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 (Track ynema), 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, i.e., 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* (after O. and R. 

 Hertwig). ltl>, Sense organ ; 

 O, O', two otoliths; Hh, audi- 

 tory cilia ; Hz, auditory cells ; 

 Nc, upper nerve-ring ; Rg, cir- 

 cular vessel. (Type of the audi- 

 tory organ of the Veslculuta.) 



N 



FIG. 181. Auditory vesicle of Gery- 

 onia (Car marina), seen from the 

 surface (after O. and H. HerUviu). 

 N and N', The auditory nerves ; 

 Of, otolith ; .ff;r, axiditory cells ; 

 //A, auditory cilia (type of the 

 auditory organ of the Track y- 

 meduste) . 



