320 THE OCEAN WORLD. 



coloured matter is furnished only at intervals, spots and patches of 

 irregular form and increasing in size with the enlargement of the 

 mantle are the consequence." 



Bivalves move about and change from place to place hy means of 

 an extensible fleshy organ called, from some of its functions, a foot ; in 

 fact, it has less resemblance to a foot than to a large tongue. It is 

 a muscular mass, capable of being pushed out from between the mantle 

 and the valves, and varies much in form ; it is in turn a hatchet, a 

 ventilator, a pole, an awl, a finger, and a sort of whip. This foot is 

 simple, forked, or fringed. In some species the tissues are spongy, and 

 capable of receiving considerable quantities of water. When the organ 

 swells, it is elongated and stiff; on the other hand, by suddenly ex- 

 pelling all the water, it gets small and pliable, and can now return to 

 its shell. This organ is represented in Fig. 128 (Donax trunculus, 



Fig. 128. Donax trunculus (Linnaeus). 



Linn.), in which it is singularly developed. This bivalve is found 

 on the sea shore in shallow water; it buries itself almost perpen- 

 dicularly in the sands. They are so abundant on the French side of 

 the Channel and on the shores of the Mediterranean, that they form 

 a considerable portion of the people's food. These bivalves have the 

 singular power of leaping to a considerable height and then throwing 

 themselves to a distance of ten or twelve inches a spectacle which 

 may be witnessed any day at low water. When abandoned by the 

 retreating tide, they try to regain the sea. If seized by the hand, in 

 order to drag them out of the sand, aided by their compressed, 

 branched, and angular feet, they give to their shell the sudden and 

 energetic movement under which the bounding action takes place. 



The shell of the Donax is slightly triangular and compressed ; its 

 length exceeds its height; it is regular, univalve, unequally lateral, 

 and its hinge bears three or four teeth on each valve. The action of 

 these feet is very simple, and is compared by lleaumur to that of 



