10 LIFE BY THE SEASHORE. 



creep under stones and into chinks and fissures of the rocks 

 as the tide ebbs, to seek protection both from sun and wind 

 and from the keen eyes of the birds. It is in search of 

 these that the shore-hunter diligently turns stones and 

 creeps under overhanging rocks, where the weeds drip and 

 the sea-squirts eject their tiny jets of water. Most of these 

 are protected from the force of the waves by the fact that 

 they are attached and sedentary, or by the shape of their 

 bodies which makes it easy for them to lurk in crevices out 

 of harm's way. 



There are still other ways in which shore animals may 

 escape the dangers associated with the ebb and the flow of 

 the tide. Thus they may avoid these dangers by their own 

 activity, following the water as it ebbs seaward, and return- 

 ing with it when it once more flows landward. These are 

 best represented on the shore by some of the Crustacea 

 such as prawns, some shrimps, various kinds of lobsters 

 and by certain fishes. In both cases, however, the power 

 of active swimming is comparatively rare in truly littoral 

 forms, probably because the strong shore currents make it a 

 danger rather than an advantage. Thus, of the shore fishes, 

 the blenny (Biennius pJiolis) remains lurking under stones 

 often quite uncovered by water, the sand-eels (Ammodytes 

 tobianus) often bury themselves in the sand, where stickle- 

 backs (G aster osteus) are also at times to be found. Among 

 the shore Crustacea, as we shall afterwards see, there is 

 evidence that in the higher forms the power of swimming 

 has been gradually lost, and the animals have been adapted 

 for life at the bottom and on the tidal rocks. This has been 

 accompanied in the crabs by a modification of the dorsal 

 shield or carapace, which has for its object the protection 

 of the gills from the risk of drying up. So carefully are 

 these protected in many crabs that the animals can live for 

 a long period out of water. In some cases, indeed, as in the 

 common shore crab, an exposure to air during a portion of 

 the day seems actually beneficial. While very many Crus- 

 tacea and a few fishes are thus rather to be reckoned among 

 the forms which lurk passively in hiding when the tide 

 ebbs, there are still a considerable number who are active 

 swimmers, and constitute the "floating population" of 

 the rocks. The capture of these can only be hoped for 



