CHAPTER XXXIX. 



REPTILIA AND AVES. 

 THE REPTILIA. 



THE Reptilia are cold-blooded animals with a scaly or ar- 

 mored skin. In many cases there are no limbs, and when pres- 

 ent they are usually weak, scarcely raising the body from the 

 ground. In most cases the trunk, even when the limbs are pres- 

 ent, is an important agent inlocomotion, as the vertebral column 

 is adapted to serpentine movements. The internal skeleton is 

 always bony, and generally the ribs are numerous and well de- 

 veloped. Each half of the lower jaw is composed of several 

 pieces joined by sutures, and the two halves are usually only 

 loosely united. The lower jaw is jointed to the skull by means of 

 the " quadrate bone," which enables the mouth to be opened 

 much wider than would otherwise be possible. The jaws are 

 usually supplied with prehensile teeth, and salivary glands are 

 present. The stomach and intestines with pancreas and liver, are 

 well developed. The heart has distinct auricles, but the ventricle 

 is but imperfectly divided, so that mixed blood is distributed to 

 the organs. Respiration is by means of lungs, which often reach 

 considerable size, not being separated from the digestive organs 

 by a diaphragm, as in the higher vertebrates. The nervous sys- 

 tem is more highly developed than among the amphibians, the 

 cerebral hemispheres being relatively large, but the senses arenot 

 specially acute. The reptilia in general are oviparous, the eggs 

 having membranous shells. During the development a mem- 

 brane called the amnion grows out from the blastoderm around 

 the embryo so as to form a sac, which is filled with fluid. In ad- 

 dition the allantois, an extremely vascular organ, is formed, 

 which serves as an embryonic respiratory organ. The appear- 

 ance of the allantois corresponds with the disappearance of 

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