CLASS III. : REPTILES (REPTILIA). 



Vertebrate animals whose body temperature varies with that of the 

 external medium ; the body covered by horny or bony plates (scutes) ; 

 breathing by lungs, and generally oviparous ; limbs in the form of legs 

 or absent. 



1. The Shape of the Body in reptiles (explain the name) is. very 

 variable. The typical forms are those of the lizard, the snake, and the 

 tortoise, which are repeated with more or less variation in the different 

 orders. 



2. Limbs. — Either one or two pairs are present or they are entirely 

 absent. (Examples.) In the perfectly- developed limb the constituent 

 bony elements are the same as in mammals (see Part I, p. 12). In pro- 

 portion as the limbs are reduced in size the body becomes more elongated, 

 and either aids in or entirely takes over the business of locomotion (see 

 lizards and snakes). The different ways in which reptiles move explain 

 the different relations of 



3. The Skeleton. — (a) Those species in which the limbs are well 

 developed possess both the shoulder and pelvic girdles (see Part L, pp. 4 

 and 11). In species devoid of hind-limbs the pelvis is absent, or only 

 persists in a very rudimentary condition. In the snakes the shoulder 

 girdle also is absent. (Give reasons for these differences.) 



(b) The vertebral column in those species which possess limbs is, as 

 in mammals, divisible into several segments. In limbless species, on 

 the other hand, it consists of a uniform series of similar vertebrae. Their 

 number varies directly with the length of the body (more than 400 in 

 some snakes), as does also their mobility. (Why? See lizards and 

 snakes.) In the tortoises the vertebras are in part anchylosed with the 

 dorsal carapace. Eibs are present in varying number, and in the snakes 

 (which see) aid in locomotion. 



(c) The skull in its general structure is like that of birds. As in the 

 latter, a quadrate bone is present. Where this bone is united to the 

 mandible by ligaments only (lizards and snakes), the mouth can be 



