THE SEA SHORE 47 



occasionally in order to moisten them. There are also 

 species of tropical fish, notably the " walking goby," which 

 are able to live out of water and even to climb trees ; thev 

 have a lung-like extension of the gill chamber, while it is 

 also reported that they breathe through their tails, for they 

 sit on the land with only their tails in water ! If held under 

 water for any length of time they are drowned. 



Locomotion and Migrations 



As we have seen, many shore animals are attached to 

 rocks or weed or else live in permanent tubes or burrows. 

 The advantage of this mode of life is clear when we consider 

 the perpetual beating of the waves on the shore as the tide 

 comes in and goes out. Of equal advantage is the burrowing 

 habit of many animals, bivalve molluscs and burrowing 

 sea urchins, for example, which are also able to move 

 about beneath the surface. Movement in shore animals 

 takes many different forms. The larger crustaceans 

 clamber over rocks and through gullies by means of their 

 strong, hinged walking legs. The starfish move steadily over 

 the surface by the concerted action of the many tiny " tube- 

 feet " each terminated by a small sucker, double rows of 

 these feet lining the grooves which run down the centre of 

 the underside of the arms. They are connected with a 

 complicated system of canals containing water which can 

 be forced out of or into the tube-feet at will, enabling them 

 in turn to fix and relax their hold. A similar mode of 

 movement is found in the sea urchins, "which also employ 

 their teeth for this purpose ! A mussel may move up the 

 side of a rock by fastening a byssus thread as far as possible 

 above it and then pulling itself up by hauling on to it. 

 Some shore fish, such as the blennies, can crawl over rocks 

 by means of their fins. The shore insects and the crustacean 

 sand-hoppers move by jumping, suddenly straightening 



