132 CCELENTERATA OR STINGING-ANIMALS. 



months occurs abundantly around the British coasts. We 

 often see hundreds gently swimming in shoals, and many are 

 washed shorewards and stranded on flat beaches. The 

 glassy disc usually measures about four inches in diameter, 

 but the maximum size is about twice as large. The jellyfish 

 feeds on small animals, such as crustaceans, which are 

 entangled and stung to death by the long lips. 



External Appearance. — The animal consists of a gelatinous 

 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 shor^ 

 hollow tentacles, by little lappets, and by a delicate muscular 

 flap or velarium. Conspicuous in bright red are the four 

 reproductive 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. 



The Three Layers. — The ectoderm which covers the 

 external surface bears stinging-cells, especially on the 

 tentacles, and to this layer belong sensory and nervous 

 cells aggregated at eight centres, also a plexus of ganglion- 

 cells beneath the skin on the under surface, and, finally, the 

 muscle-cells. According to some, the ectoderm lines part of 

 the mouth-tube or manubrium. The endoderm lines the 

 digestive cavity, is continued out into its radiating canals, 

 and is ciliated throughout. The mesoglcea is a gelatinous 

 coagulation practically without cells. 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 con- 

 trasted with that in Craspedote medusoids, but too much 

 must not be made of the contrast, for a nerve-ring is described 

 in Cubomedusse, one of the orders of Acraspedote jelly- 

 fish. In Aurelia, the sense-organs are less differentiated 

 than in many other jellyfish. Each of the eight organs, 

 protected in a marginal niche, consists of a pigmented spot, 



