220 STRUCTURE OF THE VERTEBRATES 



appendages of the male become vascular and swollen dm'ing the 

 breeding season and the annuals are able to stay submerged for 

 long periods while guarding the egg masses. 



The sea turtle, although a lung breathing amniote, has two 

 specializations which aid in respiration. Both the pharynx and 

 cloaca are vascular and have thin epithelial coverings, and by 

 keeping a constant stream of water passing into and out of 

 these openings the animals can remain immersed for long periods 

 without damage. Only a small amount of oxygen is needed, for 

 the turtle at rest has a low metabolic rate and a minimum of 

 respiratory function suffices. 



There are no accessory respiratory structures in the birds and 

 mammals, both of which have a heavy, dead skin. This is also 

 true of the completely aquatic forms. Porpoises and whales come 

 to the surface frequently for air. Some of the latter have a 

 pharyngeal air chamber w^hich gives a reserve supply to the 

 animal while under w^ater, but respiration takes place in the 

 lungs. 



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