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. 



4. Radially arranged connecting canals between the central 



stomach and the circumferential 

 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 



Fl G.65.-Eucope campanula, partly after Medm ? * 4 ln number, and 

 Haeckel A, From the surface. B, Section in the ar 6 placed CrOSS-WlSC. Ihe 

 direction a-b-c in Fig. A. a-b, Perradius ; 6-c, ad- radii in which they lie are 



-M ji ,j" /T?' CK. A i\ 

 Called PSrradll (Fig. 65, A, a-b). 



In Order to define the position 

 O f 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. 



radius ; t. tentacle ; sb, marginal vesicle ; g. gonades ; 

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

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

 umbrella; ga, jelly; ty, tentacular vessel ; b-6, main 



