GENERAL CHARACTERS OF REPTILES. 49 



markable feature in the Serpent's skeleton, besides the absence of legs, and 

 the large number of ribs and vertebrae, is the deficiency of a sternum ; through 

 the absence of this, the extremities of the ribs are free, and they become in 

 fact the fixed points, on which the animal crawls, when advancing slowly for- 

 wards, in a manner which bears a strong resemblance to the progression of the 

 Centipede. 



41. Although the configuration of the cranium varies much in the different 

 orders of Reptiles, yet there is a remarkable agreement in certain general 

 characters, and in the general degree of development. It consists of a much 

 larger number of parts than are to be found in the cranium of adult Birds or 

 Mammalia ; each principal bone being subdivided, as it were, into smaller 

 ones. 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 foetus 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 scales ; which, though generally small, are sometimes large flattened 

 plates. 



42. 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 which 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 order (Ba- 

 trachia) of the class of Reptiles ; and is sometimes made to 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 is that of Fishes. At a certain period, a metamorphosis 

 commences, in which almost every organ in the body undergoes an essential 

 change. Lungs are developed, which take the place (in regard to their" func- 

 tion) 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 disappear- 

 ing ; in some species, however, the tail remains, and the extremities 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 different kind of food on which 

 the animal has to subsist. The mode of articulation of the spinal column, also, 

 undergoes a change, which brings it to the type of that of Reptiles. On the 

 whole, there scarcely appears sufficient reason for separating these animals, in 

 their adult condition, from the class of Reptiles. The most important point of 

 difference is the nakedness of the skin, by which the Batrachia may be at once 

 distinguished, even when their external configuration approaches that of Rep- 

 tiles in general. In this manner, the common Salamander or Water-Newt 

 may be recognized as belonging to this group, though we should otherwise 

 have placed it among the Lizards ; and the Ccecilia, which has the form of 



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