STRUCTURE OF AURELIA. 143 



into its radiating canals, and is ciliated throughout. The 

 mesoglcea is a gelatinous coagulation containing wandering 

 amoeboid cells from the endoderm. The whole animal is very 

 watery, indeed the solid parts amount to not more than ten 

 per cent, of the total weight. 



Nervous System. 



The nervous system consists (a) of a special area of 

 nervous epithelium, associated with each of the eight sense 

 organs, and (b) of numerous much elongated bipolar 

 ganglion cells lying beneath the epithelium on the under 

 surface of the disc. This condition should be contrasted 

 with that in Craspedote medusoids, but too much must not 

 be made of the contrast, for a nerve ring is described in 

 Cubomedusae, one of the orders of Acraspedote jelly fish. 

 In Aurelia, the sense organs are less differentiated than in 

 many other jelly fish. Each of the eight organs, protected 

 in a marginal niche, consists of a pigmented spot, a club- 

 shaped projection with numerous calcareous " otoliths " in 

 its cells, and a couple of apparently sensitive pits or 

 grooves. We are not warranted in calling these organs 

 " optic," " auditory," and " olfactory," in Aurelia at any 

 rate. The sense organs arise as modifications of tentacles, 

 and are often called " tentaculocysts " or " rhopalia." Their 

 cavities are in free communication with branches of the 

 radial canals. 



Muscular System. 



Between the plexus of nerve cells and the sub-umbrellar 

 mesoglcea, there are cross-striped muscle fibres, each of 

 which has a large portion of non-contractile cell substance 

 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 " arms," each with a ciliated groove and stinging 

 cells, and with an axis of mesoglcea. They exhibit con- 

 siderable mobility. Their crumpled and mobile bases 

 surround and almost conceal the mouth. A short tube, the 

 " manubrium " or gullet, connects the mouth with the 



