HYDROMEDUSAE 29s 



shape, with a plain, simple marginal rim ; but when the mouth contracts, four distinct 

 lips of the usual type are formed. 



The gonads show slight variations in shape, but are usually globular. In some of the 

 larger specimens the ova are clearly visible and appear to be about ready for liberation 

 (Plate XVI, fig. 8). In the smallest specimens the gonads are visible as minute points near 

 the base of the stomach ; during the growth of the umbrella they gradually shift their 

 position from the proximal to the middle part of the radial canals, at the same time 

 increasing in size. 



The medusa in the early stages has only twenty-four tentacles, and these are all of 

 the same size, which indicates that the medusa is liberated from the hydroid with 

 twenty-four tentacles equally developed. When the medusa is about o-8 mm. in width 

 it has twenty-four large and twenty-four small tentacles, and the next series of forty- 

 eight tentacles is formed almost simultaneously between the tentacles of the first and 

 the second series. Even in the largest specimens with 100 or more tentacles the twenty- 

 four primary tentacles may as a rule be recognized by the somewhat larger size of their 

 basal bulbs, but they are not provided with a special pigmentation, as e.g. in O. nigra 

 Browne. The largest number of tentacles counted is 120, in specimens 3 -0-3 -5 mm. 

 wide. 



Obelia sp. (Plate XVI, figs. 9-12). 

 Obelia diaphana Browne, 1902, p. 281. 



Description. Adult: Umbrella disk-shaped. Stomach small, with a nearly quad- 

 rangular base. Mouth with four lips. Gonads spherical, attached to the radial canals 

 by a short stalk and situated very close to the margin of the umbrella. Tentacles about 

 150, closely packed together. Marginal statocysts small. Colour: Stomach, gonads, and 

 basal bulbs of tentacles of a pale yellowish colour (specimen in formalin). Size: 5 mm. 

 in diameter. 



The collection contains only a single specimen, upon which alone the description is 

 based. It was taken in Stanley Harbour on 10 November 1898. 



The condition of the ova shows that the medusa had reached the adult stage. The 

 ova have the appearance of being enclosed in a little sac, which is attached to the radial 

 canal by a short stalk or peduncle (Plate XVI, fig. 11). Three ofthe sacs have two opposite 

 sides somewhat flattened in; the fourth is more truly spherical. It may be a slight 

 contraction due to shrinkage in the preservation. 



The eight marginal sense organs are not in a true adradial position ; they are situated 

 nearer the perradial canals and consequently are not equidistant. Probably a single 

 statolith is contained in each vesicle, but not one is visible. Statoliths are seldom seen 

 in specimens preserved in formalin. 



The basal bulbs of the tentacles are small, either spherical or ellipsoid. The pro- 

 longation or root of the tentacle into the substance of the umbrella is cylindrical in 

 shape and is divided by three or four transverse septa (Plate XVI, fig. 10). This root- 



