152 PHYLUM CCELENTERA. 



a fixed asexual hyaroid colony, the planula settles down, 

 loses its cilia, buds out tentacles, and develops into a new 

 hydroid. 



In many hydroid colonies, as has been already noticed, 

 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 Hydractinia, are simply small 

 closed sacs enclosing the genital products (Fig. 87). 



Third Type ^CCELENTERA. The common Jelly-fish 

 AureJia aurita. Class SCYPHOMEDUS^E 



This Medusa is almost cosmopolitan, and in the summer 

 months occurs abundantly around the British coasts. It 

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

 without any pulsations. They often occur in great shoals, 

 and hunokeds 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. 73). 



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 



