REPRODUCTIVE SYSTEM OF AURELIA. 145 



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." Halfway between these, four " inter- 

 radials" are then developed. Then eight "adradials" may follow, 

 between perradii and interradii. 



Reproductive System. 



The sexes are separate. The reproductive organs — ovaries 

 or testes — consist of plaited ridges of germinal epithelium, 

 situated on the four patches already mentioned, within sacs 

 which are derived from and communicate with the floor of 

 the gastric cavity. They are of a reddish violet colour, and at 

 first of a horse-shoe shape, with the closed part of the curve 

 directed outwards. Afterwards the ridges become circular 

 and extend all round the walls of the sacs in which they lie. 



Fig. 43. — Vertical section of Aurelia. (After Claus.) 



;;/., Mouth; st.^ stomach; r.c, radial canal; A"., reproductive 

 organs; g/. gastric filaments; g.p.^ genital pocket: t., marginal 

 tentacle ; s.^ sense organ ; the shaded part is mesoglcea. 



But the sub-umbrellar surface is modified beneath each 

 genital sac in such a way that the sac comes to lie in a sub- 

 genital cavity communicating with the exterior (g-p-, Fig. 43). 

 The contractions of the umbrella produce a rhythmic move- 

 ment of the water which enters the sub-genital sacs, and 

 this constant renewal of the water suggests some respiratory 

 significance for the sacs. It must be understood that 

 the genital sacs containing the plaited ridges of germinal 

 epithelium communicate with the gastric cavity only, while 

 the sub-genital cavities containing water and enveloping the 

 genital sacs communicate with the exterior only. 



The ova and spermatozoa pass from the frills of germinal 

 epithelium into the sacs, and thence into the gastric cavity. 

 They find exit by the mouth, but joung embryos may be 



10 



