IV . INTRODUCTION. 



of the same canal. After the mouth comes the 

 pharynx, which is provided with valves, a singular 

 conformation, to prevent the return of the aliment. 

 The stomach is placed below the breast, and after 

 that the intestine, which lessens as it proceeds, till 

 it turns back upon itself, enters between the lobes of 

 the liver, and opens, for the passage of the excrement, 

 from the cavity of the chest. 



The AcSphales, which are very differently orga- 

 nized, may be divided into two sections, each of 

 which should be considered separately. 



The first comprehends only the animals that move 

 from place to place ; and these, instead of a head, 

 have two tubes, or siphons, sometimes united, some- 

 times separate, but always susceptible of being 

 lengthened, or contracted, at pleasure. The largest 

 of these tubes absorbs water and animalcula, and 

 is called the mouth ; the other, which rejects water 

 and the excrements, cannot be mistaken for the 

 anus. Their opening is generally ciliated, or 

 fringed. The mantle lines the interior of the shell, 

 and envelopes the body more or less, according to 

 the species. Between the mantle and the body, we 

 find the tracheae, which in these animals answer the 

 purpose of lungs, and are the same to them as gills 

 are to fishes. They are generally composed of four 

 membranous leaves, attached to the body of the 

 animal, and formed by a tissue of little longitudinal 



