ENDO SKELETON. 



79 



usually products of the external skin (chititi), or they may be 

 internal (cartilage, bone) and give rise to vertebrce (fig. 74 a, b). In 

 either case the body becomes divided at right angles to its long axis 

 into a series of segments, which, in the simpler cases of locomotion, 

 are homonomous (Annelids, Myriapods, Snakes). As development 

 progresses some of the muscles required for locomotion gradually lose 

 their relation to the long axis of the body, and acquire a relation to 

 secondary axes; and in this way conditions are acquired for the 

 accomplishment of more difficult and complete forms of locomotion. 

 The hard parts in the long axis of the body then lose their primitive 



FIG. 74 a Diagram of the vertebral 

 column of aTeleostean fish with verte- 

 bral constriction of the notochorcl. 

 Ch, notochord; Wk, bony vertebral 

 bodies ; J, membranous intervertebral 

 section. 



FIG. 74 b Vertebra of a fish. 5", ver- 

 tebral body. Ob, neural arch (neum- 

 pophysis) ; Ub, haemal arch (hfeinapo- 

 physis) ; D. neural spine ; D', hwmal 

 spine ; K, rib. 



uniform segmentation and partially fuse with one another to form 

 several successive regions, the parts of which are capable of a greater 

 or less amount of movement upon one another (head, neck, thorax, 

 lumbar region, etc.) In this case, however, the parts of the skeleton 

 of the chief axis are usually less movable upon one another, while, on 

 the contrary, a much more perfect locomotion is effected by the 

 extensive movements of the paired extremities or limbs. The limbs 

 likewise possess a solid skeleton, to which the muscles are attached, 

 and which is usually elongated and may be external or internal, 

 and is attached more or less closely to the axial skeleton. 



The most essential property of animals is that of sensation. This 



