230 MEDUS.E OF THE WORLD. 



coiled. The short tentacles are hardly more than cirri. No marginal sense-organs. Velum 

 well developed. 8 main radial-canals arise from the inanubnum, but each bifurcates, giving in 

 all 16 terminal-canals, which extend straight toward the circular vessel. The 8 proximal roots 

 of the radial-canals are bound to the manubriuin by groined arches. Manubrium broad and 

 disk-like. The mouth is at the extremity of a short neck, surrounded by 4 simple lips. 

 Gonads along the main radial-canals, adjacent to the stomach. Entoderm of tentacles, 

 radial-canals, and manubnum rosin-colored. Quite common at the Bahamas, but rare at the 

 Tortugas, Florida. Seen only in summer. 



This medusa is closely related to Dichotomic cannoides, but in NctocertoiJfs 8 main 

 radial-canals arise from the stomach instead of 4 as in Dichotomic, and the manner ot branch- 

 ing is quite different, even in young medusae. 



Monobrachium parasiticum Mereschkowsky. 



Monabrachium parasiticum, MERESCHKOWSKV, 1877, Annals and Mag. Nat. Hist., ser. 4, vol. zo, p. 220, plates 5, 6. WAGNER, 

 1889, Zool. Anzeiger, Jahrg. 12, p. 1 16; 1890, Archiv. Biologic, tome lo, p. 273, plates 8, 9 (details of histology). 



This interesting hydroid has only I tentacle. Its medusiform gonophore belongs appar- 

 ently to the Thaumantiadae, but the free medusa is unknown, if, indeed, it be set free. The 

 medusiform gonophore is oval and is developed in a sac which arises from the hydrorhiza. 

 The hydroid grows upon the posterior part of the living shell of Tellina calcarea, near the 

 siphon. The hydrorhiza is a flat expansion of anastomosing tubes covered with a thin chitmous 

 membrane. The polypites are about 2 mm. long, tubular, with a single tentacle. This is 4 to 

 6 mm. long, tapering, lash-like, and arises from the side of the hydranth at a point about one- 

 third the length of the polypite from the mouth. The gonophores are medusiform, about as 

 long as the polypites, and they arise from the hydrorhiza by a short stalk. The contained 

 medusa is oval with a short conical manubriuin, 4 radial-canals, each with 2 gonads upon its 

 sides. Ring-canal and 16 rudimentary tentacles. Color of colony, yellow. White Sea, Russia. 



