WORMS TO VERTEBRATES 73 



plan of structure than tlieirs must be sought if the 

 animal kingdom is to have a future. 



The future belonged to the vertebrate. To begin 

 with less characteristic organs the digestive system is 

 much like that of the annelid or schematic worm, but 

 with greatly increased glandular and absorptive surfaces. 

 The present mouth of nearly all vertebrates is prob- 

 ably not primitive. It is almost certainly one of the 

 gill-slits of some old ancestor of fish, such as now are 

 used to discharge the water which is used for respira- 

 tion. The jaws are modified branchial arches or the 

 cartilaginous or bony rods which in our present fish 

 support the fringe of gills. These have formed a pair 

 of exceedingly effective and powerful jaws. The re- 

 productive system holds still to the old type and shows 

 little if any improvement. The excretory organs, kid- 

 neys, are composed primitively of nephridial tubes 

 like those of the schematic worm or annelid, but im- 

 mensely increased in number, modified, and improved 

 in certain very important particulars. The muscles iii 

 simplest forms are composed of heavy longitudinal 

 bands, especially developed toward the dorsal surface 

 of the body to the right and left of the axial skeleton. 

 Locomotion was produced by lashing the tail right 

 and left, as still in fish. There is improvement in all 

 these organs, except perhaps the reproductive, but 

 nothing very new or striking. The great improve- 

 ment from this time on was not to be sought in the 

 vegetative organs, or even directly to any great extent 

 in muscles. 



The new and characteristic organ was not the verte- 

 bral column, or series of vertebrae, or backbone, from 

 which the kingdom has derived its name. This was a 



