THE SEA SHORE 39 



on the surface. It is extremely abundant in suitable 

 localities, such as the shore round Holy Island where a 

 population of 82,000 per acre has been estimated. An 

 animal which might be easily taken for a worm is the reddish 

 Synapta, also found in sand, which is really a relative of the 

 sea cucumbers. Allied animals — in structure, not appear- 

 ance — are the burrowing sea urchins {Echinocardium) 

 (Plate 20), to be dug near low-water mark, though, as we 

 shall see, they are commoner in deeper water. 



It is on such sandy shores that the common shrimp 

 [Crangon vulgaris) is found, though it is difficult to see 

 owing to its sandy colour and its habit of covering itself 

 with sand, leaving only the long feelers exposed. With it 

 are a variety of other crustaceans, notably the ghost- 

 shrimps (Mysids), smaller and even more difficult to see. 

 There are several kinds of crustaceans which construct 

 burrows often of great depth so that considerable industry 

 is needed in digging them. Several anemones habitually 

 live buried in sand with only the mouth disc with the sur- 

 rounding tentacles exposed. Where reefs of rock run out 

 into the sand we frequently find large colonies of the 

 peculiar reef -building worm Sabellaria (Plate 18), a creature 

 which forms a sandy tub^ not in, but above, the sand, and 

 not singly but in great numbers altogether, so that large 

 reefs of hardened sand are formed which, on examination, 

 will be found to be honeycombed with the tubes of the 

 worms which construct them. Various fishes are common 

 in shallow water on sandy shores, being occasionally left 

 behind in pools by the retreating tide ; of such are young 

 flat-fish of various kinds and the sand-eel {Ammodytes). 



We may consider the associations found in mud and in 

 estuaries together because the mouths of rivers are the 

 great site of mud deposition. The fauna bears many 

 resemblances to that found on sand, with which the mud 



