412 CAHDIACE^E ', CARDIUM, OR COCKLE. 



usually prolonged into tubes (Fig. 597) : which can, however, 

 be drawn within the shell by means of a retractor muscle. 



954. In the Cardium, or Cockle, the tubes or siphons are 

 shorter than in most of the other genera ; indeed they are some- 

 times reduced to mere openings ; and scarcely any vestige of a 

 retractor muscle exists. The foot is very large, and is capable of 

 being bent at an acute angle, and then suddenly straightened ; so as 

 to enable the animal to move from place to place by a succession 

 of leaps. But it is only occasionally, that it serves this purpose. 

 The chief use of the organ is as a boring instrument, by which 

 the animal may penetrate the sand or mud, below the surface of 

 which it is usually found. A very curious provision exists for 

 adapting it to this object. As usually seen, the foot, when 

 extended, tapers gradually to a point ; and, as its diameter is at 

 its largest point much less than the breadth of the shell, it is 

 not apparent by what means the hole that is excavated is made 

 sufficiently large for the reception of the latter. This is accom- 

 plished, however, by the distension of the foot with water, 

 through a tube which opens just within the mouth ; and thus 

 the size of the borer becomes so nearly equal to that of the 

 shell, that (its solid point first entering the sand) it is enabled, 

 by rotatory motions often repeated, to excavate a burrow large 

 enough to receive the animal with its shell. The Cardia are 

 found in all known seas; and in some they abound so much, that 

 they become very important articles of food to Man, as well as 

 to marine animals. Mr. Kirby mentions that, on the North 

 East coast of Norfolk, an alteration in the sands has taken place, 

 which has caused a great diminution of late years in the number 

 of boring bivalves ; and that the quantity of Soles and other 

 Flat Fish frequenting the coast, of which they form the principal 

 food, has consequently much decreased also. 



955. Nearly allied to the Cardium are a considerable num- 

 ber of genera of great interest to the Conchologist ; many of 

 them being remarkable for the beauty of their shells, or for the 

 curious situations in which they live. The greater number of 

 them inhabit sand or mud ; but there are several which bore 

 into rocks ; and a few that burrow in masses of coral. The 



