328 ZOOLOGY 



external skeleton is in the fact that bone of the former is typically 

 formed in and around cartilage, whereas in the latter there is no 

 cartilage. The internal skeleton consists of two portions, (i) 

 the axial, embracing the vertebral column, and (2) the appen- 

 dicular, or that supporting the appendages. 



As was seen in the arthropods the chief functions of skeletons 

 are to protect and support soft parts, and to furnish hard lever- 

 ages for muscular attachment. It is clear that any rigid skeleton 

 which entirely surrounds an elongated animal must have some 

 breaks or articulations in it. We found this to be true in 

 Crustacea and insects. Similarly in the internal skeleton in 

 vertebrates, the attached muscles would be useless if there were 

 no joints. It is clear that solid bones coming together in a joint 

 with the muscles external to them will make a stronger articula- 

 tion than hollow shells coming together like the segments of a 

 stovepipe. The arthropod joint is effective for small animals 

 but would not meet the needs of the large vertebrates. 



349. Axial Skeleton. In its simplest condition this consists 

 of the notochord which it will be remembered is derived from 

 the entoderm and lies between the alimentary canal and the 

 spinal cord (Fig. 156). In the true vertebrates, cells arising from 

 the mesodermal pockets on either side (Fig. 158) produce a 

 continuous skeleton-forming sheath about the notochord. 

 From the cells of this sheath are developed, finally, rings of 

 cartilage or bone about the notochord (centrum; pleural, centra, 

 Fig. 159, c) and about the spinal cord (spinous processes, Fig. 

 159, no). These, with certain other growths, constitute the 

 typical vertebrae. In this process the notochord often becomes 

 obliterated by the developing vertebrae. To each vertebra 

 may be attached a pair of ribs, which protect the ventral 

 structures, somewhat as the neural arch protects the nerve cord. 

 The ribs of fishes and of the higher forms are not considered 

 to be homologous structures (Figs. 159, 160). 



The axial skeleton varies from this typical condition in dif- 

 ferent parts of its course. In the head region, for example, 

 the nervous cord is immensely enlarged and the neural arches 

 are much modified, being replaced by plates and supplemented 



