INVERTEBRATE BOTTOM FAUNA 



497 



being extremely numerous in certain localities and absent in 

 others ; no doubt the currents at the bottom are responsible for 

 this, seeing that the depth and temperature are in themselves 

 entirely favourable. These enormous quantities of small 

 crustaceans must have an appreciable influence upon the shoals 

 of fishes, and in particular upon the young fishes, and this I have 

 been able to confirm by direct observation. On the northern 

 slope of the Dogger Bank we captured a number of young 

 whitings and flounders with the trawl at a depth of 38 metres 

 (temperature 10^ C), and their stomachs at first sight seemed 

 to contain only sand, but closer 

 investigation revealed small amphi- 

 pods (sand- hoppers) which thus 

 formed their principal nourishment, 

 the sand being swallowed simul- 

 taneously with them ; the stomachs 

 of the larger fishes generally con- 

 tained hermit crabs and swimming 

 crabs (Portunus). The caprellids 

 seemed to be especially associated 



with a bottom overgrown with 



Fig. 350. 

 Macrocliymm pomum, M. Sars. 



hydroids, and were found only 

 exceptionally where hydroids were 

 absent.^ 



The central portion of the 

 North Sea is poorly supplied with 

 pycnogonids (sea- spiders), there 

 being only one widely distributed 

 form [Pyaiogonum littorale), and 

 it was only found in deep water (80 to 100 metres) at low 

 temperatures (6°-7° C), where I sometimes found it, as 

 described by Sars, clinging to large sea-anemones [Urticina 

 crassicornis and Metridium diantkus), into the skin of which 

 it bores its proboscis for sucking ; a solitary specimen of 

 Nyniphon stromi was the only other pycnogonid found in deep 

 water. 



The ascidians (sea-squirts) are also poorly represented ; the 

 monascidians (simple sea-squirts) were not very conspicuous any- 

 where in the area examined, but we got large and well-developed 

 specimens of Ciona intestinalis in about 80 metres (tempera- 



1 The commonest is Caprella linearis (it seems difficult to discover any invariable difference 

 between this species and C. septenirionalis), but stray specimens occur o{ Proto pedata, mainly 

 found along the edge of the Norwegian depression, at a depth of about lOO metres, and one 

 individual o{ Protella phasma was captured at 77 metres, temperature 7.33° C. 



2 K 



