THE INTEGUMENT AND THE EXOSKELETON 51 



various parts of the body. The head may bear enlarged scales or head shields. 

 There is no dermal exoskeleton in lizards. 



2. Exoskeleton of the turtle. The exoskeleton of the turtle is somewhat 

 complicated. For the purposes of study, carapaces and plastrons which have 

 been separated by sawing through the bridges will be provided. It is also de- 

 sirable that these parts shall have been cooked to render the sutures between the 

 plates more distinct. The following description is based on the exoskeleton 

 of our common pond turtles; in other families of turtles the arrangement of the 

 scutes and plates may be slightly different. 



a) The carapace: The dorsal surface of the carapace consists of large thin 

 horny scales or scutes, whose boundaries are marked by grooves. These scutes 

 are of epidermal origin, formed from the stratum corneum. The scutes are ar- 

 ranged in five longitudinal rows, one median, and two pairs of lateral rows. The 

 median row consists of five neural scutes; on each side of this are four costal scutes 

 and the margins of the carapace are covered by a number of small marginal scutes. 

 The unpaired narrow marginal scute at the middle of the anterior end of the cara- 

 pace is termed the nuchal scute; besides this there are twelve pairs of marginal 

 scutes of which the median posterior two, behind the fifth neural scute, are often 

 called pygal scutes. Observe that the marginal scutes are continued over the 

 edge of the carapace to cover the margins of the under side. Draw the dorsal 

 surface of the carapace, showing accurately the outlines of the scutes (only half 

 need be filled in). 



Turn the carapace over and study its ventral surface. It is composed of 

 heavy bony plates of dermal origin. The vertebral column with its ribs occu- 

 pies the median line, and both are firmly fused to the carapace. The boundaries of 

 each plate are marked by jagged sutures which should be located in identifying 

 the plates. The plates like the scutes are arranged in five longitudinal rows. 

 The median row of plates, fused to the dorsal sides of the vertebrae, consists 

 of a single large anterior nuchal plate followed by eight smaller vertebral or neural 

 plates, each attached to a vertebra, followed by two postneural or precaudal 

 plates, not attached to vertebrae. On each side of the median row is a row of 

 costal plates, eight pairs of elongated plates, each attached to a rib. The margins 

 are formed of eleven pairs of marginal plates and one single unpaired pygal plate 

 in the median posterior position. Make an accurate drawing of the under side 

 of the carapace, showing bony plates and their relation to the vertebrae and ribs. 

 Do the scutes and plates of the carapace correspond ? 



A difference of opinion exists regarding the bony plates of the turtle's carapace. According 

 to one view, all of the plates are of dermal origin and the expanded vertebrae and ribs are fused 

 to the under surface of these dermal plates. According to the other view, which is probably the 

 correct one, the vertebral and costal plates are formed entirely by the expansion of the vertebrae 

 and ribs, and are therefore not dermal exoskeleton at all but parts of the endoskeleton. This 

 view also includes the conception that turtles originally possessed a set of real dermal plates 



