330 ERNST HAECKEL. 



condition of morphology. I therefore lay no more stress on 

 the following explanation than that of a provisional attempt. 

 The phylon of the Protozoa is naturally entirely excluded 

 from this consideration, as, according to our previously ex- 

 pressed opinion, no animal of this root-group rises to the 

 formation of germ-lamellae, and therefore the organs de- 

 veloped from the latter are also completely absent in the 

 Protozoa. We therefore, for instance, consider any morpho- 

 logical comparison of any part of the body of an infusorium 

 with an apparently representative (and physiologically, 

 perhaps, equally important, and therefore analogous) portion 

 of a germ-lamellar animal as quite inadmissible. As I have 

 already shown in an essay " On the Morphology of the 

 Infusoria," the intestine of the Ciliata can, for instance, be 

 looked upon as such and compared with the intestine of the 

 Metazoa. But in a morphological aspect these parts cannot 

 generally be comj)ared at all. The intestine of the Ciliata is 

 but a portion of a single highly differentiated cell ; the in- 

 testine of the Metazoa is a cavity enclosed by the many-celled 

 inner germ-lamellae. Homologies can only exist between 

 the six stem-groups of the Metazoa, which are all derived 

 from the Gastraea. 



As the most certain and universal homology which is 

 applicable throughout the whole series of Metazoa (from the 

 sponges to the vertebrates), we may take the comparison of 

 those organs which are already differentiated in the simplest 

 Metazoa (the Gastraeada and the lowest sponges), and which 

 persist in them throughout their lives in their simplest con- 

 dition ; that is, firstly, the primitive intestinal canal with its 

 epithelium (the intestinal glandular layer, the entoderm of 

 the gastrula) ; and, secondly, the most superficial covering 

 of the body (the cuticular layer or the epidermis, the exo- 

 derra of the gastrula) . With reference to this latter, it is 

 expressly to be noticed that, indeed, the originally complete 

 homology of the epidermis in the six phyla of the Metazoa 

 may be unsatisfactory and frequently disturbed, in conse- 

 quence of earlier commenced cuticular processes, by which 

 the original outer epidermis layer is changed or stripped off 

 into a transitory embryonal covering (as in Hydra, Kleinen- 

 berg), but that none the less the epidermis constantly retains 

 at least a layer of cells, and serves as a foundation for the 

 others, consequently the epidermis, as a whole, and as a 

 derivative of the simple exoderm of the gastrula, is homo- 

 logous in all the six stem-groups of the Metazoa.^ 



' The formation of many embryonal coverings, which arise ontogenetically 

 from the uppermost germ lamella (the horny layer), is perhaps to be explained 



