57 



and others on the contrary regard the Chordata as being 

 derived from some group of the Annelida, chiefly on account 

 of the similarity in their nephridia. I prefer to look upon 

 the Proto-Chordata as having arisen from a more generalised 

 form than either the Nemerteans or the Annelides. Their 

 point of origin was certainly very far back, and may have 

 been even lower down on the vermean axis than its position 

 in the table. From the striking resemblance between the 

 respiratory organs of Balanoglossus and those of some of the 

 lower Chordata it is extremely probable that the Proto- 

 Chordata started from ancestral Enteropneusta, and it has 

 been already seen (page 40) that the Enteropneusta and the 

 Echinodermata probably possessed a common ancestor out- 

 side the vermean series, consequently the Proto-Chordata, 

 the Enteropneusta, and the Echinodermata most probably 

 diverged from the main axis together as a branch which very 

 soon gave off the first ancestors of the Chordata in one 

 direction and the primitive Echinoderms in another. 



The early Proto-Chordata had probably a worm-like seg- 

 mented body (fig. 19), with a ventrally placed mouth in front 



Fig. 19. Hypothetical ancestor of the Proto-Chordata. a, anus ; b, anterior 

 respiratory part of the alimentary canal; c, notochord ; d, dorsally placed nervous 

 system ; g, supra-oesophageal ganglion forming part of brain ; i, intestine ; m, mouth ; 

 n, nephridium ; n^, modified anterior nephridia (hypophysis cerebri?) ; p, front end 

 of body formed of prae-oral lobe ; s, one of the segments or metameres. 



of which projected a large prae-oral lobe. The anterior 

 portion of the alimentary canal had become respiratory in 

 function by giving off a series of lateral diverticula which 

 were placed in communication with the exterior by a series of 

 vertical slits on the side of the body. Renal organs were 

 present in the form of paired nephridia, placing the coelom in 



