498 



DEPTHS OF THE OCEAN 



ture about 7' C), whereas along the Norwegian coasts it is 

 chiefly found in quite shallow water, where it attains its fullest 

 development. Asciciiella virginea and Styela loveni were 

 fairly widely distributed. A large globular compound ascidian 

 {Mac7'oclinum pomiun, see Fig. 350), although very local, was at 

 times very plentiful. 



The attached fauna, which, properly speaking, includes the 

 sea-squirts, is mainly represented by three groups : sponges, 

 hydroids, and bryozoans, the two last forming occasionally 

 regular little forests. On the northern slope of the Dogger 

 Bank (depth 'i^'^ metres, temperature 10° C.) there were con- 

 siderable quantities of large bush-like colonies of two species 

 of bryozoans (Fhtstra securi/rons, see Fig. 351, and Alcyonidium 



gelatinosiim\ which, ^'whFlustra 

 foliacea, are the most character- 

 istic of the North Sea bryo- 

 zoans ; they vary in relative 

 abundance, but on the Great 

 Fisher Bank Flustra foliacea 

 appears to be the predominant 

 form. Small bryozoans, some- 

 times occurring in large quan- 

 tities, are found growing on the 

 bigger species or on other 

 substances. 



Hydroids are distributed 

 over the whole area examined 

 wherever the bottom is suit- 

 able, especially where it is covered with empty shells or 

 stones. They sometimes form " communities," but are as 

 a rule scattered about here and there. Tuhilaria larynx is 

 occasionally met with in enormous quantities, and there 

 are sometimes "communities" of Tlmjaria thuja (see Fig. 

 352), Hydrallmannia falcata, Campamilaria longisshua, and 

 C. verhcillata. The species of Dicoryne and Hydractinia are 

 very often found on shells inhabited by hermit crabs. ^ The 

 hydroids in the central portion of the North Sea differ to a 

 certain extent from those found in the northern portion or 

 on the other plateaus. Tkujaria and Hydrallmannia are, how- 

 ever, common to both areas. 



Among coelenterates there are really only two forms, if we 



Fig. 351. 



Flustra sectirifrons , Pallas. 



' Dicoryne conferta, Hydractinia echinata ; other species commonly found in the North Sea 

 are Campanidaria johnstoni, Plumularia pinnata, Lafoea diimosa. 



