266 PHILOSOPHY OF ZOOLOGY. 



Vertebral Animals with cold blood. 



The cold-blooded vertebral animals, and indeed those 

 belonging to the remaining divisions of the system, have 

 their temperature, in a great measure, regulated by ex- 

 ternal circumstances. The skin is either naked, or pro- 

 tected by scales, and never exhibits either hair or fea- 

 thers. In all the tribes, the brain does not fill entirely 

 the cavity of the skull destined for its reception. Organs 

 of sight and hearing, more or less perfectly developed, may 

 be detected. The circulating system is not so perfect as in 

 the warm-blooded classes, as there is always some defi- 

 ciency either in the systemic or pulmonic vessels. This 

 condition of the circulating vessels gives rise to the follow- 

 ing division into Reptiles and Fishes. 



REPTILES, 



Furnished with a systemic heart. 



The skin of reptiles is either naked, or fortified with 

 scales or plates. Among many of the tribes it is periodi- 

 cally renewed. The cuticular secretions are few in num- 

 ber ; and they seldom serve to lubricate the skin. 



The bones, unless in the larger kinds, scarcely ever at- 

 tain the same degree of firmness as those of quadrupeds 

 and birds. Their number and connection vary exceed- 

 ingly in the different tribes. The organs of motion are fit 

 to perform almost every kind of progression. Some are 

 found with two or four feet, either divided or palmated, and 

 fitted for walking, climbing, or swimming. Others move by 

 Avhat is peculiarly termed a serpentine motion. Many species 

 inhabit the land, and not a few live in the water. To fit 

 them for residing in the latter element, some of the groups, 

 wanting feet, have their bodies compressed behind, while 

 others are furnii^hcd with fin?, dct-titute, h!)wcver, of carti- 



