REPTILES. 345 



Class VIII. — REPTILIA. 



Reptiles are colcl-bloodeil animals which broathc^ by means of lungs, and gen- 

 erally have the ventricles of the heart but incompletely separated from each other. 

 The body is protected, externally, by scales or armor plates, and the embryos are pro- 

 vided with an amnion and an allantois. A general structure is presented somewhat 

 liighcr than that of batrachians, and lower than, though strongly resembling, that of 

 birds. 



The general form of the body is that of the previous class. The trunk usually 

 plays the chief part in locomotion, while the limbs are either entirely absent, as in 

 Ophidia, or, among the lower forms, are present only as aids in the serpentine move- 

 ment. To this end the vertebral column is strong and rigid, terminating posteriorly 

 in an elongated tail, and ])resenting but feebly those regions so distinct in the birds 

 and mammals. All reptiles, however, are not of this low type; the tortoises, several 

 lizards, and many fossil forms have the limbs well developed and the vertebral column 

 more or less differentiated. 



Protection from injury with most of the smaller reptiles lies chiefly in resem- 

 blance, color, and in the shielded areas of the skin, the outgrowths of which may be 

 from either the dermis projter, as the scale-like ossifications of many lizards, or from 

 the epidermis, as the corneous plates of the crocodiles and turtles. Many serpents 

 and the remarkable lizard Heloderma are provided with poison apparatus, which 

 renders a conflict with them of the most dangerous nature. The larger reptiles trust 

 alone to sheer force for protection. 



Tlie skeleton is seldom otherwise than strong and bony, and though many fossil 

 forms, as well as the geckos and Hatteria, have bi-concave vertebral centra, as a rule 

 the bodies of the vertebrss are concave anteriorly only. Ribs are quite characteris- 

 tic, — in the serpents being the chief organs of locomotion, and in a few lizards forming 

 the support of the so-called wings. They are often united to a sternum by means of 

 sterno-costal pieces, and in the crocodiles a cartilaginous plate extends from sternum to 

 pelvis, bearing lateral processes which serve the function of ribs proper. The skull is 

 composed of well-ossified bones, the embryonic condition presented by many batra- 

 chians being supplanted, and is connected with the axial skeleton by means of a single 

 condyle. The specialized cranial structure presented by the Ophidians is of particular 

 interest, and will be spoken of in connection with that group. The limbs and their 

 respective girdles, though completely absent in most snakes, are generally present and 

 are often highly specialized. 



The nervous system is a decided advance on that presented by the previous class. 

 The hemispheres are large, .and show an inclination to overlap the portions of the 

 brain posterior to them. The cerebellum exhibits a regular advancement in develop- 

 ment, coincident with that presented by the respective representatives of the class 

 from Ophidia to Crocodilia, where .avian peculiarities are anticipated. 



As to the organs of special sense : In the serpents and a few lower lizards, eye- 

 lids are not present. When thus unprotected, the cornea is covered by a crystal-like 

 Bcale which holds a thin layer of lachrymal fluid. Hearing is of varying delicacy. A 



