HOW VERTEBRATES BREATHE 



Chordata, which it will be remembered is the large 

 group or phylum including, not only the higher vertebrata, 

 but the series of lower forms which are distributed among 

 the classes of Ascidia and variously lowly organized 

 marine creatures, the pharynx that is the anterior part 

 of the alimentary tract behind the mouth is perforated 

 by a series or many series of slits, putting it into com- 

 munication with the outside world, which in the forms 

 where this arrangement is fully developed is always 

 water. The water rushes into the pharynx by means of 

 these clefts, and parts with its dissolved air containing 

 oxygen to the small blood vessels which fringe the sides 

 of the clefts. Thus is respiration effected. That this 

 arrangement occurs in the vertebrata may be easily 

 seen by the inspection of any fish, and among the dogfish 

 and skates the details are plainer than in some others. 

 In those fishes a row of holes is to be seen along the throat; 

 and if a probe of any kind be passed through it, will be 

 found to emerge in the throat cavity. Furthermore, the 

 edges of the clefts are seen to be furnished with red tufts, 

 which are the gills, and are practically an agglomera- 

 tion of small blood vessels divided only from the water 

 which washes them by a thin membrane through which 

 the dissolved oxygen can pass. In vertebrates above 

 fishes the gills along the clefts entirely, or nearly entirely, 

 lose their function as breathing organs. It is only in 

 certain Amphibia that the clefts in question remain 

 throughout life, and then in diminished numbers ; but 

 in the tadpoles of all of these clefts are present, and 

 associated with the respiratory function. It might 

 seem therefore that this important character was only one 

 that applied to the lower vertebrata ; and in its full 

 development it does only so apply. But the study 

 of the development of animals, that is embryology, 

 has shown that in reptiles, birds, and mammals, traces 

 of these same gill clefts are to be seen ; and one of them, 



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