INVERTEBRATE BOTTOM FAUNA 469 



parent snail [Rissoa), which may often be found in enormous 

 quantities ; often also there are great numbers of another snail 

 (Akei^a biillatd), and in the mud, even where there is no zostera 

 vegetation, we frequently find species of Philine. A species 

 of attached ascidian {Ciona intestmalis, see Fig. 332), which, 

 however, is also found on laminaria, especially when growing 

 in sheltered or rather deep places, is one of the most prominent 

 animal forms of the eelgrass. Hydroids and synascidians are 

 also occasionally met with. Swim- 

 ming amongst the blades of the -^ ;, (?»',** 

 eelgrass we further find various crus- 

 taceans, of which two species of 

 prawns [Pandalics animlicornis and 

 Palcumoii) are the most noticeable. 

 They are not limited to the eelgrass, 

 however, but occur also in places 

 where zostera does not grow. The 

 list of forms to be found here is 

 far from exhausted, for I have men- 

 tioned only some of the chief ones. 

 The zostera belt is not of so much 

 importance along the Atlantic and 

 North Sea coasts of Scandinavia, as 

 it covers a very limited area in com- 

 parison with the other subdivisions of 

 the littoral zone, and it is negligible 

 indeed, when compared with the im- 

 mense tracts in the Kattegat which 

 are literally overgrown with this plant. ^--^^i 



Such in general is a picture of the 

 fauna to be found in the algat and ^. ^.^^'•^^";. , 



111 Ciona mtestinalis, L,. 



zostera vegetation of the strand-belt; (After Aider and Hancock. ) 

 though it must be understood that 



when speaking of this fauna as associated with the plants I 

 do not imply that these animal -forms can exist only upon 

 them. This is only exceptionally the case. The relation- 

 ship between them depends on the fact that, as a rule, the 

 algse afford an excellent foundation for the attached forms, 

 which find favourable conditions of nourishment wherever 

 the alg^e flourish. For we must remember that these attached 

 forms are obliged to obtain their nourishment from such 

 organisms as chance to come within their reach, and since 

 currents and waves furnish the necessary assistance, we 



