268 VERTEBRATED ANIMALS 



and their flesh preserves its irritability much longer, after having been separated from 

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

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

 mobility 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 smallness of the pulmonary vessels enables Reptiles to suspend their respiration 

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

 difficulty, and for a longer time, than Mammalia 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. Reptiles are provided with a trachea and larynx, although they have 

 not all the power of emitting an audible voice. 



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

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

 naked skin. 



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 Batrachians), they 

 have [mostly] none at all. 



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

 fecundated until after they have been excluded ; they have merely a membranous 

 envelope. 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 Reptiles which produce eggs, the Snake, for example, the young is 

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

 deposits it ; and there are even some species which may be rendered viviparous at will, 

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

 the common Snake of water. 



The amount of respiration in this class is not fixed, as in the Mammalia and Birds ; 

 but it varies according to the relative proportion of the diameter of the pulmonary 

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

 more than Frogs, &c. [though the latter, it should be observed, respire in part over the 

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

 Edwards]. Hence, the diff'erences of energy and sensibility are very much greater than 

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



Reptiles also present 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), which have a heart with two auricles, 

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

 lers formed of the ribs and sternum. 



