. HYDROMEDUSAE. 733 



umbrella. The gonads (fig. 6) extend along nearly the whole length of the radial canals, 

 leaving both ends free. In the male the portion of the canals bearing the gonads is more 

 tubular than in the female. In the female (fig. 7) the gonads hang down from the wall 

 of the sub-umbrella and are distinctly bilamellar. 



The basal bulbs of the tentacles (figs. 10 — 12) are broad hollow sacs and are characterized 

 by curling over a thickening of the margin of the umbrella. There is an excretory pore 

 ojiening from the circular canal opposite each of the basal bulbs of the tentacles and each of 

 the largest marginal bulbs. The marginal bulbs also curl over the margin of the umbrella 

 and some of them probably develop tentacles at a later stage in the growth of the medusa. 

 The sensory vesicles are very minute ; their otoliths are not visible. 



Genus Mesonema, Eschscholtz, 1829. 



Aequoridae with numerous simple, unbranched radial canals. Stomach circular, with lower 

 wall quite rudimentary. Mouth nearly as large as the diameter of the stomach and cannot 

 be closed. 



Mesonema pensile (Modeer), 1791. (PI. LV. fig. 4, PL LVII. figs. 2 — 9.) 



Mediisa, sp. Forskal (1776, p. 9, Tab. XXVIII. fig. B); Medusa coelum pensile, Modeer 

 (1791, p. 32); Aequorea mesonema, P^ron (1809); Mesonema coelum pensile, Eschscholtz (1829); 

 Mesonema pensile, Haeckel (1879). 



In Forskal's Icones rerum naturalium, 1776, there is a good figure of a medusa about 

 which nothing is stated, except in the description of the figure. There occurs this vei-y brief 

 statement, " Medusa non descripta. Color coerulescens." 



Modeer gave a short description of the medusa from Forskal's figure, and called it 

 Medusa coelum pensile. Peron placed the species in a new genus under the name of Aequorea. 

 mesonema. Eschscholtz removed it from the genus Aequorea to the genus Mesonema and 

 restored Modeer's specific name. These early authors suggested the locality for Forskal's 

 medusa to be in the Mediterranean, but it must be remembered that ForskS,l did not state 

 where he found his specimen, and that his book contains the descriptions of the animals 

 which he found in the Red Sea, as well as in the Mediterranean. 



Haeckel adopted the name Mesonema pensile for Forskal's medusa, but he gives among the 

 synonyms Mesonema coerulescens, Kolliker, 1853, and Stomohrachium mirahile, Kolliker, 1853 ; 

 both taken by Kolliker in the Mediterranean. I fail to see the connection between Forskal's 

 medusa and Kolliker 's two species. These are young and intermediate stages, probably belonging 

 to the genus Aequorea. Haeckel's description of Mesonema pensile is based upon the description 

 of three species, and consequently is of little value. 



In the Maldive collection there are four medusae which have all the characters of the 

 medusa figured by Forskal. 



Description of the Species. Umbrella almost a solid mass of jelly, rather like a plano- 

 convex lens in shape, about twice to three times as broad as high. Sub-umbrella forming 

 only a fringe round the periphery. Stomach completely rudimentary, its lower wall about 

 2 mm. in length, and its diameter about two-thirds the diameter of the umbrella. Mouth 

 circular, nearly as large as the diameter of the stomach ; non-contractile, and always wide 



94—2 



