THE GASTRAEA-THEORY, ETC. 229 



On the ground of this phylogenetic consideration Ave can 

 class together the whole of the originally bilateral descendants 

 of the Gastraca (the successors of Prothelmis) in a natural 

 main group, -which we will briefly designate Bilateria or 

 Sphenota(" wedge-animals," on account of their wedge-shaped 

 essential form in the sense of Bronn). This group includes 

 all the worms and the four highest groups of animals derived 

 from them ; the Mollusca, Echinodermata, Arthropoda, and 

 Vertebrata.i 



6. Signification of the Gastraea-Theory in respect 

 TO THE Homology of Types. 



By comparing the germ-lamellse in the different groups of 

 animals we are led to the important question, how far the 

 organs and systems of organs in general are capable of a 

 morphological comparison in the seven phyla of the animal 

 kingdom, and how far a true homology in the strictest sense 

 (i. e. homophyly) is to be carried out between them ? Those 

 who maintain Baer's and Cuvier's doctrine of types in its 

 original rigid sense, and consid(;r all the types of the animal 

 kingdom as perfectly separated morphological units, must 

 naturally answer this question generally in the negative. 

 Those, t)n the other hand, Avho regard the theory of types in 

 the light of the theory of descent, and those who admit the 

 moditication of it, which we have attempted here by the 

 Gastraea-theory, as Avell as the generalisation of the germ- 

 lamellae theory Avhich depends upon it, must, to a certain 

 extent, agree to sucli a morphological comparison. In fact, 

 Gegenbaur^ has recently expressed himself in this sense, 

 and KoAA'alevsky ^ also in his latest AA'ork. 



Although this question about the homologies of the groups 

 of animals is extremely important and interesting for com- 

 parative anatomy and phylogenesis, yet its positive solution 

 seems difficult and entangled in the present imperfect 



' In all the Vertebrata, Annulosa, and Mollusca, the dipleiiral or bilateral 

 essential form is just as undisputed as in the Vermes. But the root-lorm 

 of the Echinodermata possesses also the same ruudainentalform. According 

 to our theory of Echiuodermata we consider as sucii tlie arlicuhited worm- 

 person which has still preserved most of its independence in the " Arm" of 

 the Asterida. The radiate form of the developed specimens of Ecliinoder- 

 mata (star-shaped Cormi, composed of five or more Persons), therefore forms 

 just as little of an objeeliou as the radiate form of specimens of the Synas- 

 cidian stock (Botryllus). 



2 Gegenbaur, ' Grundziijije der vergl. Anatomic,' ed. 2, p. 82. 



=" Kowalevsky, ' Embryologische Studien an Wiirmern und Artliropodcn, 

 1S71, conclusion. 



