CLASSIFICATION OF THE ANIMAL KINGDOM. 209 



sified natures, and presenting a regular gradation from gill-breathing inhab- 

 itants of the water to true land animals respiring by means of lungs. 

 The lowest orders of this class indeed are so little removed in organic 

 structure, both internal and external, from the preceding, that it is with 

 difficulty determined to which they belong. Others, again, resemble fishes 

 in the first stage of their existence, and afterwards, when their metamor- 

 phoses are complete, possess the principal attributes of the reptile world, 

 while the highest orders, although, like all vertebrate animals, they go 

 through certain distinct changes in their youth, yet, like the two higher 

 classes of this division, Birds and Mammalia, they pass their transformation, 

 or rather series of developments when in the egg state. The class we 

 are now considering is divided into four great orders. The lowest the 

 Amphibia, is again divided into Amphibia Perennibranchiata, animals which 

 breathe by means of branchiaa all their lives, and Amphibia Caducibranch- 

 iata, which, although they respire at first by means of gills, like fishes, 

 yet, as they approach perfection, are provided with lungs, of which the 

 frog affords a familiar example. The second order is that of serpents, 

 Ophidia, the third lizards, Sauria, and the fourth Chelonia. 



The skeletons of this great class present various structural modifications 

 and additions not met with in those of fishes, which it will not be 

 necessary or possible to enter into here, though to the osteologist, from 

 the exceeding variety of shape and development of the bones met with 

 in the various genera, there is perhaps no class that presents so much 

 interest. "In their muscular development," says Professor Jones, "reptiles 

 are ordinarily slow and languid," though it is remarkable that when dead 

 the muscles retain the power of motion for an astonishing length of time, 

 even when separated from the body. The teeth of the lower orders much 

 resemble those of fishes. Those of the Ophidians are very numerous, 

 particularly in the more venomous tribes. The Saurians possess dental 

 organs that approach in their structure the type met with in the Mammalia, 

 while the Chelonians have toothless horny jaws resembling the beaks of 

 birds. The alimentary canal in reptiles is generally composed of a large 

 oesophagus and a very variably-shaped stomach, which, in one species, the 

 crocodile, closely resembles that of birds, and an intestine, usually divided 

 into two portions, representing the small and large intestines. The secre- 

 ting glands are the salivary, which are of very singular and various 

 construction, and the liver, pancreas, spleen, and vena portae, which are 

 developed in the usual way. The absorbent vessels in this class are very 

 elaborate and peculiar in their character, and become very important organs. 



With regard to their means of respiration, the different orders, as has 

 been before observed, vary considerably, the lowest breathing by means of 

 permanent gills, others possessing a branchial apparatus, which ultimately 



