24 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



3. Appendages.— The trunk bears two pairs of limbs, which have the same 

 parts as those of Neclurus. Five digits, the typical vertebrate number, are 

 present on each limb; they terminate in horny claws. The student should 

 note that the limbs depart still further from the primitive position described in 

 connection with Necturus, thus lifting the animal above the ground to a greater 

 extent. In the hind limb the thigh still extends out at right angles to the body, 

 but is slightly twisted on its longitudinal axis so that the original dorsal surface 

 is becoming anterior. The shank is directed ventrally and posteriorly, and has 

 undergone the same sort of torsion as the thigh. The foot retains the primitive 

 position. In the fore limb the upper arm is rotated in the opposite direction 

 from that observed in the thigh, so that the original preaxial surface is now 

 dorsal; the upper arm is directed posteriorly. The forearm and hand are 

 directed downward and forward, and are so rotated that their original preaxial 

 borders look inward instead of forward. 



4. Anus.— On the ventral side of the trunk behind the bases of the hind 

 limbs is the anus or cloacal aperture. Its shape, a transverse slit, is character- 

 istic of lizards and snakes. 



K. EXTERNAL ANATOMY OF THE TURTLE 



The turtle is a reptile belonging to the order Chelonia. Obtain a specimen 

 and examine. 



1. Body and skin. — The form of the body is considerably modified from 

 the typical vertebrate shape. It is divided into head, neck, trunk, and tail. 

 The head is similar in form to that of other reptiles, but the neck is unusually 

 long and flexible; the trunk is remarkably flat and broad, and the tail diminished 

 in diameter and length. The turtle is thus one of those forms in which, as in 

 the skate, the transverse axis is elongated while the anteroposterior and dorso- 

 ventral axes are shortened. The skin of the legs, tail, and other exposed parts 

 of the body is provided with small horny scales or horny thickenings as is charac- 

 teristic of reptiles. The trunk is inclosed in a hard shell, the outer surface of 

 which consists of greatly enlarged horny scales, definitely arranged. The skin 

 of the head in most of our common turtles is bare, but in some turtles is marked 

 off into large head shields. 



2. Head. — The anterior end of the head is pointed and elevated and bears 

 at its tip the two external nares, close together; this position of the nostrils 

 enables the animal to breathe air with only a slight exposure of the head 

 above water. The jaws are clothed with hard, horny beaks, teeth being absent. 

 The large eyes are provided as in the lizard with upper and lower eyelids and a 

 nictitating membrane, located in the anterior corner of the eye. Just posterior 

 to the angles of the mouth can be observed a circular area, covered by skin; this 

 is the tympanic membrane beneath which is located the cavity of the middle 

 ear. This membrane is not, however, sunk below the surface, as in the lizard. 



