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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