242 AN INTRODUCTION TO ZOOLOGY. 



The Skeleton. — Of the characters common to all 

 the classes of the Vertebrates, the most obvious at first 

 sight is that to which they owe the name Vertebrata, 

 i.e., the possession of a jointed backbone to protect 

 the central nervous cord. The existence of this 

 central dorsal nervous cord, and the special develop- 

 ment of its front end into a brain, similarly protected 

 by a skull, is the real distinctive feature of the 

 Vertebrates. The existence of internal bones forming 

 an endoskeleton, to support or protect the various 

 important structures, is, however, in contrast with the 

 usual structure of the laverfcebrata, which have their 

 hard protective structures developed on the outside of 

 the body, forming what is called an exoskeleton. 

 Whether this contrast is as striking as it seems at 

 first sight is as yet uncertain. There are, however, a 

 few bones in the Vertebrate skeleton, which belong 

 to the outside of the body and have travelled inward 

 (see p. 261). 



The skeleton of Vertebrates consists of the following 

 chief parts : — the axial skeleton, already named, 

 i.e., spinal column and skull, protecting the central 

 nervous system; the ribs, supporting the muscular 

 walls of the trunk, some of which are connected in 

 front in nearly all Vertebrates but fishes with a 

 breast-bone or sternum ; the bones of the fore- 

 limbs and hind-limbs, with the bones by which they 

 are attached to the trunk, called respectively the 

 pectoral girdle (Lat. pedor ; breast) and the pelvic 

 girdle. The parts of the pectoral girdle on each side 



