SECOND WEEK] June 135 



So when we marvel that the ribs of a turtle are on the 

 outside of its body, a second thought will show us that 

 this is just as true of us as it is of these reptiles. 



This hardening of the skin has brought about some 

 interesting changes in the body of the turtle. In all the 

 higher animals, from fishes up to man, a backbone is of 

 the greatest importance not only in carrying the nerves 

 and blood-vessels, but in supporting the entire body. In 

 turtles alone, the string of vertebra is unnecessary, the 

 shell giving all the support needed. So, as Nature seldom 

 allows unused tissues or organs to remain, these bones 

 along the back become, in many species, reduced to a mere 

 thread. 



The pieces of bone or horn which go to make up the 

 shell, although so different in appearance from the skin, 

 yet have the same life-processes. Occasionally the shell 

 moults or peels, the outer part coming off in great flakes. 

 Each piece grows by the addition of rings of horn at the 

 joints, and (like the rings of a tree) the age of turtles, 

 except of very old ones, can be estimated by the number 

 of circles of horn on each piece. The rings are very dis- 

 tinct in species which live in temperate climates. Here 

 they are compelled to hibernate during the winter, and 

 this cessation of growth marks the intervals between each 

 ring. In tropical turtles the rings are either absent or 

 indistinct. It is to this mode of growth that the spreading 

 of the initials which are cut into the shell is due, just as 

 letters carved on the trunks of trees in time broaden and 

 bulge outward. 



The shell has the power of regeneration, and when a 



