REPTILIA 



581 



blood is partially mixed with venous blood in the ventricle before 

 being sent into the aortic arches. 



Urlnog'enital System 



The excretory system consists of two kidneys, their ureters, the 

 cloaca, and a bilobed urinary bladder. The urine is voided into the 

 cloaca and stored in the urinary bladder until expelled through the 

 anus. 



The sexes are separate. The male reproductive system is made up 

 of a pair of testes, a pair of vasa deferentia, and an evertible penis 

 located on the anterior wall of the cloaca. The female genital organs 

 are ovaries, oviducts, and cloaca. 



The Nervous System 



This system is typically reptilian (see page 573). In some turtles 

 adaptations in structure of nervous tracts to the eye are known to 

 have occurred, probably in response to feeding habits. That part 

 of the peripheral nervous system which normally innervates the 

 musculature of the costal region is absent. 



The Skeleton 



The skeleton of turtles is the most characteristic of their systems 

 Embryological studies have shown that in the early stages the 

 skeleton follows the typical vertebrate plan. Soon, however, the 

 ribs rise above the limb girdles; those above the pectoral girdle 

 bend anteriorly and those over the pelvic girdle bend posteriorly. 

 The process of co-ossification goes on over a period of years and is 

 not altogether completed until maturity is reached. The result is 

 a vertebrate animal with the limb girdles enclosed by the ribs. No 

 other vertebrate follows such a plan. 



The Muscular System 



This system shows modifications corresponding to those in the 

 skeleton. The intercostal muscles of the body wall are not present 

 as such. The muscles of the neck are highly developed and adapted 

 to moving the head with great rapidity. The muscles of the legs are 

 little modified, being similar to those of other vertebrates. 



