Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 1ITDUOZOA. 23!> the muscular epithelium and the fibrous layer. The ganglion cells iu the upper nerve-ring are smaller, and the fibrilla; given off from it pass to the tentacles. The fibrilla? of the sense nerves may be derived from both rings. The marginal bodies have long been recognised as sense organs, and are either eye spots (ocelli) or auditory vesicles ; hence the Hydromedusce may be divided into two group
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 1ITDUOZOA. 23!> the muscular epithelium and the fibrous layer. The ganglion cells iu the upper nerve-ring are smaller, and the fibrilla; given off from it pass to the tentacles. The fibrilla? of the sense nerves may be derived from both rings. The marginal bodies have long been recognised as sense organs, and are either eye spots (ocelli) or auditory vesicles ; hence the Hydromedusce may be divided into two groups, the Ocellata or Vesiculata. In the Vesiculata the auditory vesicles are situated at the edge of the under side of the umbrella, and contain one or more concretions (otoliths} which are formed in the interior of cells. Peculiar sense cells surround each vesicle-like cell containing a concretion. The curved hairs of these sense cells (auditory hairs) are in contact with the con- cretion vesicle. A nerve fibrilla enters the basis of the auditory cells (fig. 183). The audi- tory organs of the Tra- c!/i/medusce are placed above the velum, and are in con- nection with the upper nerve ring ; they have the form of small projecting tentacles furnished with otoliths and auditory hairs. The tentacle may either project freely on the surface (Trackynema), or, as in Geryonia, it may be placed in a vesicle (fig. 184) which lies in the gelatinous substance of the disc and close to the edge of the latter. Separate sexes are almost invariably the rule, but it is rare to find that the colonies are dioecious, , that male and female medusoids are developed in different colonies (Tubularia). Gemma- tion has occasionally been observed among the Medusw (Saraia 2>rollfera) and division (titomobracltium mirabile). The larvae of Cuninu, which are parasitic on the Geryonidie, may also there give rise to a cluster of Duels. FIG. 183.—Sense organ on the nerve-ring and circular vessel of Octorchi
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