132 VERTEBRATE ZOOLOGY 



Note the looseness of the skin of the neck, legs, and tail, which 

 enables them to be drawn under the shell. If scales are present in 

 the skin, note their variation in size. 



The body of the turtle may be divided into four regions,— 

 the head, neck, trunk, and tail. In most turtles the soft parts of 

 all these regions are covered by horny scales which are similar in 

 texture to the outer plates of the shell. 



The head is compactly and very strongly built, with powerful 

 jaws which are armed with horny plates instead of teeth, and con- 

 trasts markedly with the heads of amphibians and fishes. The 

 mouth is ventral in position, and projecting in front of it is the 

 snout, with the paired nostrils, or external nares, at its forward end. 

 The eyes are prominent objects; they do not usually project from 

 the head, but are protected above and below by bony ridges of 

 the skull. Each is provided with three eyelids,— the thick upper 

 lid, the more movable lower lid, and the nictitating membrane, 

 a translucent third lid which can be withdrawn into the anterior 

 corner of the eye. Just back of the mouth is a slight circular de- 

 pression marking the position of the tympanic membrane, or 

 ear drum, which lies on the outer surface of the head, as it does 

 in the frog. 



The neck of the turtle is very long and flexible, a feature corre- 

 lated with the heavy, awkward trunk. 



The trunk is inclosed within the shell and bears the extremities. 

 It shows a high degree of specialization. The spinal column and 

 ribs have become flattened and help to form the shell, and the 

 trunk muscles are wanting. Two pairs of appendages are present. 

 Each leg is made up of three divisions,— a proximal, a middle, and 

 a terminal division ; these correspond to the upper arm, the fore- 

 arm, and the wrist and hand in the foreleg, and in the hind leg 

 to the thigh, the shank, and the ankle and foot. Identify these 

 divisions. Note the absence of a sole, or walking surface, on the 

 feet ; note the number of toes on each foot, and the claw on each 

 toe. See if the toes are free or are joined by a membrane. 



The tail is usually short and conical, and bears on its under side 

 the anus, the opening of the cloaca. Note if the tail can be with- 

 drawn under the shell. 



