462 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



year's skin shedding the horny covering of this part 

 slips back again, and is caught in the same way, and so 

 on. Thus is formed a series of molds of the three con- 

 solidated joints loosely caught together. Every year 

 adds another to the chain, and, if not broken or worn 

 off, the number indicates the number of skin sheddings, 

 and therefore the age of the snake. 



Turtle Shell. — Turtles are incased in an immovable 

 shell. The dorsal part is called the carapace, the ventral 

 part the. plastron. Where, then, is the jointed backbone 

 characteristic of vertebrates ? Their structure seems to 

 violate the vertebrate plan. But not so. It is only an 

 extreme modification of that plan, and is an admirable 

 illustration of adaptive modification underlying homology 

 (page 246). If we look into the interior of the carapace 

 of a complete skeleton of a turtle we see a continuous 

 series of vertebras, consolidated with one another and 

 with the shell in the region of the trunk, but movable 

 in the neck and tail. The shell of a turtle or tortoise is 

 indeed a very complex structure, consisting of three 

 parts — skeletal, dermal, and epidermal. The spinous pro- 

 cesses of the vertebra expand at the top into broad flat 

 plates, which unite with one another to form the ridge or 

 central row of plates of the carapace. The ribs also 

 expand into broad plates, which, uniting, form the slop- 

 ing under-roof on each side. Then the dermis over all 

 this is ossified into bony plates, which unite with the 

 skeleton proper to form the main part of the carapace. 

 Lastly, the epidermis completes the structure by forming 

 the horny covering over all. This last is the so-called 

 tortoise shell so much prized. Between this and the bony 

 shell, of course, there is a Malpighian layer, which by 

 transformation forms the horny layer. Similarly the 

 plastron is formed by the union of dermal bony plates 

 with the ventral ribs (like those of an alligator), and 



