528 COLLEGE ZOOLOGY 



External Features. The shell of the turtle is broad and 

 flattened, and protects the internal organs. Even the head, 

 limbs, and tail can be more or less completely withdrawn into 

 the shell. The neck is long and very flexible. The head is 

 flattened dorso-ventrally and triangular in shape. The mouth 

 is large, but, instead of teeth, horny plates form the margin of 

 the jaws. The nostrils, or external nares, are placed close to- 

 gether near the anterior end of the snout. The eyes, situated one 

 on each side of the head, are each guarded by three eyelids: (i) 

 a short, thick, opaque upper lid; (2) a longer, thin lower lid; and 

 (3) a transparent nictitating membrane, which moves over the 

 eyeball from the anterior corner of the eye. Just behind the 

 angle of the jaw on either side is a thin tympanic membrane. The 

 limbs usually possess five digits each ; most of the digits are armed 

 with large claws, and connected one with another by a more or 

 less complete web. The skin is thin and smooth on the head, 

 but thick, tough, scaly, and much wrinkled over the exposed 

 parts of the body. 



Internal Anatomy and Physiology. THE SKELETON. - 

 Since the life of the turtle is influenced so strongly by the 

 skeleton, this system will be described first. 



The exoskeleton (Fig. 439) consists of a convex dorsal portion, 

 the carapace (c), and a flattened ventral portion, the plastron 

 (Hyp, Hpp, Xp) ; these are usually bound together on each side 

 by a bony bridge (at M) . Both carapace and plastron are usually 

 covered by a number of symmetrically arranged epidermal plates 

 forming a shield; the plates do not correspond either in number 

 or arrangement to the bony plates beneath them. The number 

 and shape of the plates vary according to the species, but are 

 usually constant in individuals of the same species. The horny 

 shields of the " Hawk's-bill Turtle " (Fig. 447) furnish the tortoise- 

 shell of commerce. B eneath the shields are a number of bony plates 

 formed by the dermis and closely united by sutures (Fig. 439). 



The endoskeleton may, as in other vertebrates, be divided into 

 an axial portion and an appendicular portion. The skull 



