VERTEBRATA. 319 



four limbs — the hinder the longer, and the first developed. 

 They have four fingers and five toes. The tongue is long, 

 and, fixed by its an- 

 terior end, it can be 

 rapidly thrown out as 

 an organ of prehen- 

 sion.'" The eggs are 

 laid in the water en- 

 veloped in a glairy 

 mass ; and the tadpoles ^"*- '"-^™^ <'"""'>■ 



resemble the TJrodelans, till both gills and tail are absorbed. 

 Frogs {JRana) have teeth in the upper jaw, and webbed 

 feet ; Toads {Bvfo) are higher in rank, and have neither 

 teeth nor fully webbed feet. The former have been 

 known to live sixteen years, and the latter thirty-six. 



Class III. — Reptilia. 



These air-breathing, cold-blooded Vertebrates are dis- 

 tinguished from all Fishes and Amphibians by never hav- 

 ing gills, and from Birds by being covered with horny 

 scales or bony plates. The skeleton is never cartilaginous ; 

 and the skull has one occipital condyle. The vertebrae are 

 ordinarily concave in front ; and the ribs are well devel- 

 oped. With few exceptions, all are carnivorous ; and teeth 

 are always present, except in the Turtles, where a horny 

 sheath covers the jaws. The teeth are never fastened in 

 sockets, except in Crocodiles. The jaws are usually very 

 wide. The heart has three chambers, save in Crocodiles, 

 where the ventricle is partitioned. But in all cases a 

 mixture of a^^rterial and venous blood is circulated. The 

 lungs are large, and coarsely cellular. The limbs, when 

 present, are provided with three or more fingers as well 

 as toes. 



There are about three tliousand species of living Eep- 

 tiles, and of these there are four main groups : the first 



