STRUCTURE OF JELLY-FISH 



i8s 



cent, of the total weight. Yet some jelly-fish (species of 

 Rhopilema) are used as food in Japan ! 



Nervous system. — The nervous system consists — {a) of 

 a special area of nervous epithelium, associated with each of 

 the eight sense organs, and {h) of numerous much-elongated 

 bipolar ganglion cells lying beneath the epithelium on the 

 under surface of the disc. This condition should be con- 

 trasted with the double nerve-ring in Craspedote medu- 

 soids, but too much must not be made of the contrast, for a 

 nerve-ring is described in Cubo-medusae, one of the orders 

 of Acraspedote jelly-fish. In Aurelia the sense organs are 

 less diff^erentiated than in 

 many other jelly-fish. 

 Each of the eight organs, 

 protected in a marginal 

 niche, consists of a pig- 

 mented spot, a club-shaped 

 projection with numerous 

 calcareous " otoliths " in 

 its cells, and a couple of 

 apparently sensitive pits or 

 grooves. The sense organs 

 arise as modifications of 

 tentacles, and are often 

 called " tentaculocysts " 



or " rhopalia." The.r ^^ ^-ZllI^rRotrl''"'*'' 

 cavities are in tree com- 

 munication with branches 

 of the radial canals. 



Muscular system. — 

 Between the plexus of nerve cells and the sub-umbrellar 

 mesogloea there are cross-striped muscle fibres, each 

 of which has a large portion of non-contractile cell sub- 

 stance attached to it. They lie in ring-like bundles, and 

 by their contractions the medusa moves. Unstriped 

 muscle fibres are found about the tentacles and lips. 



Alimentary system. — The four corners of the mouth are 

 extended as four much-frilled lips, each with a ciliated 

 groove and stinging cells, and with an axis of mesogloea. 

 They exhibit considerable mobility. Their crumpled and 

 mobile bases surround and almost conceal the mouth. A 



Showing four genita pockets in centre, 

 much -branched radial canals, eight peri- 

 pheral niches for sense organs, and peri- 

 pheral tentacles. 



