572 



DEPTHS OF THE OCEAN 



The Acraspeda include the common jelly-fish, and excepting the 

 genus Pelagia they all go through an alternation of generations. The 

 free-swimming medusae produce eggs, the larvae fixing themselves to the 

 bottom and developing a zoophyte differing from the hydroid-zoophyte 

 in that it produces only one kind of bud ; the division is transverse, the 

 medusae not being'produced, as in the hydroida, by evagination (Fig. 404). 



In northern waters, for instance on the coast banks and in the fjords 

 of Scandinavia, the brown stinging jelly-fish Cyanea capillata and the 

 transparent jelly-fish Aurelia aurita are the most important species ; in 

 the southern part of the North Sea we find the blue Cyanea lamarckzana, 

 which annually drifts up to the Skagerrack and the west coast of Norway. 

 Distantly related to these is Rhizostoma octopus, which is similarly dis- 



Fig. 404. 

 Development of Aurelia aurita froni the o\um. The upper series shows the development of the 

 larva (planula) into Scyphostoma ; the lower series shows stages in the formation of small 

 medusas by division. (After Hatscheli, from Hertwig. ) 



tributed and occurs in Scandinavian waters as a visitor. The oceanic 

 genus Pelagia, as already indicated, has a direct development, and is 

 thus holopelagic (see Fig. 405). Of certain smaller groups resembling 

 the Trachymedusae, I may mention the gehera Atolla, PeripJiylla (Fig. 

 406), and Nausithoe, which are wholly oceanic forms widely distributed 

 mainly in deep water. 



During the cruises of the " Michael Sars " the distribution of medusae 

 in the Norwegian Sea and in Norwegian coast waters has for years 

 been investigated, and Damas, who is working up the material, has 

 found 64 species, of which 14 are new to science ; some are shallow- water 

 forms, and others belong to the deep fauna of the fjords. In 1900 I 

 noted the occurrence of Cyanea capillata all over the warm part of the 

 Norwegian Sea, and later on the drift of this form from the coasts has 

 been traced, as also the drift of Cyanea lamarckiana from the North Sea to 

 the west coast of Norway (see Chapter X.). 



