THE GASTllAliA-TIIEOllVj ETC. 235 



sponges to the vcrtebrata, every wlicre honiolofi^ous, and every- 

 where is derived directly from the entoderm of the gastrula. 

 To be sure, Kowalevsky has lately arrived at the opinion 

 that the intestinal glandular layer of insects forms an excep- 

 tion, and is much rather to be regarded as a special new 

 formation sui generis (' Embryologische Studien an Wiir- 

 mern,' 1871, p. 58). This view seems to me untenable. If 

 any organ can be homologous in all six phyla of the Metazoa, 

 it is certainly the intestinal canal, with its outer covering 

 epithelium, the intestinal glandular layer. On the other 

 hand, the question of the homology of the openings of the 

 intestine, the mouth, and anus, is at present still quite ob- 

 scure, and so much only is certain that the opening of the 

 mouth is not always the same. The original oral opening 

 of the gastrula, the rudimentary mouth or the Prostoma, 

 seems only to have descended to the Zoophyta, and, per- 

 haps, to a part of the Vermes. It, nevertheless, seems to 

 reappear in the Rusconian anus of the Vertebrata. On the 

 other hand, the oral openings of the Vertebrata, the Arthro- 

 poda, and the Echinodermata, are peculiar new formations, 

 and certainly not homologous with the rudimentary mouth. 



7. The Phylogenetic Signification of the Onto- 

 genetic Succession of the Systems of Organs. 



The regularly graduated series in which the system of organs 

 appear one after another in the different groups of animals 

 during ontogenesis, furnishes us with a sure key, according 

 to the biogenetic principle, to the historical series in which 

 the animal systems of organs have developed themselves after 

 each other and from each other, during the long and slow 

 course of the organic history of the earth. This palseonto- 

 logical seniority of the systems of organs, as it is empiri- 

 cally found a posteriori from the facts of ontogenesis, com- 

 pletely anticipates on the whole the demonstrations which 

 could be formed on the subject d priori by physiological 

 reflection and by philosophical consideration of forces at work 

 (Causal-Momente). 



In the first place, it follows from the comparison of the 

 gastrula, and of the bilamellar cell-condition which represents 

 it in the most dissimilar groups of animals, that two primary 

 systems of organs, the inner intestinal system, and the outer 

 tegumentary system, are simultaneously differentiated in 

 the first series, in the oldest Metazoa, the Gastraeada. The 

 original and perfectly simple stomachic cavity or primary 

 intestine of the Gastraea is, indeed, the oldest organ of the 



VOL. XIV. NEW SER. Q 



