L'60 



CCELENTEBATA. 



acteristic of the group. The striped muscles of the sub-umbrella 

 are strongly developed to correspond with the great size of the body ; 

 and beneath them the supporting lamella is usually thrown into a 

 number of closely aggregated circular folds, thus causing a consider- 

 able increase in the surface on which the muscular epithelium with 

 its circularly arranged fibres are placed. 



The generative organs have the form of horse-shoe shaped frills 

 which project into four widely open cavities in the sub-umbrella, 

 the sub-genital pits. These cavities are not developed in some ex- 

 ceptional cases (Nausitfwe, Discomedusa). The geiminal epithelium, 



FIG. 20Q.Aurelia aurita, seen from the oral surface. MA, The four oral arms with the 

 mouth in the centre ; Gk, The genital frills ; G H, Openings of the sub-genital cavities ; 

 Rk, Marginal bodies ; EG, Radial vessels ; T, Tentacles on the margin of the disc. 



which is always embedded in the gelatinous substance, is covered 

 with an endodermal layer, and is probably itself ari endodermal 

 product (fig. 200). Development takes place by alternation of gene- 

 rations. In rare cases (Pelagia) the development is simplified, and 

 the larva passes directly into the Ephyra, missing out the attached 

 Scyphistoma and the Strobila stage (Krohn). 



1. Semceostomece. Discophora with large central mouth sur- 

 rounded by four large often multi-lobed oral arms. The form of the 



