rom Scolecida by formation of
dorsal nerve-tube and chorda, and resembling tailed
larvae of Ascidians.)
4. Acrania. (With metameric segmentation. Including
Amphioxus.)
5. Monorrhina. (Cyclostomes.)
6. Selachia.
7. Dipneusta.
8. Sozobranchia. (Amphibia with permanent gills.)
9. Sozura. (Tailed Amphibia.)
*10. Protamnia.
*11. Promammalia.
12. Marsupialia.
13. Prosimiae.
14. Menocerca. (Tailed apes.)
15. Anthropoides.
16. Pithecanthropi.
17. Homines.

It will be noticed that except for the hypothetical forms (marked with
an asterisk), which are themselves generalised classificatory groups,
the ancestral forms belong to long-recognised classes. The whole course
of the evolution follows well-worn systematic lines. This is typical of
Haeckel's phylogenetic speculations.

A more abstractly morphological scheme of the evolution of Vertebrates
is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and
ancestral stages are arranged in parallel columns thus:--

Cytula. Cytaea (Protozoa).
Morula. Moraea (Coenobium of Protozoa).
Blastula. Blastaea (_Volvocina_, etc.).
Depula (invaginated blastula). Depaea.
Gastrula. Gastraea (cf. _Olynthus_, _Hydra_, and
primitive Coelentera).
Coelomula (with one pair Coelomaea (cf. _Sagitta_, _Ascidia_,
of coelom-pockets). and primitive Helminthes).
Chordula (with medullary Chordaea (_cf._ Ascidian larva and
tube and chorda). larva of Amphioxus).
Spondula (with segmented Prospondylus (Primitive Vertebrate).
mesoderm).

This scheme differs from the earlier one chiefly in taking into account
certain advances, notably as regards the cytology of the fertilised ovum
and the true nature of the coelom, which had been made in the interval
of some twenty years.

Haeckel's Gastraea theory, though it exercised a great influence upon the
subsequent trend of phylogenetic speculation, was by no means
universally accepted _telle quelle_. Opinions differed considerably as
to the primitive mode of origin of the two-layered sac which was very
generally admitted to be of constant occurrence in early embryogeny. Ray
Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a
"Planula" theory, according to which the ancestral form of the M