IV 



PHYLUM CCELENTERATA 



171 



It was mentioned above that in the free medusa the gonads 

 appear through the transparent umbrella as coloured horse-shoe- 

 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. 127, 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 

 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 located in the ectoderm, 

 and are always discharged directly on the exterior. <The sexes 

 are lodged in distinct individuals. 



oif.i 

 A 7 



FIG. 129. Aurclia aurita. A, small portion of jeclge of umbrella, showing the relations of 

 the tentaculocyst ; B, vertical section of the same region (diagrammatic), h, hood ; 

 /. lithite ; mg. Ip. marginal lappet ; oc. ocellus ; olf. 1, olf. 2, olfactory pits. (Altered 

 from Lankester.) 



Lying parallel with the inner or concave border of each gonad 

 is a row of delicate filaments (Figs. 127, 128, ^./),formed of endoderm 

 with a core of mesoglcea and abundantly supplied with stinging- 

 capsules. These are the gastric filaments or phacellae : their 

 function is to kill or paralyse the prey taken alive into the 

 stomach. No such endodermal tentacles are known in the 

 Hydrozoa. 



Muscular and Nervous Systems. The contractions of the 

 bell by which the animal is propelled through the water are 

 effected by means of a muscular zone round the edge of the sub- 

 umbrella. The nervous system is formed on a different plan 

 from that of the hydroid medusse. Extending over the sub- 

 umbrellar surface between the superficial epithelial layer of 

 ectoderm and the muscular layer is a plexus of simple nerve-fibres. 

 This presents radial thickenings, most strongly developed 

 externally in the per-radii and inter-radii, corresponding to the 



