74 



COMPARATIVE ANATOMY 



CHAP. 



and becomes the gelatinous dise of the Medusa, an elastic passive 

 organ for locomotion and support. The partial fusing of the inner 

 wall of the oral portion of the body with that of the aboral portion 

 considerably reduces the gastric cavity lying between them, which 

 originally spread throughout the whole extent of the disc. There 

 remain only : 



1. the cavity of the gastric peduncle into which the mouth leads. 



2. A central stomach above the oral peduncle. These two parts 

 form the main intestine. 



3. A peripheral "canal at the edge of the disc (the circumferential 

 canal) which is continued into the tentacles. 



canals between the central 

 stomach and the circumferential 



4. Eadially arranged 

 71 



canal. These canals become 

 nutritive gastro- canals, which 

 also serve the purpose of blood- 

 vessels (which are wanting) and 

 convey food from the central 

 stomach to the organs at the 

 disc's edge. These two portions 

 form the peripheral intestine. 

 In some Craspedote Medusce, 

 besides the radial canals and 

 the circumferential canal, the 

 endoderm from the centre to 

 the circumference persists, its 

 two layers being pressed to- 

 gether by the strongly de- 

 veloped oral or subumbrellar 

 and aboral or exumbrellar jelly, 

 thus forming the so - called 

 vascular lamellae or eatham- 

 mal plates, the layers of which, 

 separating in radial strips, form 

 the radial canals. 



The radial canals in simple 

 Medusce are 4 in number, and 

 are placed cross -wise. The 

 radii in which they lie are 

 called perradii (Fig. 65, A, Orb). 

 In order to define the position 

 of other tentacles, canals, and 

 organs, the radii exactly half 



way between the perradii have been called interradii. 



Half way between the 4 perradii and the 4 interradii lie the 8 



adradii (b-c) ; half way between the 8 former and the 8 latter lie the 



16 subradii. 



Fio. 66 . — Euoope campanulata, partly after 

 Haeckel A, From the surface. B, Section in the 

 direction a-b-c in Fig. A. c-6, Perradius ; 6-c, ad- 

 radius ; t, tentacle ; s6, marginal vesicle ; g, gonades ; 

 mr, gastric peduncle ; r, radial canals ; v, velum ; 

 ri, circumferential canal ; ex, exumbrella ; su, sub- 

 umbrella ; ga, jelly; tg, tentacular vessel; 6-6, main 

 axis. 



