THE SEA SHORES 1 79 



arc not worms at all but molluscs, tunnel into wood and feed 

 upon it, like the larvae of boring insects in the forest trees. 

 Most of the unattached bivalves can move about, though 

 rather slowly; a few are quite active, like the razor-shells, 

 and the scallops are more active still and can even swim. 



Many worms, while not attached themselves, live in tubes 

 of their own construction attached to other objects or partly 

 rooted in the mud. 



Of animals which live wholly exposed or hiding away in 

 burrows, holes and crevices and among the roots of plants 

 there are multitudes of conchs, whelks, drills, periwinkles and 

 other snail-like creatures, of crustaceans of very many sorts, 

 and jointed worms of many different types, together with 

 nemerteans, some of which are very large, priapulids, sipun- 

 culids and flatworms. Fishes, of course, are everywhere. 



Besides the abundance of attached animals, especially of 

 the colonial types, the coastal regions are mainly characterized 

 by the great development of three groups of animals which are 

 almost or quite unrepresented in the open sea. These are the 

 bivalve and gastropod or snail-hke molluscs and the echino- 

 derms, including the star-fishes, the brittle-stars, the sea-urchins, 

 the sea-cucumbers or holothurians, and the crinoids. The 

 first two are most abundant along the shore, becoming much 

 less common in deeper water, while the echinoderms rapidly 

 increase in relative abundance with increasing depth. Practi- 

 cally all the members of these three groups are sluggish ani- 

 mals, most of them able only to crawl slowly, though a few 

 bivalves and star-fishes and the comatulids can swim feebly 

 for short distances; some, like oysters and most stalked cri- 

 noids, live permanently' attached to other objects. 



To mention in detail the economic interrelationships of all 

 these creatures would be an overwhelming task; and, indeed, 

 very little is known about them. But let us consider one or 

 two examples. 



The common mussel, like oysters and other bivalves, is 

 preyed upon by the common star-fish which is often most 



