CHAPTER FORTY-FOUR 



THE STRUCTURE OF THE VERTEBRATES 



Theverte- i. VERTEBRATE animals may be defined as those 

 skeleton possessing a vertebral column ; but as we have already 

 seen, they possess other important characters, some of 

 which are shared by types lower in the evolutionary 

 series. The skeleton of a vertebrate, or endoskeleton 

 (internal skeleton), consists of numerous separate parts, 

 which support the muscular and other tissues of the 

 body, and protect the more important organs, such as 

 the brain, heart, and lungs. In the lowest vertebrates, 

 such as the sharks, the skeleton is wholly cartilaginous, 

 consisting of gristle which can be easily cut with scis- 

 sors or knife. In the bony fishes, such as the salmon or 

 perch, and in all the higher vertebrates, hard bone is 

 formed. This bone, however, is laid down in cartilage, 

 or sometimes (e.g., the flat bones of the skull) in mem- 

 brane, being formed by cells which secrete lime salts. 

 Thus even man has first a cartilaginous skeleton, and 

 it is only in the course of development that it is replaced 

 by bone. The process of becoming bone is called ossifi- 

 cation. 



From Zittel's " Palteontologie " 



FIG. 120. Skeleton of a perch, showing a loosely articulated skeleton of a relatively 



primitive kind. 

 328 



