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 
unseomented 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 ccelom) 
and blood in their interior. It is difficult to say 
which of the still living Ccelomati 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, 
E1cHTH SracE: Sack Worms (Himatega). 
Under the name of Sack worms, or Himatega, we here 
allude in the eighth place to those Ccelomati, out of which 
the most ancient skull-less Vertebrata were directly devel- 
oped. Among the Ccelomati of the present day, the Ascidians 
