238 



CCELENTEKATA. 



often only after a long period of free life, in which they become 

 much larger and undergo a metamorphosis, reach sexual maturity. 



The Medusae belonging to the order Hydromedusa? are, with but 

 few exceptions, distinguished from the Acalephce (Scyphomedusag) 

 by their smaller size although certain forms, for example Aequorea, 

 may attain such a size as to have a diameter of more than a foot and 

 by their simpler organization. The number of their radial vessels is 

 smaller (4, 6, or 8), their sense organs (marginal bodies) are not 

 covered by folds of membrane (hence Gymnophthalmata Forbes), and 

 they have a muscular velum (hence Craspedota Gegenbaur) (fig. 182). 

 The generative products are always formed from the ectoderm, and 

 originate on the walls of the radial canals or of the rnanubrium, but 



never, as in the Acalepha, 

 in diverticula of theerastric 

 cavity. 



The hyaline gelatinous 

 substance of our Medusae 

 is, as a rule, structureless, 

 and contains no cellular 

 elements ; there may, how- 

 ever, be fibres running per- 

 pendicularly through it 

 (Liriojje). These fibres are 

 probably derived from 

 cell processes of the ecto- 

 derm and entoderm, and 

 have arisen contemporane- 

 ously with the gelatinous 

 disc, which is itself to be 

 looked upon as an excretion 

 product of the adjoining 

 ectoderm and entoderm epithelium. 



The nerve-ring is placed at the edge of the disc at the point of 

 insertion of the velum. It is covered by a sense epithelium com- 

 posed of small cells bearing sense hairs, and has the form of a double 

 fibrous cord containing ganglion cells. The larger upper nerve-ring 

 runs above the velum, while the weaker nerve-ring, on the other 

 hand, is placed below it. The lower nerve-ring is composed of larger 

 fibres and larger ganglion cells ; bundles of fibrilla? pass off from it 

 to supply the muscles of the velum and subumbrella, where they 

 form a sub-epithelial plexus interspersed with ganglion cells, between 



FIG. 182. Plduhiduun ntriubile represented from the 

 underside of the umbrella. T', Velum ; O, mouth ; 

 Ov, ovary ; Ob, auditory vesicle ; Sf, tentacles on 

 the margin of the disc ; Sw, marginal swellings. 



