CHAPTER IV 



ENDOSKELETON AND VOLUNTARY MUSCLE 



Many invertebrate animals are soft-bodied and lack a hard, 

 rigid supporting or protecting skeletal structure. 

 If a skeleton is present, it is in the form of an 

 exoskeleton enveloping the body. An internal 

 supporting hard framework is absent. The 

 invertebrate skeleton is integumentary in both 

 origin and location. In sharp contrast, all verte- 

 brates possess an internal skeleton regardless of 

 the fact that in some cases an exoskeleton may 

 also be present. The vertebrate type of skeleton 

 is called an endoskeleton because it is situated 

 internal to the body muscles. In the inverte- 

 brate type of skeleton the muscles are internal to 

 the skeleton (Fig. 29). 



The endoskeleton is composed of cartilage or 

 bone, or a combination of both, and in its 

 development it is closely associated with the 

 development of the voluntary musculature of the 

 body. Such a skeleton provides an internal 

 framework to which the body muscles are 

 attached. Joints between the parts of the 

 skeleton permit movement of the parts upon or 

 about one another, while the movement itself is 

 produced by the contractions of the attached 

 29_Rei a - muscles. Skeleton and muscle are intimately 

 tion of muscle to related functional as well as structural compo- 



hard parts in the „ , -i 1 r i_ 1 



leg of an insect, nents of a system responsible lor body move- 

 (AfterBeriesejrom me nts, controlled through nervous connections 



Skull, LaRue and . , , 



Ruthven; Animal by the central nervous system. 

 Biology.) t wo general regions are recognized in the 



endoskeleton of vertebrates: (1) the axial skeleton, which includes 

 the skull and vertebral column or backbone, and (2) the appen- 

 ds 



