AURELIA. 



129 



Internal 

 Features. 



The mouth passes by a short oesophagus into a gastric 

 cavity which is produced into four pockets in the inter-radii. 

 Each pocket contains on its oral wall a horse- 

 shoe-shaped. ^(9;/^^, and near the middle a row 

 of gastric filame?its which assist in digestion. 

 The gastric cavity is continued outwards towards the edge 

 of the disc by numerous vascular ca?ials. The eight primary 

 branched canals are the four per-radial and the four inter- 

 radial. Between these there are the eight secondary un- 

 branched canals or ad-radials. All the canals open into a ring- 

 canal round the edge of the disc. The gastric cavity and the 

 canals are ciliated. They are derived from the coelenteron, 

 as in Obelia. In the inter-radii, immediately below the 

 gonads, are four suh-geiiital pits, each opening on the oral 

 surface by a pore. 



The mesogloea between the two layers is a thickened 

 jelly which in this case contains scattered cell-elements. 



Fig. 61. — Median Longitudinal Section through the Inter- 

 radial Plane of Aurelia. (Diagrammatic.) 



Gastric Filaments. 

 Sub-genital Pit. 1 Stomach, 



Gonad. 



Inter-radial Canal. 

 Ring-canal. 



Lappet. 

 Tentaculocyst, 



Oral Tentacle. 



Mouth. 



There is no nerve-ring, but there is a diffuse nerve-plexus 

 concentrated round the sense-organs or tentaculocysts. 

 These latter are complex and appear to unite the senses of 

 sight, hearing and smell in different parts. 



Development. — Atirelia is dioecious and the sexual elements 



are discharged by the mouth. A free-swimming planula larva (Chapter 



V. ) settles down on rocks or weeds and forms the hydra-ttiha, a minute 



hydra-like individual. It is a two-layered sac, with a mouth at the 



M. TO 



