66 



MEDUSA OF THE WORLD. 



bulbs large, each with an abaxial, ectodermal ocellus. Manubrium cylindrical, with about 

 one-third of its length extending beyond the velar opening. A short, conical axial canal above 

 the stomach. Gonad extends from the base to the distal end of the manubnum. No medusa- 

 buds. Stomach brownish, gonads yellowish-white, tentacle-bulbs and axial canal red. 

 Helgoland and British coasts. 



The hydroid has a thin-branching hydrorhiza, from which a number of club-shaped 

 hydranths arise singly. These hydranths are each about 2 mm. long, with a proximal circlet 

 of 4 to 6 short knobless tentacles, and above 2 or 3 more or less irregular circlets of large 

 knobbed tentacles, usually with 4 tentacles in each circlet. The medusa-buds are produced 

 from the sides of the hydranth above the basal circlet of tentacles. The hydranths are red. 

 The hydroid and medusa are described in detail by Hartlaub, 1895. 



The English and Helgoland forms differ considerably and they may be regarded as 

 varieties one of the other (see Hartlaub, 1895, p. 157). The hydrorhiza of the English form 

 anastomoses, while that of the Helgoland hydroid branches sparingly and does not anasto- 

 mose. In the English form the 

 knobbed tentacles are in whorls of 

 4 each, whereas in the Helgoland 

 hydroid there areotten 5, occasionally 

 6, tentacles in each whorl. In the 

 English form the medusae are said 

 to arise from the bases of the ten- 

 tacles, while in the Helgoland form 

 they arise from the sides of the 

 polypite between the knobless basal 

 circlet and the lowest circlet of 

 knobbed tentacles. Finally the Hel- 

 goland medusa has a largeaxial canal 

 above the stomach, and this appears 

 to be absent in the English medusa. 



Genus HYDRICHTHYS Fewkes, 1888. 



H\drichthys, FEWKES, 1888, Bull. Mus. Comp. 

 Zool. at Harvard Coll., vol. 13, p. 224; 

 1888, Annals and Mag. Nat. Hist., ser. 6, 

 vol. i, p. 364. HARGITT, 1904, Bull. U. S. 

 Bureau of Fisheries, vol. 24, p. 32. 



( ?) Plotocnide, HARTLAUB, 1907, Nordisches Plank- 

 ton, Nr. 12, p. 68. 



GENERIC CHARACTERS. 



The medusa resembles Sarsia, 

 but is without ocelli upon the ten- 

 tacle-bulbs. The mature medusa has 

 not been determined. The hydroid is 

 firmly attached to the side of a fish 

 (Sertola zonata), and is probably 

 parasitic, and degenerate in many 

 respects. All of our knowledge of 

 this remarkable form is derived from 

 Fewkes, 1888. It differs widely from 

 all other known forms of Tubulanan 

 hydroids. 



In default of knowledge of the 

 mature medusa we must remain in 

 doubt concerning its true place in 

 our classification. It is possible that 

 Sarsia flamrnea Hartlaub may be the mature stage of this medusa. 



\ 



FIG. 31. Young medusas of Hydrichthys rnirus, from Fewkes, in Bull. 

 Museum Comp. Zool. at Harvard College. 



