268 VERTEBRATED ANIM.VLS 



and their flesh preserves Its irritabihty much longer, after having been separated from 

 the rest of the body, than is the case with the preceding chxsses. Their heart pulsates 

 for many hours after it lias been detached, and its loss does not deprive the body of 

 mobihty for a still longer period. It has been remarked of some which have the 

 cerebellum extremely diminutive, that this circumstance has some reference to their 

 disinclination to move. 



The smallncss of the pulmonary vessels enables llcptilcs to suspend their respiration 

 without arresting the course of the blood, and thus to remain submerged witli less 

 difficultv, and f.jr a longer time, than IVIammalia or Birds. The cells of their lungs 

 are not so numerous, as they contain fewer vessels within their precincts, and they are 

 also much larger, these organs having sometimes the form of simple sacs, merely a 

 little cellular. 



For the rest, Heptiles are provided with a trachea and laryn.x, although they have 

 not all the power of emitting an audible voice. 



Their blood not being warm, tbev consequently do not require teguments capable 

 of retaining heat ; and they are accordingly covered with scales, or simply with a 

 naked >kin. 



The females have a double ovary and two oviducts, and the males of several genera 

 have a forked or double penis, but in the last order (that of the Batruchians), they 

 have [mostly] none at all. 



No Reptile inculiates its eggs. In several genera of Batrachians, these are not 

 fecundated until alter the)* have been excluded ; the}' have merely a membranous 

 envelojie. The young of this last order have, on quitting the egg, the form and gills 

 of Fishes ; and certain genera retain these organs even after the developement of their 

 lungs. In other ReptdLS which produce eggs, tlie Snake, for example, the young is 

 already formed and considerably advanced within the egg at the time the jsirent 

 deposits it ; and there are even some spiecies which mav be rendered viviparous at will, 

 by retarding the deposition of their eggs, as M. Geoftroy exemplified by depriving 

 the common Snake of water. 



Tlie amount of respiration in this class is not fixed, as in the I\Ianimalia and Birds ; 

 but it varies according to the relative proporti(jn of the diameter of the pulmonary 

 artery, as compared with that of the aorta. Thus, Tortoises and Lizards respire much 

 more than Frogs, <S;c. [though tlie latter, it should be observed, respire in part over the 

 wdiole damp skin, as conclusively ascertained by the experiments of Dr. Milne 

 Edwards]. Hence, the diflferences of energy and sensibility are very much greater than 

 those between one IMamraalian and another, or one Bird and another. 



Reptiles also [iresent more varied forms, characters, and modes of gait, than the 

 two preceding classes ; and it is in their production more especially, that Nature 

 seems to have tried to imagine grotesque forms, and to have modified in every possible 

 way the general plan adopted for all vertebrated animals, and for the oviparous classes 

 in particular. 



A comparison of the extent of their respiration with their organs of movement has 

 led M. Brongniart to divide them into four orders, which are as follow : — 



The Chelonians (or Turtles and Tortoises), wdiieh have a heart with two auricles, 

 and the body of which, supported by four limbs, is enveloped by t-,vo plates or buck- 

 lers formed of the ribs and sternum. 



