HYDROMEDUSAE 273 



Genus Hybocodon L. Agassiz (1862) 



Generic characters. Codonidae with a single tentacle or a single group of two or 



more tentacles, perradially situated, and three rudimentary perradial basal bulbs 



without tentacles. Margin of the umbrella not at right angles to the longitudinal axis 



of the umbrella, but sloping towards the side carrying the tentacle or group of tentacles. 



Hybocodon unions (Browne) (Plate XV, figs. 2-3). 



Amphicodon iiniciis Browne, 1902, p. 276. 

 Hybocodon unicus Mayer, 1910, p. 42. 



Specific characters. Adult (Plate XV, fig. 2) : Umbrella bell-shaped, a little higher 

 than broad. Manubrium cylindrical and nearly as long as the umbrella cavity. Gonads 

 surround the tube-like manubrium and extend from near the base of the manubrium to 

 close to the mouth. One solitary tentacle, situated between two rudimentary basal 

 bulbs; three perradial bulbs without tentacles. Colour: Gonads and basal bulbs of a 

 pale yellowish brown (specimen in formalin). Size: Umbrella 3 mm. in height and 

 2 mm. in width. 



The collection contains only one single specimen taken in Stanley Harbour on 

 20 November 1898. 



The umbrella has fairly thin walls and a narrow velum. The diameter of the ring 

 canal is much smaller than the maximum diameter of the umbrella. The exumbrella is 

 covered with scattered nematocysts, which are not arranged in longitudinal rows. The 

 margin of the umbrella is slightly oblique, so that the side of the umbrella bearing the 

 tentacle is longer than the opposite side. The manubrium has a small apical knob and a 

 small apical stalk, the remains of the umbilical canal which originally connected the 

 medusa to its hydroid. The proximal part of the manubrium, free of gonads, consists 

 of large vacuolated cells (as in Hybocodon prolifer). The solitary tentacle is situated on a 

 very small bulb, which is placed between two large rudimentary basal bulbs containing 

 nematocysts, but not bearing tentacles (Plate XV, fig. 3). In H. prolifer there is at first 

 a solitary tentacle, and later on two more tentacles appear, one on each side of the 

 primary tentacle; in the Falkland specimen the two rudimentary bulbs mark their 

 position. The other rudimentary perradial bulbs are very small and have a long tapering 

 process which is attached to and curls over the margin of the umbrella. The three 

 rudimentary perradial bulbs as well as the two large nematocyst pads flanking the 

 solitary tentacle resemble the corresponding structures in H. prolifer, but are not 

 continued upwards on the exumbrella as lines of nematocysts. The tentacle is long ; in 

 the present specimen it is coiled into a close spiral and so much contracted that the 

 arrangement of its nematocysts can only be seen by a closer examination ; the nemato- 

 cysts form complete rings around the tentacle just as in H. prolifer and other medusae 

 of the same group {Euphysa, Steenstrupia). 



