|
| / n | | | |
| < | | | |
| a , | Synamoebae Gregarinae | |
| o s | (Ontogeny : Morula). | | |
| z r | | | | |
| o e | | \_____ ______/ |
| t y | | \/ |
| o a | | Amoebina. |
| r l | | | |
| P | \____________ _____________/ |
| > m | \/ |
| i r | _Amoebae_ ? ? ? |
| < e | (Ontogeny : Ovulum). | | | |
| g | | | | | |
| | | | | | |
| o | _Monera_ Monera. |
| N | (Ontogeny : Monerula). |
| | |
|______|______________________________________________________________|

The scheme is in many respects an interesting and important one. The
great contrast between the Protozoa, or animals with neither gut nor
germ-layers, and the Metazoa, which possess both structures, is for the
first time clearly brought out. The derivation of all the Metazoa from a
single ancestral form, the Gastraea, leads to the conclusion that the
types are not distinct from one another as Cuvier and von Baer supposed,
but agree in the one essential point, in the possession of an
_archenteron_ (Lankester, 1875), and an ectoderm and endoderm which are
homologous throughout all the Metazoan phyla. Finally, in the separation
of the sponges, Coelenterata and Acoelomi as animals lacking a body
cavity or coelom[435] from the four higher phyla, which are essentially
Coelomati, there is contained the germ of a conception which later
became of importance.

Somewhat similar views as to the importance of the germ-layer theory for
the phylogenetic classification of animals were published by Sir E. Ray
Lankester in 1873.[436] He distinguished three grades of animals--the
Hom