6 4 



THE SEAS 



ing small bivalves. Several kinds of worms habitually live 

 in sand, while of crustaceans the commonest are the 

 swimming crabs, which have the terminal joint of the fifth 

 pair of legs flattened and paddle-shaped, enabling them to 

 swim upwards. Especially adapted for a buried existence 

 is the masked crab, Corystes (Fig. 10), which lives with 

 only the extreme tip of its long, upwardly directed feelers 

 exposed. Each of these has two rows of stiff hairs which, 



when the feelers are placed 

 together, interlock so that 

 a channel is formed down 

 which water can be drawn . 

 Other devices for obtaining 

 water for respiration while 

 burrowing are the long 

 siphons of the bivalves, 

 and the rows of vibrating 

 spines which keep up a cir- 

 culation of water through 

 the burrows of Astropecten 

 and the burrowing urchins. 

 Burrowers are also 

 typical of a muddy bottom. 

 There are anemones of 

 which only the mouth and 

 tentacles appear above the 

 mud, a variety of worms 

 which burrow or live in parchment-like tubes and also 

 scavengers like the interesting sea mouse, Aphrodite, which 

 attains a length of six or seven inches and has a broad back 

 covered with mouse-coloured felting beneath which are scales, 

 while from the sides of the body spring clusters of spines 

 and beautiful iridescent hairs. The big sea cucumber or 

 trepang (Holothuria) (Plate 23), lives chiefly in mud 



Fig. 10. — The Masked Crab, Corystes, 

 which lives in sand (slightly reduced). 



