HYDROZOA. 49 



cesses of the lower portion of the central cavity, 

 immediately above the bases of the radiating 

 canals, and, being usually of some bright colour, 

 form a conspicuous cross shining through the 

 thickness of the disc. 



But in RJiizostoma, Cephea, and Cassiopeia, sl 

 different arrangement prevails, which is best de- 

 scribed in the words of Professor Huxley. 



"In the Rkizostomiike, a complex, tree-like 

 mass, whose branches, the ' stomatodendra,' end 

 in, and are covered with, minute polypites inter- 

 spersed with clavate tentacula, is suspended from 

 the middle of the umbrella in a very singular way. 

 The main trunks of the dependent polypiferous 

 tree, in fact, unite above into a thick, flat, quadrate 

 disc, the ' syndendrium,' which is suspended by 

 four stout pillars, the *dendrostyles,' one springing 

 from each angle, to four corresponding points on 

 the under surface of the umbrella, equidistant 

 from its centre. Under the middle of the um- 

 brella, therefore, there is a chamber whose floor 

 is formed by the quadrate disc, while its roof is 

 constituted by the under wall of the central cavity 

 of the umbrella, and its sides are open. The re- 

 productive elements are developed within radiat- 

 ing, folded diverticula of the roof of this genital 

 cavity " {fig. 7, a). 



This is, without doubt, the most complicated 

 structural product presented bj the class, and its 

 description forms a not inappropriate conclusion 

 to the preceding general survey of their organisa- 

 tion. 



The majority of Hydrozoa are dioecious, the 

 same hydrosoma not bearing both male and female 



E 



