84 THE EVOLUTION OF MAK. 



spoken, and the tailed Ascidian larva. We will for the 

 present call this kind of Worm, which was primarily dis- 

 tinguished by the possession of a notochord, the Chorda- 

 animal (Chordonium). The Ascidians on the one hand, and 

 the Vertebrates on the other, developed, as two diverging 

 branches, from these Chorda-animals. The common parent- 

 form of the Chorda-animals themselves was a coelomate form, 

 which finally must have descended from the Acoelcmi, and 

 from the Archelminthes. 



Many connecting intermediate forms must, of course, have 

 existed between these two groups of Worms, between the 

 Primitive Worms and the Chorda-animals. Unfortunately, 

 however, zoological knowledge is at present especially im- 

 perfect with regard to these important intermediate forma 

 of the multiform Worm tribe. For very evident reasons, 

 none of these Worms could leave fossil remains. For, like 

 the great majority of other Worms, they had no hard parts 

 in their bodies. Most even of the known fossil Worms 

 are worthless, for they tell us little or nothing of the most im- 

 portant structural features of the soft body. Fortunately, 

 however, we can in great measure satisfactorily fill the con- 

 siderable palseontological gap in this part of our pedigree, 

 with the help of the Comparative Anatomy and Ontogeny of 

 Worms. If, on the one hand, we examine the structure and 

 mode of development of the lower Worms from the Gliding- 

 Worms (Turbellaria), and, on the other hand, the Anatomy 

 and Ontogeny of the Ascidians, it is not difiicult, step by 

 step, to re-construct in imagination the connecting inter- 

 mediate forms, and to insert a series of extinct ancestral 

 forms between the Acoelomi and the Chordonia. This 

 series of forms under the name of Soft-worms {Scolecida) 



