, THE INVERTEBRATA 



I opens into a spacious gastric cavity which is divided by four 

 |ns, the interradial mesenteries^ into four broad chambers which 

 to be perradial. The mesenteries are vertical walls projecting 

 frpnii tke body wall and composed of endoderm with an internal layer 

 ofimaiogloea. They have a free edge centrally, while on each side a 

 vdrtlcal series of 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 subumbral pit. 



The Stauromedusae only exist as individuals of this structural 

 type, superficially more like a polyp than a medusa, but usually sup- 

 posed to be a medusa, and the egg develops into an individual exactly 

 resembling the parent. 



The vast majority of the Scyphomedusae belong to the subdivision 

 Discomedusae, which includes our type Aurelia aurita (Fig. 131), 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 subgenital 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 

 consists 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 



