r secretion (kidneys), and generation (sexual organs)--this branch is
the prothelmis, the common primary worms (vermes). Like the turbellaria
of the present day, the whole surface of their body was covered with
cilia, and they possessed a simple body of an oval shape, entirely
without appendages. These acoelomatous worms did not as yet possess a
true body cavity (coelom) nor blood. No member of the next higher
animals are in existence, neither are there any fossil remains, owing to
the soft nature of their body. They are therefore called soft worms, or
scoleceda. They developed out of the turbellaria of the sixth stage by
forming a true body cavity (a coelom) and blood in their interior. The
nearest still living coelomati is probably the acorn worms
(balanoglossus). The form value of this stage must, moreover, have been
represented by several different intermediate stages.

Out of the four different groups of the worm tribe, the four higher
tribes of the animal kingdom were developed--the star-fishes
(echinoderma) and insects (arthropoda) on the one hand, and the molluscs
(mollusca) and vertebrated animals (vertebrata) on the other. Out of
certain coelomati, the most ancient skull-less vertebrata were
directly developed. Among the coelomati of the present day, the
ascidians are the nearest relatives of this exceedingly remarkable worm,
which connect the widely differing classes of invertebrate and
vertebrate animals. To these animals have been given the name of
sack-worms (himatega). They originated out of the worms of the seventh
stage by the formation of a dorsal nerve marrow (medulla tube), and by
the formation of the spinal rod (chorda dorsalis) which lies below it.
It is just the position of this central spinal rod or axial skeleton,
between the dorsal marrow on the dorsal side and the intestinal canal on
the ventral side, which is most characteristic of all vertebrate
animals, including man, but also of the larvae of the ascidia.

We now come to the second half of the series of human ancestors. The
skull-less animal lancelet, which is still living, affords a faint idea
of the members of this group (acrania). Since this little animal, in its
earliest embryonic state, entirely agrees with the ascidia, and in its
further development shows itself to be a true vertebrate animal, it forms
a direct transition from the vertebrata to the invertebrata.

[Illustration: FIG. I.--Appendicularia, seen from the left side, _m_