6 LABORATORY DIRECTIONS 



familiar with the structure of the amphibian skin examine a 

 demonstration slide. Distinguish between the outer epidermis 

 and the deeper dermis. The glands, although derived from the 

 epidermis, lie wholly within the dermis. 



The nostrils communicate with the mouth cavity. Compare 

 with the shark in this respect. The three pairs of external gills 

 do not correspond to the internal gills of fishes. Gill slits (how 

 many.?) are present and open into the pharynx. Water, 

 however, contrary to the condition in fishes, does not ordinarily 

 pass through the slits during respiration. Watch the respira- 

 tory movements of the external gills of a living Necturus if one 

 is available. The paired appendages, although consisting of 

 arms and legs with digits on the hands and feet, correspond to 

 the fins of fishes. Claws are absent. Find the cloaca at the 

 junction between body and tail. 



Lizard — Class Reptilia 



Lizards are closely related to snakes and belong to the same 

 order. Their horny scales are derived from the epidermis and 

 thus are not homologous to the bony scales of fishes. The 

 presence of a dry scaly skin is characteristic of reptiles; there are 

 very few skin glands. Claws are present on the digits. The 

 nostrils, as in amphibians, open by internal nares into the mouth 

 cavity. There are three eye-lids: an upper, a lower, and a 

 nictitating membrane (a thin membrane lying in the inner corner 

 of the eye when not in use). The depression In the skin behind 

 the eye may be considered as the beginning of an external ear. 

 The tympanic membrane stretches across the bottom of the 

 depression, covering the cavity of the middle ear. Fishes and 

 Necturus possess Internal ears only. The comparative anatomy 

 of the ear will be discussed later. 



Turtle — Class Reptilia 



The shell, which consists of a dorsal carapace and a ventral 

 plastron, is made up of bony scutes covered with epidermal 

 scales. Are the scales of the same size and shape as the scutes 

 beneath ? Teeth are absent; the jaws are covered with a horny 

 epidermis. 



