106 A COURSE ON ZOOLOGY. 



species are very brilliant, and the length of those found 

 in warm climates may exceed sixteen or twenty inches ; 

 in such countries they live in hedges and bushes. 



The common lizard may be taken as a type for the 

 study of the organization of all reptiles. It has four feet, 

 supporting a long, slender body, terminated by a long 

 tail, and is covered with a scaly, naked skin that bears 

 neither hair nor feathers. At a certain season of the 

 year the lizard gets a new skin, and throws off the old 

 epidermis. 



On dissecting the lizard we find that there is no divi- 

 sion between the thoracic and abdominal cavities : there 

 is no real diaphragm. The digestive apparatus is simple, 

 and after the mouth, which is armed with short teeth, we 

 find a straight oesophagus, a stomach, and an intestine, 

 ending in a cloaca, as in the birds. 



The circulatory apparatus differs from those which we 

 have already studied : there are but three cavities in the 

 heart, two auricles and one ventricle. In this ventricle 

 the arterial and venous blood are mixed, so that the cir- 

 culation, instead of being complete, as in mammals and 

 in birds, is incomplete, that is, there is a mixture of the 

 two kinds of blood. 



The blood-corpuscles are elliptical, like those of birds, 

 but they are much larger, and have a distinct nucleus. 



There are two aortic arches, one right and one left, 

 which unite back of the heart to form a single vessel. 



The respiratory function is accomplished by lungs, but 

 these are much less developed than in the animals we 

 have studied. ^ 



The skeleton is not unlike those we have examined. 

 The skull is flat and depressed. The vertebrae are con- 

 cavo convex, that is, at the posterior extremity they 



