454 VERTEBRATE LIFE AND ORGANIZATION 



The kidney tubules of reptiles are modified in such a way that less 

 water is initially removed from the blood than in amphibians, and much 

 of the water that is removed is later reabsorbed by other parts of the 

 kidney tubule and by the urinary bladder. In some reptiles the nitrog- 

 enous waste products are excreted as uric acid, which is much less 

 soluble and less toxic than ammonia or urea. Urea and ammonia are 

 characteristic excretory products of fresh-water vertebrates. The urine 

 of animals excreting uric acid typically has a pastelike consistency. The 

 reptilian kidney also differs from that of lower vertebrates in being 

 drained by a duct called the ureter instead of by the Wolffian duct. The 

 latter becomes a genital duct in males, and is lost in females. 



The reptilian body shape is better adapted to land life than the 

 amphibian. The neck is longer and the first two cervical vertebrae are 

 specialized to permit the head to move independently of the rest of the 

 body as the animal feeds. The tail is more slender than in the lab- 

 yrinthodonts and salamanders. This reflects the decreasing importance 

 of fishlike lateral undulations of the trunk and tail in locomotion, and 

 the increasing importance of the limbs. Well formed claws, which are 

 basically modified horny scales, are borne upon the toes. The more 

 powerful hind legs require a pelvic girdle that is attached more firmly 

 onto the vertebral column. Reptiles typically have two sacral vertebrae 

 whereas amphibians have only one. 



Improved locomotion and increased agility also involve a more 

 elaborate muscular system, nervous system and sense organs. The delicate 

 tympanic membrane is protected by lying deep within a canal, the 

 external auditory meatus, and the eye is further protected through 

 the evolution of a third, transparent eyelid known as the nictitating 

 membrane. 



The dry, horny skin of reptiles reduces cutaneous respiration to a 

 negligible amount, but an increase in the respiratory surface of the lungs 

 not only compensates for this, but also provides for the increased volume 

 of gas exchange necessitated by a general increase in activity. Mech- 

 anisms for moving air into and out of the lungs are also more efficient. 

 Instead of pumping air into the lungs by froglike throat movements, 

 reptiles decrease the pressure within their body cavity, and atmospheric 

 pressure drives in air. A subatmospheric pressure is created around the 

 lungs during inspiration by the forward movement of the ribs and 

 the concomitant increase in size of the body cavity. The contraction of 

 abdominal muscles and the elastic recoil of the lungs force out air. 

 Circulatory changes, discussed in a later chapter, further separate the 

 oxygenated and unoxygenated blood leaving the heart, and make the 

 oxygen siipjily to the tissues more effective. 



Major changes have come about in the method of reproduction. 

 Male reptiles have evolved copulatory organs which introduce the sperm 

 directly into the female reproductive tract. Fertilization is internal, and 

 the delicate sperm are not exposed to the external environment. A large 

 quantity of nutritive yolk is stored within the egg while it is still in the 

 ovary. As the eggs j)ass down the oviduct after ovulation, they are 

 fertilized, and additional substances and a shell are secreted around 



