THE ANCESTRY OP THE CHOKDATA. 95 



mesoblast already arose thus^ and when new bundles of 

 muscles formed in the adult began to arise in the larva they 

 arose in the same manner as the primitive mesoblast. That 

 provision is made for the production of inore mesoblast than 

 that of the original fourteen pairs of pouches is shown by the 

 presence of mesoblastic pole-cells in Amphioxus (Hatschek). 

 In any case the existence of Balanoglossus proves that the 

 notochord, gill-slits^ and Chordate nervous system were present 

 together before the myotomes were formed. 



The Grill- slits. — It is unfortunate that the facts of the 

 Enteropneusta seem to throw no new light on the original 

 meaning of gill-slits. That they do not do so tends, however, 

 to show that probably gill slits were from the first developed 

 as such, and not as modifications of any previously-existing 

 organ, as has been sometimes held. 



The folded skeletons of the gill-slits of Balanoglossus are re- 

 markable in their resemblance to those of Amphioxus. Until the 

 development of these latter is fully known no further com- 

 parison can be instituted. It is clear from their origin in 

 Balanoglossus that no "myotomes" are obliterated between 

 them (as has been suggested by some, with the hope of in- 

 creasing the symmetry of the body), for plainly their repeti- 

 tion preceded that of the myotomes. 



The Excretory System. 



Upon the origin of the excretory system of Vertebrata 

 nothing can be affirmed from a study of Balanoglossus. The 

 excretory systems of Vertebrata cannot be easily derivable 

 from anything found in either Balanoglossus, Ascidians, or 

 Amphioxus. The absence of any regular excretory system in 

 these three forms may, perhaps, be correlated with the extraor- 

 dinary development of their respiratory systems, which may 

 possibly assist in this function. The one fact which is de- 

 rivable from the morphology of Balanoglossus, Ascidians^ and 

 Amphioxus, is that it is nearly certain that the excretory 

 system of other Chordata has been developed within the group. 



The Pituitary Body and Proboscis Pore. — Though 



