130 THE LOG OF THE SUN 



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. Occa- 

 sionally 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 distinct 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 tropi- 

 cal turtles the rings are either absent or indis- 

 tinct. It is to this mode of growth that the spread- 

 ing 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 portion is crushed or torn away the in- 

 jured parts are gradually cast off, and from the 



