152 THE INVERTEBRATA 



are said to be perradiaL The mesenteries are vertical walls projecting 

 from the body wall and composed of endoderm with an internal 

 layer of mesogloea. They have a free edge centrally, while on each 

 side a vertical series oi gastric filaments project into the enteron, and a 

 parallel series of gonads stand nearer the body wall. The perradial 

 chambers do not quite extend to the edge of the bell : a circular canal is 

 cut off from the rest of the enteron. Also in the interradial position 

 and penetrating the whole length of the mesentery is an ectodermal 

 invagination, the subumhral pit. 



The Stauromedusae only exist as individuals of this structural type, 

 superficially more like a polyp than a medusa, but usually supposed to 

 be a medusa, and the eg'g develops into an individual exactly re- 

 sembling the parent. 



The vast majority of the Scyphomedusae belong to the subdivision 

 Discomedusae, which includes our type Aurelia aurita (Fig. 128), the 

 commonest British jellyfish, but one whose distribution is world 

 wide. 



It has a similar external appearance to that of Obelia^ save for the 

 difference in size, the margin of the bell being surrounded by very 

 numerous short tentacles. The manubrium is well developed and the 

 corners of the mouth are drawn out into four long frilled lips along 

 the inside of which are ciliated grooves leading into the gullet. The 

 gullet is very short and opens into the endodermal stomach. This is 

 produced into four interradial pouches in the lining of which the 

 genital organs develop as pink horseshoe-shaped bodies. Parallel to 

 the internal border of the gonads there is a line of gastric filaments 

 which project freely into the lumen of the pouch. The endodermal 

 cells of which they are composed contain batteries of thread cells 

 which kill any living prey taken into the stomach. The gastric pouches 

 of Aurelia occupy the position of the mesenteries of Lucernaria, and 

 the suhgenital pits occurring underneath the gonads and lined by ecto- 

 derm correspond to the subumbral pits of the simpler form. The 

 broad perradial pouches in Lucernaria have disappeared owing to the 

 great growths of the mesogloea and the restriction of the gastric 

 cavity to a central position. There is, however, an extensive canal 

 system running from the gastric cavity to the circular canal which is 

 all that represents the former extension of the gastric cavity. It con- 

 sists of eight branched and eight unbranched canals: four of the 

 branched canals are interradial and four perradial: the eight alter- 

 nating unbranched canals are called adradial. 



In this elaborate "vascular" system there is a circulation of fluid 

 produced by the cilia of the lining epithelium working in definite 

 directions (Fig. 129). The water drawn in by the mouth passes first into 

 the gastric cavity and then the gastric pouches ; thence by the adradial 



