Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 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,
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 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-epithe
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