100 
CHARLES L. BOULENGER. 
cells of the superficial layer (Text-fig. 3). I was not able to 
observe the formation of the circular canal; there can be no 
doubt, however, that this organ arises as in other medusae by 
the fusion of the four primitive radial canals at their distal 
extremities. 
The velum becomes perforated at a comparatively early 
stage, the last organ to appear being the manubrium, which 
Moore correctly described as forming a rounded boss projecting 
into the subumbrella cavity (PI. 15, fig. 8). There seems, 
however, to be considerable variation in the exact time of 
appearance of the manubrium, some quite advanced medusa- 
buds being devoid of this organ, although the mouth and the 
elongated endoderm cells of the future manubrium are always 
indicated (Text-fig. 3). 1 
7. The Phylogeny op the Medusoid Individual in the 
HYDROMEDUSiE. 
* Until the publication of Goette’s memoir on the development 
of the gonophores of the Hydromedusae theVe was a general 
agreement as to the relations between the medusoid and 
o 
hydroid individuals of this group of Ccelenterates. The 
manubrium of the medusa was considered to be homologous 
with the hypostome of the hydroid, and the umbrella to be 
derived from the tubular body of the latter by a process of 
radial expansion followed by the inter-radial fusion of the 
gastric lining of its cavity. The tentacles are carried out- 
wards by this process of expansion but remain as a circlet 
round the hypostome, now the manubrium. 
The development of the medusa-buds, as formerly under- 
stood, distinctly favoured this theory, and Weismann, in his 
classical monograph on the origin of the sex-cells in the 
Hydromedusae (14), showed how the ontogenetic evidence 
could be made to fit in with such a phylogenetic origin of 
medusae. To quote his own words 3 : 
1 Cf . also Gunther’s figure (1), pi. 19, fig. 14. 
3 The quotation is taken from Goette s memoir (6), p. 261. 
