158 COMPARISONS OF STRUCTUKE IN ANBIALS. 



meut, it is still a cutaneous production, and 

 not an integral portion of the skeleton. 



From a consideration of the tortoise, boxed 

 up in its own skeleton, let us now turn to 

 the snake, with a view to ascertain the 

 mechanism of its vertebral column. These 

 long, tortuous creatures are destitute of a 

 breast-bone and of limbs, except, that, in 

 the boa, rudiments of posterior limbs in the 

 form of spurs, consisting of a few bones 

 acted upon by muscles, are seated just an- 

 terior to the base of the tail. These spurs 

 aid the tail in grasping ; for in these snakes 

 this part is strongly prehensile, and often 

 firmly lashed round some fixed object, while 

 the huge reptile lies in wait for its unwary 

 victim. 



The skeleton of the snake is very simple, 

 but at the same time very elegant : it consists 

 only of the head, the vertebral column, and 

 the ribs. From the extreme flexibility of 

 these animals, we are naturally prepared to 

 expect certain modifications of structure, in 

 the osseous fabric, whence this freedom of 

 motion arises ; but here, as in all similar 

 cases, simplicity pervades the whole. Three 

 points may be noticed : the sub-division of 



