of his ingenious " plunger aquarium, in rearing 

 the medusae of Obelia dichotoma, which were set free from the hydroids within the aquarium. 

 He finds that the medusa is set free with 16 tentacles, no gonads, and 8 adradial hthocysts. 

 The medusae became mature at the end of 3 weeks and were then about 2.5 mm. wide wit n 

 almost flat, disk-like bell, and 60 to 84 tentacles. The cores of the tentacles project inward into 

 the gelatinous substance as a semiglobular or oblong root, usually without a "transverse sep- 

 tum" (/. e., usually not more than one cell in length). There are 8 adradial hthocysts on the 

 inner sides of the tentacle-bulbs, each containing a single concretion. The stomach is short and 



