172 



ZOOLOGY 



endodermal structure : when mature, its products — ova or sperms 

 — are discharged into the stomach and pass out hy 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. 



Lying parallel with the inner or concave border of each gonad 

 is a row of delicate filaments (Fig. 126, 127, g.f), formed of endoderm 

 with a core of mesogloea 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 



Fig. 128. — Aurelia aurita. A, small portion of edge of umbrella, showing tlie relations of the 

 tentaculocj'st ; B, vertical section of the same region (diagrammatic), h, hbodj /, lithite ; 

 mg. /j), marginal Lappet ; oc',^ ocellus ; olf, i, olf. 2, olfactory pits. (Altered from tankester.) 



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 medusae. 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 

 position of the marginal notches and sense-organs. About the 

 base of each of the latter are special groups of nerve-cells. A 

 slight ring-like thickening of the plexus extends round the margin 

 in the neighbourhood of the marginal canal. 



The sense organs (Fig. 128) are lodged in the marginal 

 notches in close I'elation with the nerve-patches : like the latter, 

 therefore, four of them are per-radial and four inter-radial. Each 

 consists of a peculiar form of sense-club or tcntaculocyst, containing 



