48 THE SEAS 



out the tail or hind end of the body. Periwinkles, whelks 

 and all their allies really glide over the surface, waves of 

 movement passing over the broad flat " sole " of the large 

 fleshy " foot." Anemones and flatworms glide in a similar 

 manner. Apart from the fish, many animals swim, the 

 lobsters by sudden movements of the tail which propel 

 them quickly backwards through the water, while many 

 worms are able to swim to a greater or less extent by un- 

 dulatory movements of the long body, some, most highly 

 specialised, having paddle-like flaps on the side of the body. 

 Mention will be made of the swimming crabs and of the 



_ 4 



Fig. 7. — Razor-Shell (Solen) ; 1-4 showing successive stages in burrowing in sand 



(after Fraenkel). 



swimming scallops in the next chapter. The peculiar 

 movements of boring animals are discussed in Chapter VI. 

 The burrowing bivalves work their way through the sand 

 by the action of the muscular foot, the most specialised 

 case being that of the razor-shell, which drives its pointed 

 foot directly downwards, anchors it there by forcing in 

 blood which makes the end swell out and then, by a sudden 

 cod traction, draws the shell down to the level of the foot, 

 repeating the process as often as need be and burrowing 

 so quickly that one has to drive a spade in very suddenly 

 to capture it (Fig 7.). Burrowing worms, crustaceans 



