534 



HISTORY OF THE HUMAN BODY 



through two lateral rows of paired gill-slits, which are sup- 

 ported by a branchial skeleton very much like that of Am- 

 phwxus. 



This characteristic of the possession of pharyngeal gill- 

 slits is extremely significant, for nowhere else, except in ver- 

 tebrates and in their allies, above considered, do such organs 

 occur, and investigators have naturally been led through these 

 to seek for the other essential vertebrate characters of a noto- 



FIG. 147. Balanoglossus. [After Loos, in LEUCKART charts.] 



chord and a dorsal nervous system. In this search they have 

 been to a qualified extent successful, although these characters 

 are far from appearing with the same distinctness as in the 

 case of the gill-slits. The nervous system consists in general 

 of a diffuse net-work of nerve fibers lying in the depth of the 

 surface epithelium and occurring everywhere, a very low type 

 of nervous system. This net-work is, however, reinforced and 



