MKDUS^E. 



29 



visible signs of Medusa-buds in any of the specimens, and, if this species does 

 reproduce asexually, then some buds should be present in the young stages. It is quite 

 probable that only Eleutheria dichotoma in this genus has Medusa-buds. 



The number of the tentacles increases with age, and they are closely packed 

 together round the margin of the umbrella. It is very likely that the number of 

 tentacles present when the Medusa is liberated from its Hydroid corresponds to the 

 number of radial canals, one tentacle being opposite each canal. The tentacles opposite 

 the radial canals in the later stages have their ocelli further in from the margin 

 (Plate III., fig. 2), indicating that they are the oldest of the series. Each tentacle is 

 bifurcated or branched, and the bifurcation is visible soon after the first appearance of 

 the tentacle. The upper branch comes off close to the umbrella, and, when fully 

 developed, is provided with ten to twelve clusters of nematocysts arranged laterally 

 in pairs, and a terminal cluster of nematocysts is also present. When the branch is 

 expanded (Plate III., fig. 1) the clusters are far apart and form an alternating series, 

 but in a contracted branch (fig. 4) their arrangement is pinnate. It is by the 

 position of these clusters of uematocysts that this species can be easily distinguished 

 from Eleutheria vallentini, which has two or three clusters on the upper (aboral) side, 

 and occasionally one on the under side. The lower branch of the bifurcation is 

 without clusters of nematocysts, and it terminates in a slight enlargement, the 

 adhesive disc or sucker, which is composed of specialised ectoderm cells. The 

 tentacles are hollow and the endodermally lined lumen extends along both the 

 branches. The basal portion of each tentacle is covered on its under side with an 

 extra thick layer of ectoderm containing nematocysts (Plate III., fig. 3), but there 

 is no enlargement of the nature of a basal bulb. Both Eleutheria dichotoma and 

 E. vallentini have a continuous band of nematocysts round the margin of the 

 umbrella. This band is absent from E. hodgsoni, but it is represented by isolated 

 patches of nematocysts on the basal portion of the tentacles. 



LEPTOMEDUS^l. 



FAMILY LAODICID^E. 



PTYCHOGENA, A. Agassiz, 1865. 



Generic diameter. Laodicidse with four radial canals; with a central stomach 

 and mouth ; with the basal bulbs of the tentacles without ocelli (Browne, 1907). 



PTYCHOGENA ANTARCTICA. 



* 



(Plate II., figs. 6-9). 

 Ptyckogena antarc/ica, Browne, 1!)07, p. 474. 



In my preliminary notes on the 'Southern Cross' Hydrozoa I alluded to this 

 Medusa under the name of Laodice. Later on, when I revised the Laodicidse, it 

 was placed in the genus Ptychogena, and a brief description of the species was given. 



z 2 



