JELLYFISHES 



105 



of muscle fibers is a diffuse and branching network of 

 nerve cells. 



A clear idea of the mutual relations of the epithelium, 

 nerve-net, and muscle layer in such a jellyfish as Rhizo- 

 stoma can be obtained from a section across its sphincter 

 at right angles to the contained muscle fibers (Fig. 30). 

 Such a section would of course 

 have a radial position in the 

 jellyfish as a whole. On its 

 outer face is the layer of epi- 

 thelial cells that covers the sub- 

 umbrellar surface of the jelly- 

 fish. Next within is the nerve- 

 net with its contained nerve 

 cells and branching fibers, 

 some of which can be traced in 

 among the epithelial cells and 

 others among the musole fibers. 

 Deepest of all is the band of 

 muscle fibers of the sphincter 

 muscle cut at right angles and 

 penetrated more or less by 

 nerve fibers from the nerve- 

 net. This sequence of tissues from the outer surface 

 inward is the same as that met with in sea-anemones, 

 in which the epithelium is most superficial, the nerve-net 

 next, and the muscles deepest. Although the conditions 

 thus far described are chiefly those in the acraspedote 

 medusae, they reproduce in essentials the structure of the 

 craspedote forms except in two particulars. In the cras- 

 pedote jellyfishes there are no well-defined marginal 

 bodies, but the sense organs of the edge of the bell are 

 more diffuse in their arrangement. Further, the cras- 



Fia. 30. Section at right angles to 

 the sphincter of the bell of Rhizostoma; 

 the subumbrellar surface is uppermost; 

 e, epithelium of the subumbrellar sur- 

 face; n, nervous layer; m, muscle layer. 

 (After Bethe, 1903.) 



