external gilU 





caudal fin 





branchial opening 



NECTURUS 



CRYPTOBRANCHUS 



Figure 2-8. Two large American salamanders. 



Another group of living amphibians is that of the Anura, 

 the frogs and toads. The last group, the Gymnophiona or 

 Apoda, contains legless, worm-like forms. 



Reptiles 



Several reptiles are readily available: turtles, alligators, 

 and lizards (Figure 2-9). These show much of this group's 

 range of variation. All have a scaly skin, two pairs of five- 

 digited limbs with claws, and a long, tapered tail, with the 

 exception of the turtle. The turtle has a heavy shell which 

 is quite distinctive of the group. In most reptiles the eye can 

 be covered by eyelids, largely the lower lid, or by a nictitat- 

 ing membrane, an inner membrane that can be drawn over 

 the eye. In the snakes the margins of the eyelids fuse, and 

 a clear scale in the lower lid, the brille, permits good 



vision. Such a clear area in the lower eyelid is observed in 

 some lizards of burrowing habit. There is a single external 

 nasal opening lying forward on the snout. The mouth is 

 generally large and armed with teeth. Again the turtle is 

 the exception, for it has horny jaw margins. 



Birds 



The pigeon or chicken can be used as a representative of 

 this group (Figure 2-10). Feathers cover all of the body ex- 

 cept the bill, the lower leg (tarsometatarsus), and toes. The 

 feathers are modified on different parts of the body to serve 

 various functions — protection, insulation, and flight. The 

 bill is covered with a horny sheath and at the base has a 

 soft, fleshy lump, the cere. The nostril is in or below the 

 cere. The eye is surrounded by a rim of skin in which hair- 



tympanic membrone 



nostril 



IGUANA 



SPHENODON 



Figure 2-9. The lizard and Tuatara. 



28 • THE VERTEBRATES AND THEIR EXTERNAL ANATOMY 



