THE DIGESTIVE AND RESPIRATORY SYSTEM 303 



Essentially different in principle from all of the respiratory 

 methods thus far mentioned is that which utilizes for the 

 purpose some portion of the wall of the alimentary canal, a 

 method employed sporadically among invertebrates, especially 

 the echinoderms, and forming the essential system in verte- 

 brates and allied forms. In this, which, by using the term in 

 its most comprehensive sense, may be called intestinal respi- 

 ration, the function is usually located near one end of the ali- 

 mentary canal, for the purpose of obtaining the respiratory 

 medium, and the wall at this place is richly supplied with 

 capillaries, through which the interchange of gases takes 

 place. The respiratory medium, which may be either air or 

 water, is kept in motion by a system of involuntary or semi- 

 voluntary muscles, and the motion thus generated is usually 

 rhythmic in character. 



In the vertebrates, as well as in those invertebrates that 

 probably represent their ancestors, the respiratory function is 

 located in the pharynx and the respiratory current is primarily 

 taken in at the mouth and driven out through a series of lateral 

 openings, the gill-slits.* 



These latter, as seen in the worm-like Balanoglossus, and 

 in Amphioxus, as well as in the embryos of true vertebrates, 

 are seen to be metameric in character, a pair for each somite, 

 and to be arranged in a single row along each side ; but in the 

 more specialized group of Tunicata, these rows of slits which 

 appear in the larva become secondarily modified by the forma- 

 tion of numerous cross-bars, so that ultimately the entire 

 pharynx comes to resemble a grating or a loosely woven 

 basket. 



The number of these slits is very large in both Balanoglos- 

 sus and Amphioxus, but has suffered a considerable reduction 



* Aside from the respiration at the anterior end of the canal there are 

 a few isolated instances of respiratory action in other parts of its extent. 

 Thus in the teleost Cobitis, an Eastern Hemisphere carp, some respira- 

 tory function is possessed by the intestine ; and in turtles, two lateral 

 bladders, opening from the cloaca in association with the median, or 

 allantoic bladder, are used for aquatic breathing. 



