tiU INTRODUCTION. 



the skin is naked. Some of them are pisciform in their fatal 

 state, and as they advance to maturity undergo a true metamor- 

 phosis. The organs of digestion greatly vary; the circulation 

 is simple and the respiration partial, that is, the heart, otherwise 

 very variable, transmits the blood into an artery of which one 

 branch only goes to the lungs, the result of which is, that in 

 each circuit of the blood, a part of it only is submitted to res- 

 piration. Their lungs are shaped like bags, or at least have 

 large cells. They can suspend the respiratory process, with- 

 out stopping the circulation: their blood is cold. The quantity 

 of respiration is not the same in, this class, the pulmonary ar- 

 tery not being in all, in the same ratio with the aortic trunk 

 from which it arises. They have a trachea and larynx, al- 

 though voice is not common to all: the alligator is another 

 exception. The females have a double ovary and two ovi- 

 ducts. Some males have a bifurcated penis, others have none. 

 None of them hatch their eggs. The irritability of the mus- 

 cles is such that it continues long after they are separated 

 from the nervous system, and even from the rest of the body. 

 Their sensations are obtuse. Their nostrils traverse the face, 

 but the ear is incomplete, being confined to a vestibule con- 

 taining soft bones, the semi-circular canals, and in some a 

 rudiment of a cochlea. We also find under the skin the rudi- 

 ments of the bone of the tympanum. The crocodiles alone have 

 an external auricular opening. The brain, which is small, may 

 he taken away as well as the head, and motion still continue. 

 Many remain torpid during a part of the year. 



From their great variety of organization reptiles have been 

 divided into several families. 



The chelonia or tortoises, have a heart with two auricles, 

 each of which receives a different blood, and with one ventri- 

 cle, having two unequal and communicating cavities, in which 

 the blood from the auricles is mingled. These animals are 

 enveloped by an upper shell formed by the ribs and vertebral 

 laminae, and by an under one formed by the sternum, both 

 covered by the skin, and by a horny or scaly matter exuded 

 from it. In respiration, the air is drawn in by the nostrils, and 

 forced into the larynx by a sort of deglutition. The male has 

 a simple canalated penis. The female lays very hard eggs. 



