8 LIFE BY THE SEASHORE. 



protrudes. Touch these stars, and they instantly disappear, 

 ejecting a feeble jet of water as they do so. If by means oi 

 hammer and chisel you investigate the rock, you will find 

 that the stars are the breathing-tubes or siphons of a little 

 bivalved Mollusc, called Saxicava, on account of its rock- 

 boring habits. The little creature remains permanently 

 within its rocky burrows. When the rock is covered by 

 water it protrudes its red tubes, and through them both 

 feeds and breathes ; when the tide ebbs, or enemies threaten, 

 it withdraws the tubes, and is safe. Another, and in some 

 ways an even more interesting rock- 

 boring Mollusc, is Pholas, of which one 

 species is common in the soft fissile rock 

 called shale by geologists. While walking 

 over stretches of shale you may often 

 notice that it is perforated by numerous 

 round holes. When the rock is covered 

 by water these holes are filled by a brown 

 fringe, with some superficial resemblance to 

 a sea-anemone. At a touch the fringes 

 vanish like a shot. The shale is very soft, 

 and can be readily pulled up in great 

 blocks, when you will find that the holes 

 are the openings of the burrows of Pholas, 

 FIG. \.-Phoias crispata, a . wllite Bivalve, with a shell which gapes 

 from under surface, to widely, and is beautifully toothed and 

 tSSv!SKSi sculptured. In the Firth of Forth, where 

 /, foot ; s, siphon. beds of shale are abundant, the rock is 

 often simply riddled by Pholas burrows. Other species of 

 the genus burrow in hard rocks, and are then much less 

 easy to extricate. 



Far more numerous than the rock-borers are the burrowers 

 in sand, which if it does not form so secure a resting-place 

 as the solid rock is one more easily obtained, and is taken 

 advantage of by many animals. Objection may be taken to 

 the word "many," in view of the fact that children often dig 

 in the sand for hour after hour, and yet rarely come upon a 

 living creature. But the explanation is simple. Animals 

 which burrow in sand almost invariably live on sand ; they 

 can therefore only live in sand which is impregnated with 

 organic particles. Such sand occurs usually in the vicinity 



