REPTILES. 345 



Class Yni. — REPTILIA. 



Reptik's arc cold-blooded animals which breathe by means of lungs, and gen- 

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

 The body is jirotected, externally, by scales or armor phstes, and the embryos are pro- 

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

 higher than that of batraeliians, and lower than, th<>u<;h strongly resembling, that of 

 birds. 



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

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

 Oj>hitlia, or, among the lower forms, are present oidy as aids in the ser])entine move- 

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

 in an elongated tail, and presenting but feebly those regions so distinct in the birds 

 and mammals. All reptiles, however, are not of this low type; tlie 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 sliielded areas of the skin, the outgrowths of wdiich may be 

 from either the dermis proper, 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 remark.alilc lizard JIdoderma 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. 



The skeleton is seldom otherwise tlian 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 vertebrae are concave anteriorly only. Ribs are quite characteris- 

 tic, — in the ser]ients 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 stt'rnum to 

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

 composed of well-ossilied bones, the embryoiuc condition presented by many batra- 

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

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

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

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

 are often highly si)ecialized. 



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 jiortions of the 

 brain i^osterior to them. The cerebellum exhibits a regular advancement in develop- 

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

 from Ophidia to Crocodilia, where avian peculiarities are anticipated. 



As to the organs of s])ecial 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 

 scale which holds a tliin layer of lachrymal fluid. Hearing is of varying delicacy. A 



