PHYSIOGRAPHY OF THE SEA-SHORE 39 



and their action has been well reviewed by Jehu (191 8). The 

 bivalve Pholas operates at extreme low water, each generation 

 cutting a series of auger holes 2-4 inches deep, the weakened 

 intermediate portions being readily broken down. Sponges of 

 the genus Cliona (C celata being the most important species) 

 attack shells and calcareous rocks and riddle the surface with 

 small holes, which intercommunicate below. As to how the 

 boring is accomplished, the theories of foot suction, rotation 

 of shell valves, hard particles in the foot, acid secretion, 

 spicula action, etc., have been either not proved, or dis- 

 proved ; probably, in the case of Cliona, certain special cells 

 are protruded through the epidermis and absorb calcareous 

 matter (as the osteoblasts of the vertebrates absorb bone). 



A small Annelid worm, Leucodora, destroys chalk, etc., 

 and also perforates sandstone, shale, and mica-schist, and at 

 St. Andrews occurs in soft blue shale. In some French 

 localities 250,000 to 300,000 per square metre are said to 

 have been enumerated. Of others, the tube-worm Sabella,thQ 

 Mollusc Saxicava, and the Echinoderm Strongyloce?itrotus 

 may be noted. The latter occupies thousands of depres- 

 sions at about low-water mark on the coasts of Clare, Cork, 

 and Donegal Bay (Ireland) ; in the nummulitic limestone of 

 Biarritz Fischer noted from 20 to 100 on the floor of troughs 

 of rock ; they also occur in the walls of pot-holes, and in 

 sandstone, granite, lava, etc. 



To sum up : diversity of rock surface and a sufficiency 

 of shelter, which modify wave action and erosive power, are 

 the most favourable factors influencing shores of all types, 

 from the biological standpoint. Uniformity of physical 

 conditions over considerable areas and distances leads, 

 however, to the most pronounced and well-marked (if more 

 limited) faunas. 



