228 ERNST HAECKEL. 



groups mentioned, which admit of no sharp division, and, 

 moreover, cases occur in the lower phyla of the Metazoa, in 

 which nearly related forms of one stock must be placed 

 in different groups of Blasteria. Although most of the 

 Acalephai (Hydromedusge, Ctenophorse, Corals) are j)robably 

 blasteria, yet Diblasteria are among their lower forms 

 (Hydra), and probably many Tetrablasteria are among their 

 higher forms. Among the Acoelomi (Plathelminthes) pro- 

 bably many Triblasteria, or even Diblasteria, may be found 

 among the predominating Tetrablasteria forms ; and so in 

 other cases. 



On these and other grounds it appears much more preferable 

 to employ only characters drawn from the phylogenesis of the 

 Metazoa as the leading principle for their further division, in 

 which the stereometric (radial or bilateral) essential form of 

 the parts of the body plays a decisive part. The further 

 development of the gastrula here appears next defined. 

 Following this I have already arrived at the opinion (in the 

 ' Biology of Calcareous Sponges ') that the descendants of 

 the Gastraea, as the common root-form of all the Metazoa, 

 subsequently divided into two branches, the Protascus, which 

 is to be regarded as the root-form of all the Zoophyta, and the 

 Prothelmis, which is to be regarded as the common root-form 

 of all the five higher groups of animals. The division of 

 these two principal branches is quite mechanically dependent 

 on the two difi'erent modes of life to which the descendents of 

 the monaxial (neither "radiate" nor "bilaterar^) Gastraea first 

 adapted themselves. The one group resigned the freely 

 moving habits of the swimming Gastraea, attached itself by 

 the pole of the axis of its body opposite to its mouth, and 

 then developed eo ipso further into the so-called "radiate type" 

 (Zoophyta). The other grouj) of the descendant of the 

 Gastraea retained the power of moving freely from place to 

 place, proceeded from the swimming method of moving to 

 creeping on the sea-bottom, and developed eo ipso into the 

 so-called " bilateral type " (the five higher groups of animals. 

 Vermes and Typozoa). I therefore regard only on the one 

 side the fixed habits of life in the root-form of the Zoopdiyta 

 (Protascus) as the mechanical " acting cause " of their radiate 

 type, or, more correctly expressed, of their actinote (regularly 

 pyramidal) essential form ; and, on the other side, the creep- 

 ing habits of life in the root-form of the worms (Prothelmis) 

 as the mechanical causa efficiens of its bilateral type, or, more 

 correctly expressed, of its dipleural (amphithect- pyramidal) 

 fundamental form. This has been inherited from the worms 

 by the four highest stem-groups of animals. 



