86 



THE EVOLUTION OF MAX. 



teresting points connecting this with Ascidians and the 

 Skull-less Animals (Acrania) 

 were first accurately observed 

 and explained by Gegenbaur. Al- 

 though this singular Balanoglossus 

 is in many other respects peculiar 

 in its organization, so that Gegen- 

 baur rightly ranked it as the re- 

 presentative of a special class 

 (Enteropneusta), yet the structure 

 of the anterior section of the in- 

 testinal tube is exactly similar to 

 that of Ascidians and Skull-less 

 Animals (&), a gill body, the walls 

 of which are pierced on either side 

 by gill-openings and are supported 

 by gill-arches. Now, although the 

 Acorn- worm in other points of its 

 structure may differ very con- 

 siderably from those extinct Soft- 

 worms (Scolecidce), which we must 

 regard as direct ancestors of our 

 race, and as intermediate links 

 between the Primitive Worms 



FIG. 186. A yonng Acorn-worm (Bal- 

 anoglossus). (After Alexander Agassiz.) 

 r, acorn-like proboscis ; h, collar ; k, gill 

 openings and gill-arches of the anterior in- 

 testine, in a long row one behind another 

 on each side ; d, digestive posterior intes- 

 tine, filling the greater part of the body- 

 cavity ; v, intestinal vessel, lying between 

 two parallel folds of skin j o, anas. 



