IV 



PHYLUM CCELENTERATA 



appear through the transparent umbrella as coloured horseshoe- 

 shaped patches. Their precise position is seen by cutting away a 

 portion of the ex-umbrella so as to expose one of the gastric 

 pouches from above (Fig. 113, A). It is then seen that the 

 gonad (ffon) is a frill-like structure lying on the floor of the 

 pouch and bent in the form of a horse-shoe with its concavity 

 looking inwards, i.e. towards the mouth. Being developed from 



'0/t 



FIG. 114. Aiirelia aurita. A, side view, one-foil thof the umbrella cut away ; B. diagrammatic 

 vertical section, ectoderm dotted, endoderm striuted, mesoglcea black ; circ. c. circular canal ; 

 g.f. gastric filaments ; r/on. gonad ; rj. p. gastric pouch ; <7/. gullet ; h. hood ; ;'./. c. inter-radial 

 canal ; mrj. lp. marginal lappet ; mth. mouth ; or. a. oral arm ; s.y. p. sub-genital pit ; st. 

 stomach. 



the floor of the enteric cavity, the gonad is obviously an 

 endodermal structure : when mature, its products ova or sperms 

 are discharged into the stomach and pass out by the mouth. 

 Here, then, is an important difference from the Hydrozoa, in 

 which the generative products are usually ectodermal, and are 

 always discharged directly on the exterior. The sexes are lodged 

 in distinct individuals. 



