PROGENITORS OF MAN. 283 



Seventh Stage : Soft Worms (Scolecida). 



Between the Turbellaria of the preceding stage and 

 the Sack Worms of the next stage, we must necessarily 

 assume at least one connecting intermediate stage. For the 

 Tunicata, which of all known animals stand nearest to the 

 eighth stage, and the Turbellaria which most resemble the 

 sixth stage, indeed both belong to the lower division of the 

 unsegmented Worms ; but still these two divisions differ 

 so much from one another in their organization, that we 

 must necessarily assume the earlier existence of extinct 

 intermediate forms between the two. These connecting 

 links, of which no fossil remains exist, owing to the soft 

 nature of their bodies, we may comprise as Soft Worms, or 

 Scolecida. They developed out of the Turbellaria of 

 the sixth stage by forming a true body-cavity (a coelom) 

 and blood in their interior. It is difficult to say 

 which of the still living Coelomati are nearest akin 

 to these extinct Scolecida, it may be the Acorn-worms 

 (Balanoglossus). The proof that even the direct ancestors 

 of man belonged to these Scolecida, is furnished by the 

 comparative anatomy and the ontogeny of Worms and of 

 the Amphioxus. The form value of this stage must more- 

 over have been represented by several very different inter- 

 mediate stages, in the wide gap between Turbellaria and 

 Tunicata. 



Eighth Stage : Sack Worms (Himatega). 



Under the name of Sack worms, or Himatega, we here 

 allude in the eighth place to those Coelomati, out of which 

 the most ancient skull-less Vertebrata were directly devel- 

 oped. Among the Coelomati of the present day, the Ascidians 



