142 CCELENTERA. 



In many Hydrozoa, as has been already noticed, the 

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

 to the parent hydroid. These fixed " gonophores " show 

 many stages of degeneration ; some, notably in the floating 

 colonies of Siphonophorae, differ little structurally from true 

 medusoids, while others, as in Hydractinia^ are simply small 

 closed sacs enclosing the genital products. (Fig. 49.) 



Third Type of CCELENTERA. The common Jelly fish 

 Aurelia aurita. Class SCYPHOZOA. Sub-Class Scypho- 

 medusae or Acraspeda. 



This medusa is almost cosmopolitan, and in the summer 

 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 may be twice as large. The jelly fish 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 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 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 



