PLANT AND ANIMAL ASSOCIATIONS 49 



of rocky shores can be conveniently studied in groups 

 according to the positions occupied. Farran (19 14) 

 arranges those of Blacksod Bay, Co. Mayo, as follows : 

 A. Animals exposed to the air on stones, rock faces, or 

 weeds ; B. Animals found under stones ; C. Animals living 

 in crevices of the rocks ; D. Those living in Laminaria roots. 

 Group A. includes such species as the barnacle {Balanus 

 halanoides), the periwinkle (Littorina), dog-whelk (Purpura) ^ 

 Monodonta crassa, Gibbula umbilicata, and limpet {Patella). 

 Low down but exposed come the sponges Halichondria 

 panicea and Hymeniacidon caruncula. On overhanging 

 rock faces, and not so Hable to desiccation, are the " purse 

 sponge " {Sycon compressum) and " dead men's fingers " 

 (Alcyonium digitatum). Still lower are Spongelia fragiltSy 

 Spirorhis, the bivalves Pecten varius and Mytilus edulis, and 

 the Tunicate Phallusia mentula. On Fucoid fronds : 

 Helcion pelluctdum, Lacuna divartcata, L. parva, L. pallidula, 

 and again Spirorbis. 



B. Under stones, on sand, and near high- water mark 

 are the Amphipods Orchestia, Gammarus, Talitrus, and the 

 Isopod Ligia, with beetles, mites, and springtails. Lower are 

 Lineus ruber and other Nemerteans, the Polychaet Ctrratulus 

 cirratus and numerous Oligochaets, and the common shore 

 crab Carcinus. Nearer low-water mark stones in muddy 

 sand or gravel shelter sand-burrowers, with Polychaets pre- 

 dominant ; Sthenelais boa, Nereis cultrifera, Lanice conchilega^ 

 etc., also Tubifex benedeni and the Crustacean Nebalia bipes. 

 These communities are small, however, compared with 

 those near low-water mark under loose stones partly sub- 

 merged, or at extreme low-water mark. The lists include 

 20 Porifera, 2 Coelenterata, 9 Echinodermata, 5 Turbellaria, 

 9 Nemertea, 39 Polychasta, 13 Crustacea, and 35 Mollusca. 



C. In crevices of the schist, which usually contain a thin 

 layer of mud, are many individuals, chiefly representatives 

 of classes of worms : Polychaeta, Nemertea, and Gephyrea 

 (30 species in all), with the rock-boring bivalve Saxicava, and 

 Kellia. 



D. Lammaria roots of three species digitata, saccharina, 



£ 



