INVERTEBRATE BOTTOM FAUNA 481 



of the year.^ Other sublittoral species again are plentiful every- 

 where throughout the whole sublittoral zone, but rarely descend 

 below its lower limit, so that we find at a depth of 100 to 200 

 metres a mixed fauna, consisting partly of forms that have here 

 reached their upper or lower limit of vertical distribution, and 

 partly of forms which find here the most favourable conditions 

 of life. The sublittoral zone accordingly ranks first in number 

 of species. 



The continental deep-sea zone for all practical purposes The 

 coincides with the deeper parts of the fjords, whereas out among deep"efz^one 

 the skerries, with their comparatively shallow water, we either 

 do not find it at all or else meet with it merely in very limited 

 areas. A feature of the fjords is their very great depth, usually 

 increasing as we proceed inwards, and in their deepest parts, so 

 far as the nature of the bottom and the physical character of 

 the water are concerned, we get what are practically Atlantic 

 conditions. 



In the fjords the greatest depth is met with along the 

 middle and in the innermost portions, and may be put on an 

 average at 400 to 800 metres." The sides of the fjords descend 

 in some places practically perpendicularly into deep water, in 

 other places forming more or less extensive submarine plateaus 

 and terraces. At various depths, especially in the seaward 

 portions, there are cross ridges, which frequently consist of hard 

 bottom. The material covering the floor in deep water is 

 almost invariably a soft, viscous, grayish clay or mud. It is 

 the animal life existing upon and in this mud which I shall now 

 describe. 



The mud-fauna of the deeper parts of the fjords resembles the 

 sand-fauna in the littoral zone, inasmuch as it consists mainly of 

 burrowing forms, or at any rate of forms which to some extent 

 burrow into the mud to obtain their nourishment. When we 

 sift the mud brought up by the trawl or dredge, we obtain a 

 number of curious little bodies (round, star-shaped, rod-like, 

 conical, etc.), composed of sand or particles of mud. These 

 creatures are rhizopods (foraminifera). By putting out 

 extremely fine thread-like prolongations of their protoplasm 

 through one or more openings in their covering, they attract to 

 themselves small organic particles in the mud which furnish 



^ Thus Helland-Hansen has fixed the summer limit along the coasts at 75 metres, and the 

 winter limit at 150 metres. 



'■^ In some fjords, such as the Sogne and Hardanger fjords, the depth is in places 1000 metres 

 or more. 



2 I 



