GENERAL CHARACTERS OF REPTILES. 57 



This condition exactly corresponds with that, which may be observed during 

 the process of ossification in higher Vertebrata ; for each of the larger bones 

 of the cranium is formed from several centres of ossification ; so that, if the 

 cranium of a fetus or young infant be macerated, it will fall into a number of 

 pieces nearly corresponding with those of the Reptile's skull. The different 

 orders of Reptiles have a close agreement in various other points ; especially 

 in the degree of development of their several organs of nutrition. Thus, in 

 all of them, the lungs, though commonly of large size, are so little subdivided, 

 as really to expose but a small extent of surface. The glandular structures, 

 too, are formed upon a much more simple type, than is characteristic of the 

 warm-blooded Vertebrata. They all agree, moreover, in having the body 

 covered with stales ; which, though generally small, are sometimes large flat- 

 tened plates. 



31. Between Fishes and true Reptiles, there is a group that remarkably 

 combines the characters of both ; being composed of animals which come 

 forth from the egg in the condition of Fishes, but Avhich afterwards attain a 

 form and structure closely corresponding with that of true Reptiles. This 

 group, consisting of the Frog and its allies, is sometimes associated as an or- 

 der (Batrachia} of the class of Reptiles ; though it should probably take rank 

 as a distinct class, the AMPHIBIA. The Tadpole or larva of the Frog is in 

 every essential respect a Fish. Its respiration and circulation, its digestion 

 and nutrition, its locomotion and sensation, are entirely accordant with those 

 of Fishes. The body is destitute of members for progression, but is propelled 

 through the water by the lateral undulations of the spinal column, which is 

 articulated in the same manner as that of Fishes. At a certain period, a me- 

 tamorphosis commences in which almost every organ in the body undergoes 

 an essential change. Lungs are developed, which take the place (in regard 

 to their function) of the gills ; and the latter are atrophied. The auricle of 

 the heart is divided into two ; and the circulation is performed on the plan of 

 that of the true Reptile. Two pairs of members are usually formed, to which, 

 when they are fully developed, the power of progression is committed, the 

 tail disappearing; in some species, however, the tail remains, and the extre- 

 mities are small. The digestive system undergoes a remarkable alteration ; 

 the intestinal canal, which was previously of enormous length in proportion 

 to the body, being now considerably shortened, in accordance with the differ- 

 ent kind of food on which the animal has to subsist. The mode of articu- 

 lation of the spinal column also, undergoes a change, which brings it to the 

 type of that of Reptiles. The most obvious point of difference in external 

 characters, between the higher Amphibia and true Reptiles, is the absence of 

 scales or plates on the skin of the former. In this manner, the common Sala- 

 mander or Water-Newt may be recognised as belonging to the Batrachia 

 though its form would otherwise lead us to place it among the Lizards; and 

 the Coecilia, which has the form of the Serpent, is in like manner known to 

 be really allied to the Frog. An acquaintance with the history of these ani- 

 mals confirms such an arrangement, by showing that the Salamander and the 

 CoBcilia undergo a metamorphosis ; breathing by gills, and having the general 

 structure of Fishes, in the early part of their lives. 



32. Besides those animals, however, which attain the condition of perfect 

 Reptiles, this group contains several, whose development is arrested, as it 

 were, in an intermediate or transition state ; their adult form presenting a re- 

 markable mixture of the characters of the two classes, which they thus con- 

 nect. This is the case in the Proteus, Siren, and other less known species, 

 which retain their gills through the whole of their lives, whilst their lungs are 

 at the same time developed ; so that, as they can respire in either air or wa- 

 ter, they are the only true amphibious animals. In their general organization, 



