252 MEDUS/E OF THE WORLD. 



This variety is closely related to Obelia geniculata, but is said to be distinguished by the 

 stoutness of the main stem, the great number of medusae developed in a single calycle, and 

 the wavy outlines of the somewhat bottle-shaped reproductive calycles. The young medusa 

 has 24 tentacles and 4 gonads close to the manubrium. The ten- 

 tacles are somewhat larger than in the young medusa of O. genicu- 

 lata. It is found growing on stems of Laminar 1 a in small branching 

 tufts from 25 to 50 mm. in height, at Nahant, Massachusetts. 



Obelia geniculata may be at once recognized as the creeping 

 hydroid with short, unbranched, erect, zigzag stems, growing in 

 great quantities over the fronds of Laminaria. Hardly a piece ot 

 kelp upon our coast is free from this hydroid. It is a conspicuous 

 object on account of its light coloration standing out in contrast with 

 the dark-brown of the kelp. 



The hydroid called Eucope fusiformis by A. Agassiz appears 

 to be identical with O. geniculata, but it gives rise to medusae which. 

 Fig. 133— Obelia "diaphana" w i len liberated, have 48 long, slender tentacles and 4 linear gonads 

 A^si^N^n !w- near middle of the 4 radial-canals. It appears thus to be a variety 

 ican Ac'alepha:. of 0. geniculata, which gives rise to medusae in a more advanced 



condition of growth than is usual in the typical hydroid. It occurs on the coast of Massa- 

 chusetts. 



Obelia griffini Calkins. 



Obelia griffini, Calkins, 1899, Proc. Boston Soc. Nat. Hist., vol. 28, p. 357, plate 4, figs. 18-18C, plate 6, fig. i8d. 



The hydroid forms a colony about 25 to 50 mm. high. Stems profusely branched, growing 

 on creeping stolons. Branches regularly alternate, about 0.5 mm. apart. The main stems 

 have 6 rings at the base and above the joints up to the middle of the internodes. Hydrothecae 

 0.25 to 0.35 mm. long and borne on stems 0.1 mm. long, ringed throughout (about 9 rings). 

 Rim plain. Deep and gracefully curved hydrothecae. The diaphragm is a simple partition. 

 The gonothecae are 0.8 to I mm. long and 0.25 to 0.3 mm. wide, and nearly uniform in diam- 

 eter. 6 to 8 medusa develop at the same time. The oldest medusa-buds have about 24 

 tentacles. Puget Sound. Pacific coast of the United States. 



Obelia fragilis Calkins. 



Obelia fragilis, Calkins, 1899, Proc. Boston Soc. Nat. Hist., vol. 28, p. 355, plate 3, figs. 15-15B, plate 6, fig. 15c. 



Hydrocaulus clinging, never erect. Hydrothecae bell-shaped, alternate. Margins sinuous. 

 Branches annulated throughout. Gonophores unknown. Port Townsend, Puget Sound, 

 Pacific coast of United States, on Aglaophenia. In other respects similar to 0. griffini. 



Obelia surcularis Calkins. 

 Obelia surcularis, Calkins, 1899, Proc. Boston Soc. Nat. Hist., vol. 28, p. 355, plate 3, figs. 14-14B, plate 6, fig. 14c. 



Hydrorhiza branched and creeping; never more than one branching stem. Stems erect 

 and not over 26 mm. long. Branches regularly alternate, turned upward, generally ending in 

 long filiform tendrils, slightly expanded 'at the end, and in some cases bearing 1 or 2 hydro- 

 thecae. 6 to 8 annulations at base of main stem and 3 to 4 above each branch. The stems 

 of the hydrothecx are often ringed throughout and taper from point of origin to calycle. 



The hydrotheca is slightly swollen at the center and has a slightly everted and smooth 

 margin. The diaphragm is a simple shelf projecting inwards from the cup, and the basal 

 chamber is not separated from cavity of stalk. 



The gonothecae are borne in axils and also on the branches on short ringed stalks. They 

 expand rapidly from base to end, and the extremity is flat with a very shallow, flat projection 

 containing the aperture. Medusas develop on all sides of the blastostyle, as many as 28 devel- 

 oping at once. The older medusa-buds have about 24 tentacles. The hydrothecae are 0.3 mm. 

 long, 0.25 mm. wide at margin. Peduncles are 1.2 to 5 mm. long. Gonotheca 0.75 to 0.9 

 mm. long and 0.15 mm. wide at the widest point. The hydranths have about 24 tentacles. 



Found on grasses at Port Townsend, Puget Sound, Pacific coast of the United States. 

 Distinguished by its numerous long tendrils or stolon-like branches. 



