150 ERNST HAECKEL. 



I am now about to attempt, in the following pages, 

 materially to improve that first genealogical sketch of the 

 natural system, and with the aid of the biogenetic principles 

 on the one hand, and the fundamental germ-lamella theory 

 on the other, to establish a theory, to which I attribute a 

 causal importance, for the natural system of the animal 

 kingdom, for the comprehension of the development of its 

 " types," and for the natural relationship of its main groups, 

 and which I will, briefly designate, in a word, as the Gastraea- 

 theory. The real purport of this Gastraea-theory depends on 

 the conception of a true homology of the primordial rudiment 

 of the intestine [Urdarm], and of the two primary germ- 

 lamellse in all animals except the Protozoa, and may be 

 briefly summed up in the following words : — " The entire 

 animal kingdom divides into two chief divisions : the older, 

 lower group of the Protozoa (Urthiere), and the younger, 

 higher group of the Metazoa(Darmthiere). The main group 

 of the Protozoa or Urthiere (animal Monera and Amoeba, 

 Gregarina, Acineta, Infusoria) always increases only by the 

 development of the animal individuality of the first or second 

 order (Plastide or Idorgan) ; the Protozoa never form germ- 

 lamellse, never possess a true intestinal canal, and, especially, 

 never develop a differentiated tissue ; they are probably of 

 polyphyletic origin, and branch ofi" from many different 

 primevally generated Monera. The main group of Metazoa, 

 or Darmthiere (the six races of Zoophyta, Vermes, Mollusca, 

 Echinodermata, Arthropoda, and Vertebrata) is, on the con- 

 trary, probably of monophyletic origin, and arises from a 

 single common root form, the Gastraea, which has sprung 

 from a Protozoan form ; it always multiplies by developing 

 the animal individuality of the third or fourth order (Person 

 or Cormus) ; the Metazoa always form two primary germ- 

 lamellse, always possess a true intestinal canal (a few retro- 

 graded forms only excepted), and always develop differen- 

 tiated tissues J these tissues always arise from the two 

 primary germ-lamellse only Avhich have been transferred as an 

 inheritance of the Gastraea of all the Metazoa, from the 

 simplest sponge up to the man. Next, the group of Metazoa 

 divided again into two sub-groups, first the Zoophyta (or 

 Coelenterata), which, in consequence of their habits in life, 

 form the so-called radiate type ; and, secondly, the Bilateria 

 (or Sphenota), which, in consequence of their crawling habits 

 of life, form the so-called " bilateral type." Among the 

 Bilateria, the lower worms (Acoelomi) agree with the 

 Zoophyta in the want of the coelom (body-cavity), and of a 

 circulatory system ; and then, again, from these primary 



