2 REPTILES. 



that any of the higher animals would perish under its severity, and for the most part 

 being lovers of wet and swampy situations, the Reptiles swarm within the regions near 

 the equator, and in the rivers or vast morasses of the tropical countries the very soil 

 appears to teem with their strange and varied forms. Indeed, the number of Reptiles 

 to be found in any country is roughly indicated by the parallels of latitude, the lands 

 near the equator being the most prolific in these creatures, and containing fewer as 

 they recede towards the poles. 



Some Reptiles inhabit the dry and burning deserts ; but the generality of thesfe 

 creatures are semi-aquatic in their habits, are fitted by their structure for progression 

 on land or in water, and are able to pass a considerable time below the surface without 

 requiring to breathe. 



This capacity is mostly the result of the manner in which the circulation and aeration 

 of their blood is effected. 



As has been shown in the two volumes on Mammalia and Birds, the heart in 

 these animals is divided into a double set of compartments, technically termed auri- 

 cles and ventricles, each set having no direct communication with the other. In the 

 Reptiles, however, this structure is considerably modified, the arterial and venous 

 blood finding a communication either within or just outside the two ventricles, so that 

 the blood is never so perfectly aerated as in the higher animals. The blood is con- 

 sequently much colder than in the creatures where the oxygen obtains a freer access 

 to its particles. 



In consequence of this organization the whole character of the Reptiles is widely 

 different from that of the higher animals. Dull sluggishness seems to be the general 

 character of a Reptile, for though there are some species which whisk about with 

 lightning speed, and others, especially the larger lizards, can be lashed into a state of 

 terrific frenzy by love, rage, or hunger, their ordinary movements are inert, their gest- 

 ures express no feeling, and their eyes, through bright, are stony, cold, and passionless. 

 Their mode of feeding accords with the general habits of their bodies, and the pro- 

 cess of digestion is peculiarly slow. 



Most of the Reptiles possess four legs, but are not supported wholly upon them, 

 their bellies reaching the ground and being dragged along by the limbs. One or two 

 species can support themselves in the air while passing from one tree to another, much 

 after the fashion of the flying squirrels ; and in former days, when Reptiles were appar- 

 ently the highest race on the surface of the earth, certain species were furnished with 

 wing-like developments of limb and skin, and could apparently flap their way along 

 like the bats of the present time. 



Excepting some of the tortoise tribe, the Reptiles are carnivorous beings, and many 

 of them, such as the crocodiles and alligators, are among the most terrible of rapacious 

 creatures. In this class of animals we find the first examples of structures which trans- 

 mute Nature's harmless gifts into poison, a capacity which is very common in the later 

 orders, such as the spiders and insects, and is developed to a terrible extent in some of 

 the very lowest beings that possess animal life, rendering them most formidable even 

 to man. 



The skeleton of a true Reptile, from which class the Batrachians, i. e. the frogs, 

 salamanders, and their kin are excluded, for reasons which will presently be given, is 

 composed of well-ossified bones, and is peculiarly valuable to the physiologist. It is 

 well known to all who have studied the rudiments of anatomy, that each bone is 

 formed from several centres, so to speak, consisting of mere cartilaginous substance at 

 its earliest formation, and becoming gradually ossified from several spots. 



In the young of the higher animals these centres are only seen during their 

 very earliest stages, and are by degrees so fused together that all trace of them is 

 obliterated. But in the Reptiles it is found that many of the bones either remain in 

 their separate parts, or leave so distinct a mark at the place where they unite, that 

 their shape and dimensions are clearly shown. In the head of the adult crocodile, for 

 example, the frontal bone is composed of five distinct pieces, the temporal of at least 

 five pieces, and each side of the lower jaw-bone is composed of either five or six por- 

 tions united by sutures. 



