INVERTEBRATE BOTTOM FAUNA 559 



mspidata, far south in the Norwegian depression, is probably 

 also due to the same cause, as they have most likely been 

 carried there by one of these blended currents and have 

 managed to adapt themselves to more boreal conditions. 

 That larvae may be transported in quantities to areas where 

 they are unable to develop was proved during the autumn 

 of 1903, when the fjords near Bergen were found to be full 

 of Actinotrochae. (larvae of P/ioronis, a form related to the 

 bryozoans, which occurs in the south parts of the North Sea 

 and other southern waters), but in the following year repeated 

 dredgings failed to reveal a single full-grown animal either 

 there or anywhere else on the coast of Norway, 



Currents also carry nourishment to the bottom-animals and 

 sweep away the finer particles of mud and other soft substances, 

 leaving, in sounds especially, nothing but the bare rock, or 

 perhaps a slight covering of coarse sand and shells. This 

 enables attached forms to thrive, since the current prevents 

 their being buried, and at the same time supplies them with 

 the nourishment they require. 



It is strange that a few boreal forms are peculiar to the 

 plateaus and do not enter the fjords, for the fjords and plateaus 

 have most of their forms in common. Whether it is due to 

 the fact that these peculiar forms develop at a time when the 

 Atlantic water, in which they probably live during both their 

 larval and full-grown stages, does not penetrate into the fjords, 

 or whether the physical conditions of the fjords are in some 

 way uncongenial, is unknown. Similarly we are unable to 

 explain why a number of boreal forms, which are widely dis- 

 tributed elsewhere, avoid the North Sea and Skagerrack, or 

 why some plateau-forms enter fjords north of Stat, like the 

 Trondhjem fjord, but are absent from fjords farther south. 



Distribution is of course very much affected by the character Effect of 

 of the sea-floor, since whole groups of animals are limited by their j°p°s,"s 

 structure or mode of living to some particular kind of bottom. 

 No doubt there are forms which appear to be equally at home 

 everywhere, but there are others again which are extremely 

 exacting in their requirements. This is especially the case with 

 burrowing forms, like the lancelet and numbers of mussels and 

 worms, and as a result we find, when conditions are favourable, 

 that extensive stretches of the bottom are occupied by one or 

 more of these. Some forms like sponges and corals, belonging 

 to groups most of whose members are attached and therefore 

 confined to rocky bottom, have developed special organs in the 



