INTRODUCTION. V 



trunks, very closely united by others, which run in 

 a transverse direction : these trunks have a hole at 

 top, by which the water enters, and their ends are ge- 

 nerally fringed. We have a very familiar example 

 of these tracheae in the beard of an oyster. 



Beneath the trunks is seen a thick muscle, which 

 is capable of being lengthened, and moved on all 

 sides, or may be entirely shut within the shell. This 

 is called the foot ; but although it, in part, fulfils the 

 functions of the same organ, in the animal of uni- 

 valve shells, it bears no resemblance, except in being 

 flat beneath. It is used by the animals to make 

 a groove, deep enough to sustain its shell, in a ver- 

 tical position, when it moves from one place to 

 another, or to dig beneath the sand, or mud, when 

 it wishes either to avoid the cold, or hide itself 

 from its enemies. 



The AcSphales of the second section have no 

 siphons ; both their openings are simple holes, con- 

 cealed under the doublings of the mouth and the 

 tracheae. Part of these animals have an organiza- 

 tion similar to those of the preceding section ; but 

 some have a foot, while others have none. 



Those without siphons, which have a foot, are 

 either like the animals of the first section, that is, can 

 form a groove, and move in the sand, or are solely 

 designed to spin the thread, or byssus, with which 



