THE HYDROMEDUSAE 



49 



The continuation of the tentacles along the exumbral surface into a 

 " root," which occurs in both of the freshwater genera, although not quite 

 of the character known in Trachomedusae and Narcomedusae, is never- 

 theless suggestive, and the presence of some- 

 thing corresponding to perouia points in the 

 same direction. 



As regards the character of the sense organs, 

 which are of great diagnostic value throughout 

 the class, Limnocodium and Limnocnida agree 

 with each other in possessing similar organs, of 

 a type not known in any other Hydromedusan. 

 These organs (Fig. 54) resemble tentaculocysts 

 in possessing an endodermal axis, but differ 

 from them in position and in not secreting 

 an otolith ; they lie each in a closed vesicle 

 lined by ectoderm and surrounded by meso- 

 gloea. The vesicle in Limnocnida is situated 

 in the exumbral nettle-ring at the base of the 

 velum, and in Limnocodium, in the base of 

 the velum itself, into which latter it is con- 

 tinued as a long canal. It may perhaps be 

 eventually shown that a modification of cordyli 

 in one direction has resulted in the produc- 

 tion of these organs, in another in the forma- 

 tion of tentaculocysts. 



Microhydra and Protohydra. 



These two forms of uncertain position 

 need only brief mention. 

 They agree with the hy- 

 droid of Limnocodium in 

 the absence of tentacles. 

 While Microhydra (Ryder, 

 24) reproduces by lateral 



FIG. 55&. 



Protohydra Leuckartii, expanded, 

 contracted, and in strobilation 

 (after Greef). 



gemmation, Protohydra (Greef, 24) 

 undergoes a process of transverse 

 strobilation. 



Tetraplatia (Tetraptermi) volitans. 



Yia. 55c. 



Tetraplatia volitans (after Viguier). 



This remarkable organism 

 (Viguier, 47) of marine habitat has been recorded by four observers only. 



