1 86 



PHYLUM CCELENTERA 



short gullet or *' manubrium " connects the mouth with the 

 digestive cavity in the centre of the disc. From this central 

 chamber sixteen gastro-vascular canals of approximately 

 equal calibre radiate to the circumference, where they open 

 into a circular canal, with which the hollow tentacles are 

 connected. Eight of the radial canals are straight, but the 

 other eight are branched, and thus in an adult Aurelia the 

 total number of canals is large. These canals are really due 

 to a partial obliteration of the gastric cavity by a fusion of 

 its ex-umbrellar and sub-umbrellar walls along definite 

 lines. They are all lined by ciliated endoderm. 



Where the gullet passes into the central digestive cavity, 



there are four strong 

 pillars of thickened sub- 

 umbrellar material. Be- 

 side these pillars, there 

 are four patches where 

 the sub-umbrellar sur- 

 face remains thin. These 

 are the gastro - genital 

 membranes, lined in- 

 ternally by germinal 

 epithelium (Fig. 94, R.). 

 To the inside of these 

 genital organs, within 

 the digestive cavity, are 

 four groups of mobile gastric filaments {g.f., Fig. 94), 

 which are very characteristic of jelly-fish. In appear- 

 ance these are very similar to the small tentacles of the 

 margin, and, like them, are hollow. They are covered with 

 endoderm — with ciliated, glandular, muscular, and stinging 

 cells. 



The body is mapped out into regions by the following convention : 

 The first tentacles to appear in the larva are four in number, and 

 correspond to the four angles of the mouth ; the radii on which they 

 appear are called " perradial," marked by the four lips. Half-way 

 between these, four " interradials " are then developed, marked by the 

 gonads and gastric filaments. Then eight " adradials " may follow, 

 between perradii and interradii, marked by the eight unbranched 

 radial canals. 



Reproductive system. — The sexes are separate. The 

 reproductive organs — ovaries or testes — consist of plaited 



Fig. 94. 



-Vertical section of Aurelia. — 

 After Claus. 



m.. Mouth ; st., stomach ; r.c, radial canal ; 

 R., reproductive organs ; g.f., gastric fila- 

 ments ; g.p., sub-genital cavity ; /., marginal 

 tentacle ; s., sense organ ; the shaded part is 

 mesogloea. 



