280 



THE RISE OF ANIMAL LIFE 



verfcbratft 



invertebrate 



Fig. 13-2. Comparative study of the vertebrate and invertebrate. 



A second characteristic of the chordates 

 is the presence of an internal supporting 

 rod, or skeleton, the notochord. This may 

 be thought of as a precursor to the vertebral 

 column in vertebrates, but it must not be 

 considered identical. Although the noto- 

 chord is found in the embryos of all verte- 

 brates, it persists only in the adults of the 

 most primitive. The notochord is made up 

 of a gelatinous matrix, surrounded by a 

 tough, outer sheath, which is inadequate 



to support a large animal in water, much 

 less on land. In all higher forms, therefore, 

 it is replaced by the more rigid vertebral 

 column. As a change from an aquatic to 

 a terrestrial environment took place, pro- 

 vision for support had to be even more 

 elaborate, for tlie weight of the body for- 

 merly supported by buoyancy in the water 

 now had to be borne entirely by the skele- 

 ton. 



The third charateristic is the presence of 



