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EVOLUTION OF ANIMALS 



Part V 



Fig. 35.14. Florida soft-shelled turtles (Trionyx) are highly active aquatic 

 turtles in which bony plates are reduced or absent, and the outer covering is a 

 leathery skin. (Photograph by Isabelle Hunt Conant.) 



enough air chambers to furnish an abundance of exposure to air. In spite of 

 the unyielding shells, turtles appear to breathe somewhat like mammals. 

 Muscles in each leg-pocket operate like the diaphragm of a mammal, their 

 contractions enlarging the body cavity and allowing the lungs to expand with 

 air. During expiration, the viscera press against the lungs and deflate them. In 

 many aquatic turtles, the walls of the pharynx contain numerous blood vessels 

 over which water is sucked in and expelled so that the whole structure acts as a 



gill. 



All female turtles produce eggs either with leathery or brittle shells. These 

 are usually laid in holes dug by the female in soil or in decaying vegetation in 

 which heat aids the incubation. The number varies in different species up to 

 about one hundred. Incubation periods range between two and three months, 

 being greatly affected by humidity and temperature. 



Ancient Reptiles 



The story of the great Age of Reptiles is told by their fossilized remains 

 and by certain descendants that have changed little since then. During that 

 age reptiles became at home on land, in water, and in the air. Some were small, 

 but many were giants such as have never existed since. For this period of some 

 140 million years reptiles dominated the earth, but in spite of them birds, 

 small mammals, insects, and flowering plants were becoming established. The 

 reptilian promise of bird life seems to have been dramatic and convincing 



