IV 



PHYLUM CCELENTERATA 



171 



out. Some observations seem to show that the short tube 

 described above as a gullet and a pair of the gastric pouches 

 are lined, not by endoderm, but by an in-turned portion of the 

 ectoderm, but this matter cannot be considered as definitely 

 settled. 



It was mentioned above that in the free medusa the gonads 

 appear through the transparent umbrella as coloured horseshoe- 



B 



I. PC 



FIG. 127. Aurelia aurita. A, side view, oiie-fomth of the umbrella cut away ; B, diagrammatic 

 vertical section, ectoderm dotted, endoderm strmted, mesogloea black, circ. c. circular canal ; 

 g. f. gastric filaments ; cion. gonad ; ft. p. gastric pouch ; gul. gullet ; h. hood ; i.r. c. inter-radial 

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

 stomach. 



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. 126, A). It is then seen that the 

 gonad (gon) 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 

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



