INVERTEBRATE BOTTOM FAUNA 489 



commonest forms. Ophhtra ciliaris, too, is far more plentiful 

 in the Skagerrack, and the gasteropod, Nassa reticidata, occurs 

 in quantities in the littoral zone of the Skagerrack, but is 

 comparatively rare on the North Sea coast. 



I have noticed also a difference between the fauna which 

 patronises Laminaria hyperborea and the fauna associated with 

 the two other species oi Laminaria. It is only the first named 

 with its stiff thick stalks which is densely crowded with attached 

 forms, whereas the comparatively thin pliant stalks of the other 

 two are either entirely neglected or only made use of to an 

 inconsiderable extent, with the result that there are nearly 

 always far more individuals in the L. hyperborea belt than in 

 either of the other two laminaria communities. 



I have already stated that the natural conditions prevailing 

 on the different coasts affect the character of the fauna much 

 more in the littoral zone than at greater depths. Where, for 

 instance, there is nothing in the way of foundation for attached 

 forms, we must expect to find a fauna more suited to another 

 kind of environment. Thus on many North Sea coasts, where 

 the long shallow shores consist merely of sand, like the "vader" 

 of Schleswig and Holland, upon which the waves do not break 

 with any violence, there are immense stretches where practically 

 the sole inhabitants are the lug-worm [Arenicola), a tunnelling 

 amphipod [Corophium grossipes), and one or two other forms. 

 In such sandy stretches the fauna differs entirely from that 

 found along rocky coasts, and only occasionally do we get 

 attached forms where piles, stone quays, or other suitable 

 foundations happen to occur. The animal life differs again on 

 the sandy Danish coasts, which are unprotected by a line of 

 outer islands, and are therefore exposed to the full force of the 

 breakers, where the constant disturbance produced by the waves 

 upon the sandy bottom is distinctly unfavourable to plant and 

 animal life ; consequently the upper littoral zone on such coasts 

 rarely harbours many forms. On the other hand, at slightly 

 greater depths, and in fjords or similar enclosed areas, we get 

 the conditions requisite for the development of Zostera vege- 

 tation with its special fauna. We may see how much the 

 topography of the bottom affects the development of animal life 

 by studying the conditions on the Kattegat coast of Denmark ; 

 wherever reefs, overgrown by algae, occur amidst the eelgrass, 

 we may be certain of finding a fauna consisting of chitons, snails, 

 bryozoans, and hydroid polyps. 



The littoral fauna in the southern portion of the North Sea 



