38 THE SEAS 



but can move about and burrow by means of a " foot " — 

 a muscular organ, usually wedge-shaped, which can be 

 protruded from between the two halves of the shell and by 

 its sudden movements enables the animal to progress by a 

 series of jerks on or beneath the surface of the sand. 

 Amongst the commonest of these are the cockle {Cardium 

 edule), which lives near the surface, several species of Venus, 

 one of the clams, which have ridges round the somewhat 

 globular shell the better to enable them to grip the sand, 

 and a number with more flattened shells, such as Tellina, 

 Macoma, Gari and the beautiful, highly-polished Donax, all 

 of which live deeper down than the cockle. The long razor- 

 shells {Solen) may occasionally be dug at low tide ; in it the 

 two halves of the shell form a cylinder with the two ends 

 open, the lower one enabling the foot, and the upper one the 

 two siphons concerned with the circulation of water and the 

 supply of food, to be protruded. 



Worms there are in plenty, usually with a delicate, 

 coloured ring of tentacles round the head end. They live 

 buried with only these tentacles exposed for the capture of 

 their fine food, withdrawingthem on theapproach of an enemy 

 or when the tide retreats. Of such are the Terebellids, to 

 be recognized by their sandy tubes the upper ends of which, 

 fringed with fine filaments, project above the surface, also 

 Amphi trite (Plate 77), a fine red worm with a bush of 

 sinuous tentacles. Other w^orms of a carnivorous habit, 

 such as Nephthys, are found, while, commonest of all, is the 

 lug-worm, Arenicola, whose castings are so common dotted 

 over the sandy shore (Plate 18). Like the earth worm on 

 land, it spends its time swallowing the sand in which it 

 lives, passing it out in the form of the familiar castings. 

 The head of the animal lies at the bottom of a small depres- 

 sion and the mouth continually draws in sand which, since 

 the burrow is usually U-shaped, is conveniently disposed of 



