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 cœlom) and blood in their interior. It is difficult to say which of the still living Cœlomati 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 moreover have been represented by several very different intermediate 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 Cœlomati, out of which the most ancient skull-less Vertebrata were directly developed. Among the Cœlomati of the present day, the _Ascidians_ are the nearest relatives of these exceedingly remarkable Worms, which connect the widely differing classes of Invertebrate and Vertebrate animals. That the ancestors of man really existed during the primordial period in the form of these Himatega, is _distinctly proved_ by the exceedingly remarkable and important agreement presented by the ontogeny of the Amphioxus and the Ascidia. (Compare Plates XII. and XIII., also pp. 152, 200, etc.) From this fact the earlier existence of Sack Worms may be inferred; they of all known worms were most closely related to our recent Tunicates, especially to the freely swimming young forms or larvæ of the simple Sea-squirts (Ascidia, Phallusia). 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 larvæ of the Ascidia. The form value of this stage nearly corresponds with that which the larvæ of the simple Sea-squirts possess at the time when they show the beginning of the dorsal marrow and spinal rod. (Plate XII. Fig. _A 5_: compare the explanation of these figures in the Appendix.)