TRIDACNA. CARDIACE.E. 411 



whole regiment is scarcely an exaggeration, as the flesh of these 

 animals is commonly used as food where they abound, and is by 

 no means unpalatable. The remarkable difference of the shell 

 of this animal at different epochs of its growth, has given rise 

 to the formation of many species which have no real existence. 

 It is only when immature, and when the shell is comparatively 

 light, that the animal is attached by a byssus. This cord, how- 

 ever, seems rather to be a musculo-tendinous prolongation of the 

 foot itself, than a fibrous tissue secreted by it like the byssus of 

 the Pinna ; it is so tough as to require to be chopped with a hat- 

 chet, in order that the shell may be detached. As the animal 

 approaches adult age, however, and has by successive layers 

 very much increased the weight of the shell, the byssus, being 

 no longer required to secure it from injury, disappears, and the 

 groove in the shell is filled up with a solid deposit. When thus 

 free, it is said to be taken with a long pole, which is introduced 

 between the valves when open ; the animal immediately closes the 

 valves upon it, and does not quit its hold until it is landed. In 

 some of the numerous and remarkable fossil species of this sec- 

 tion, one or both of the valves are spiral. 



SECTION B. CARDIACS^. 



1037. This Order, including the Cockles and their allies, con- 

 tains several genera, which, in the smallness and delicacy of 

 many of their shells, and in the comparative activity of the 

 animals that form and inhabit them, offer a remarkable contrast 

 to those of the previous group. The shells are all equivalve, 

 or nearly so ; they are furnished with a regularly-toothed hinge, 

 often of great complexity and beauty ; and there is always a 

 double adductor muscle. The foot is here more largely de- 

 veloped than in any of the previous Orders, and it is a very im- 

 portant organ to the animals, not only serving to excavate bur- 

 rows in the sand or mud of the sea-bottom, but also enabling its 

 possessors to leap with considerable agility. In some cases also 

 it forms a creeping disc. The respiratory orifices are prolonged 



