568 



DEPTHS OF THE OCEAN 



(4) An abyssal layer (1500 to 5000 metres), in which the large Challengeridse 

 {Frotocystis fiaresi, P. thomsoni) are found. 



The multicellular animals are all represented among the pelagic 

 forms, from the medusae to the fishes. 



Commencing with the Coelenterates we may mention the Medusae, 

 the Siphonophores, the Ctenophores, and the larval Actiniae. 

 Medusre. The Medusae are generally bell-shaped or globular, with a more or 



less transparent jelly-like body. On the edge of the bell some forms 

 have a band-shaped fold or moulding (" craspedon "), and accordingly 

 the medusae are divided into two main groups : Craspedota with a 

 craspedon, and Acraspeda without a craspedon. 



The Craspedota comprise four groups : Anthomedusae, Leptomedusae, 



Fig. 395. 



Tiiscaretta globosa (Borgert), subsp. chiini, Haecker (about "^). 



(From Haecker. ) 



Trachymedusae, and Narcomedusae, of which the first two are mero- 

 pelagic and the last two holopelagic. The meropelagic forms pass 

 through an " alternation of generations," i.e. the eggs produced by the 

 medusae develop into larvae which attach themselves to the bottom and 

 grow into hydroid polyps or zoophytes ; by " budding " the zoophytes 

 produce small medusae, which lead a swimming pelagic life and produce 

 eggs. Fig. 396 shows a colony of hydroids with different stages of 

 medusae developing, and Fig. 397 shows one of the medusae just after 

 leaving the colony. The Craspedota are therefore termed hydroid 

 medusae or hydromedusae, although they include two groups with no 

 alternation of generation and no bottom stages, which are supposed 

 to be descended from neritic forms. The hydromedusae having an 

 alternation of generations are represented by a vast number of species in 

 the surface waters off all coasts where the temperature is not too low. 

 They do not occur far from land nor in deep water. Their pelagic life 



