2/4 COMPARATIVE ZOOLOGY 



guides in the classification of animals ; while their 

 durability renders them available to the paleontologist 

 in the determination of the nature and affinities of ex- 

 tinct species, of which they are often the sole remains. 

 Even the structure is so peculiar that a fragment will 

 sometimes suffice. 



4. Deglutition, or how Animals Swallow. In the 

 lowest forms of life, the mouth is but an aperture open- 

 ing immediately into the body substance, and the food is 

 drawn in by ciliary currents (Figs. 9, n). But in the 

 majority of animals, a muscular tube, called the gullet, or 

 esophagus, intervenes between the mouth and stomach, 

 the circular fibers of which contract, in a wavelike man- 

 ner, from above downward, propelling the morsel into the 

 stomach. 85 In the higher mollusks, arthropods, and ver- 

 tebrates, deglutition is generally assisted by the tongue, 

 which presses the food backward, and by a glairy juice, 

 called saliva, which facilitates its passage through the 

 gullet. 86 Vertebrates have a cavity behind the mouth, 

 called the throat, or pharynx, which may be considered 

 as a funnel to the esophagus. 87 In air breathers, it has 

 openings leading to the windpipe, nose, and ears. In 

 man, as in mammals generally, the process of deglutition 

 is in this wise : the food, masticated by the teeth and 

 lubricated by the saliva, is forced by the tongue and 

 cheeks into the pharynx, the soft palate keeping it out 

 of the nasal aperture, and the valvelike epiglottis falling 

 down to form a bridge over the' opening to the wind- 

 pipe. The moment the pharynx receives the food, it is 

 firmly grasped, and, the muscular fibers contracting 

 above it and left lax below it, it is rapidly thrust into 

 the esophagus. Here, a similar movement (the peristal- 

 tic) strips the food into the stomach. 88 The rapidity of 

 these contractions transmitted along the esophagus may 

 be observed in the neck of a horse while drinking. 



