138 CCELENTERA. 



a fixed asexual hydroid colony, the planula settles down, 

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

 hydroid. 



In many Hydrozoa, as has been already noticed, the 

 sexual persons are not set free, but remain as buds attached 

 to the parent hydroid. 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. 67). 



Third Type of Ccelentera. — The common Jelly-fish 

 — ■ Aurelia aurita. Class Scyphozoa. Sub - Class 

 Scyphomedusse or Acraspeda. 



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 

 muscular flap or velarium. Conspicuously bright are the 

 four reproductive organs, which lie towards the under sur- 

 face. 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. 59). 



The three layers. — The ectoderm which covers the 

 external surface bears stinging cells, sensory and nerve cells, 

 and muscle cells. According to some, the ectoderm lines part 

 of the mouth-tube or manubrium. The endoderm lines the 



