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 accomplish ed*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 



