|______| _________|_________________________|______| | | | | | | __________| | | | | | | . | | | | t | Planaeada Acinetae. Ciliata. | | u | (Ontogeny : Planula). | | | | g | | \_________ _________/ | | > | | \/ | | i o | | Infusoria. | | / 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.