TYPES OF CfELEtfTERA AURELIA AURITA. 



External appearance. --The animal consists of a gela- 

 tinous disc, slightly convex on its upper (ex-umbrellar) 

 surface, and bearing on the centre of the other (sub- 

 umbrellar) surface a four-cornered mouth, with four long 

 much-frilled lips. The circumference of the disc is fringed 

 by numerous short hollow tentacles, by little lappets, and 

 by a continuation of the sub-umbrella forming a delicate 

 flap or velarium. Conspicuously bright are the four re- 

 productive organs, which lie towards the under surface. 

 Nor is it difficult to see the numerous canals which 

 radiate from the central stomach across the disc, the eight 

 marginal sense organs, and the muscle strands on the lower 

 surface (Fig. 67). 



The three layers. - The ectoderm which covers the 

 external surface bears stinging cells, sensory and nerve cells, 

 and muscle cells. The ectoderm seems also to be invagin- 

 ated to form the gullet or stomod?eum. The endoderm 

 lines the digestive cavity, is continued out 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. Yet some jelly-fish (species of 

 Rhopalema) 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 (b) 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 medusoids, 

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

 ring is described in Cubomedusas, one of the orders of 

 Acraspedote jelly-fish. In Aurclia 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. The sense organs arise as modi- 

 fications of tentacles, and are often called " tentaculocysts ' ; 

 or "rhopalia." Their cavities are in free communication 

 with branches of the radial canals. 



