122 



THE ANATOMY OF IXVERTEBRATED ANIMALS. 



way ; or by the transverse fission of the hydriform product 

 of the development of the impregnated ovum. 



In some of these (e. g., Carraarina^ Polyxenia^ ^ginopsis, 

 Trachynema)^ the disk is similar to the nectocalyx of one of 

 the medusoids of the Hydrophora ; and, like it, is provided 

 with a velum. But in the rest {Lucernaria^ and the Stega- 

 nophthalmata) the disk is either devoid of a velum, or pos- 

 sesses only a rudiment of that structure, and is termed an 

 umbrella. The edges of the umbrella are divided into lobes 

 by marginal notches in which the lithocysts are lodged. 

 Moreover, in these, the mineral particles of the lithoc3^sts are 

 numerous, and not inclosed in seperate sacs. The lithocysts 

 are often covered by hood-like processes of the umbrella, 

 whence they have been termed " covered-eyed " or Stega- 

 noplitlialmata. 



Lucefnaria is fixed by the aboral side of its umbrella 

 (Fig. 13, 5), by means of a longer or shorter peduncle. The 

 umbrella is divided into eight lobes, at the extremities of 

 each of which there is a group of short tentacles. The 



Fig. 18.-I. Aurelia aurita : X, the prolonged angles of the mouth ; G. ireiiital cham- 

 II.Slc^r'^iefofTsegment of the disk, to show the arrangement of the radiati^^^ 

 canals: the aperture of a genital chamber and the j.laited .^f.''it^J^™^™r^^^ 

 showing through its ventral wall ; and a lithocyst witli its protective hood (jn). 



hydranth stands up in the centre of the umbrella, and its: 

 cavity communicates with a central chamber, whence four 

 wide chambers pass into the lobes. These chambers are 

 •separated by septa, the free central edges of which are beset 

 with slender tentacles. The reproductive organs are double 



