INVERTEBRATE BOTTOM FAUNA 543 



800 metres, though it is quite possible that certain forms may 

 be met with at 600 metres. We have not yet acquired sufficient 

 knowledge of the factors regulating vertical distribution to be 

 able to divide the different parts of the Atlantic into vertical 

 zones, and a division of this kind will, I fancy, always be more 

 or less a matter of personal opinion. Besides, it is undeniable 

 that forms which properly belong to the abyssal fauna may find 

 their way to the lower parts of the archibenthal zone, and that 



Fig. 388. 

 Salenia hastigera, Agassiz. Reduced. "Michael Sars," 1910, Station 88, 3120 metres. 



archibenthal forms may go down into the abyssal region, while, 

 given favourable conditions, certain littoral and sub - littoral 

 forms may descend below the upper limits of the archibenthal 

 belt. In any case there is no clearly defined boundary between 

 archibenthal and abyssal areas. 



Real abyssal forms are, for instance, the following : Deima Abyssal forms. 

 fastosum (see Fig. 384), Peniagone wyvillii (see Fig. 385), 

 Oneirophmtta sp. (see Fig. 386), Freyella sexradiata (see Fig. 

 387), and Salenia hastigera (see Fig. 388), the last mentioned 

 being found, however, also in the archibenthal zone. 



I have already stated, with regard to the horizontal dis- 



