GENERAL STUDY OF TYPICAL CHORDATES 23 



in the case of the fore limb; thigh, shank, ankle, and foot, in the hind limb. Both 

 hand and foot bear four digits (fingers, toes), although five is the typical verte- 

 brate number; the first digit is the one which is missing. The position of the 

 limbs with reference to the body is very primitive, especially in the case of the 

 hind limb, and should be carefully studied. Note that the hind limb projects 

 out at right angles to the body, all of its parts on a plane parallel to the ground. 

 In this primitive position the limb has an anterior or preaxial border, a posterior 

 or postaxial border, and dorsal and ventral surfaces. In the fore limb, however, 

 the forearm is bent downward, and the hand is directed slightly forward. This 

 alteration of position is brought about chiefly by a torsion of the upper arm, 

 whose former preaxial surface now looks dorsally. The preaxial border of the 

 forearm is turned medially, its postaxial border laterally. As a result of these 

 changes the animal is able to lift itself to a slight extent above the ground. 



The flattened tail is bordered by a tail fin which differs from the fins of fishes 

 in that it contains no fin rays. The tail is diphycercal, that is, truly symmetrical 

 with respect to the vertebral column. 



4. Anus. At the junction of the trunk and tail in the median ventral line 

 is the large anus or cloacal aperture, with fimbriated borders. Amphibia, like 

 the elasmobranchs, have a cloaca. 



J. EXTERNAL ANATOMY OF A LIZARD 



The lizards are typical reptiles, class Reptilia, order Squamata. 



1. Body and skin. The body is characteristically vertebrate in form, 

 thickest in the middle, tapering to each end. It consists of head, neck, trunk, 

 and a long tail. It should be recalled that a neck is absent in fishes and 

 Amphibia; its appearance is correlated with the assumption of the land habitat. 

 The body is completely clothed in horny scales, which are thickenings of the outer 

 layer of the skin (epidermis) and are not homologous with the scales of fishes. 

 The scales are larger on the head where they are known as head shields. 



2. Head. The head bears the usual three pairs of sense organs. The 

 external nares or nostrils are located at the tip of the head and lead, as in the 

 Amphibia, into the mouth cavity where they open by the internal nares. Pos- 

 terior to the nares are the eyes, each provided with an upper and lower movable 

 eyelid and with a third eyelid, the nictitating membrane, a thin transparent 

 membrane concealed from view when not in use in the anterior corner of the 

 eye where it should be sought with a forceps. Halfway between the eye and 

 the base of the fore limb is a slight depression in the skin across the bottom of 

 which stretches a thin membrane. This membrane is the tympanic membrane 

 or drum membrane, and it covers a cavity, the tympanic cavity or middle ear. 

 The depression may be regarded as the beginning of an external ear. The lizard 

 therefore possesses a middle and an external ear in addition to the internal ear 

 present in fishes and Necturus. 



