REPTILIA, 257 



SAUROPSIDA. 



CHAPTER XXVI. 

 CLASS III. EEPTILIA. 



We commence now the second great primary division of the Verte- 

 hrata — namely, that of the Sauropsida, comprising the Reptiles and 

 the Birds. These two classes, though very unlike in external ap- 

 pearance, are united by the following characters : There are never 

 at any period of life gills or branchise adapted for aquatic respira- 

 tion ; the red corpuscles of the blood are nucleated (fig. 16.3, h, c) ; 

 the skull articulates with the vertebral colunm by means of a single 

 articulating surface or condyle ; each half of the lower jaw is com- 

 posed of several pieces, and is jointed to the skull, not directly, 

 but by the intervention of a special bone (the so-ciiUed " quadrate 

 bone"). 



These being the characters by which, amongst others. Reptiles 

 and Birds are collectively distinguished from other Vertebrates, it 

 remains to see what are the characters by which the Reptiles are 

 distinguished, as a class, from Birds. In all Reptiles the blood is 

 cold — that is to say, very slightly warmer than the temperature of 

 the external medium in which they live. The integument generally 

 secretes scales, with or without bony plates, but in no case do the 

 integumentary appendages take the form of feathers. The heart 

 consists oftw^auricles and a ventricle, which in most is partially 

 divided into two chambers by alWnconiplete partition, and in a few 

 is completely divided. In any case, however, more or less of the 

 impure venous blood is mixed with the pure arterial blood which 

 circulates over the body. There is no division between the cavities 

 of the thorax and abdomen, and the lungs are not connected with 

 air-sacs placed in various parts of the body. The limbs may be 

 wanting, or rudimentary, but in no case are the fore - limbs con- 

 structed upon the type of the " wing " of birds, and in no living 

 Reptile is there the bone which is known in Birds as the " tarso- 

 metatarsus." 



