184 PHYLUM CCELENTEKA 



the sexual members are not set free, but remain as buds 

 attached to the parent. These fixed " gonophores " show 

 many stages of degeneration ; some, notably in the floating 

 colonies of Siphonophora, differ little structurally from true 

 medusoids, while others, as in Hydr actinia, are simply small 

 closed sacs enclosing the genital products (Fig. 84). 



Third Type of Ccelentera. — The common Jelly-fish 

 — Aurelia aurita. Class ScYPHOMEDUSi^ 



This Medusa is almost cosmopolitan, and in the summer 

 months occurs abundantly around the British coasts. It 

 swims by pulsating its disc, and also drifts along at rest 

 without any pulsations. They often occur in great shoals, 

 and hundreds may be seen stranded on a small area of flat 

 sandy beach. The glassy disc usually measures about four 

 inches in diameter, but may be twice as large. The jelly- 

 fish feeds on small animals, such as copepod crustaceans, 

 which are entangled and stung to death by the long lips. 



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- 93)- 



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 stomodaeum. 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 



froin the endoderm. The whole animal is very watery ; 



indeed, the solid parts amount to not more than 10 per 



