THE GASTRAEA-THEORY, ETC. 227 



selves by the shrinking apart of the two muscular layers. 

 The four highest groups of animals, the Echinodermata, 

 Arthropoda, Mollusca, and Vertebrata, are diverging descend- 

 ants of the four different forms of Coelomati. It is not difH- 

 cult to derive these four typical phyla from the common 

 root-grouj) of the worms. Their comparative anatomy and 

 ontogenesis still shows us, even now, that they have near 

 relatives among the Coelomati. The Annelida lead to the 

 Arthropoda and Echinodermata, the Bryozoa (?) to tlic Mol- 

 lusca, the Tunicata (Ascidia) up to the Vertebrata (vide 

 Lecture 18 in the ' Natiirliche Schopfungsgeschichtc). If 

 we wish to regard the coelom (which has originated by sepa- 

 ration of the animal and vegetative muscular layer) and the 

 cells which belong to it (ccelora-epithelia, lymph-cells, blood- 

 cells) in Jaeger's sense as representatives of a special fifth 

 layer, an intermediate fifth germ-lamella, we should have to 

 refer the Acoelomi only (Plathelminthes), and perhajjs a 

 portion of the Acalephae, to the Tetrablasteria. On the other 

 hand, all the animals provided with a ccelom (the Coelomati 

 and the four highest groups of animals) would form a special 

 fifth main group : Pentablasteria, with five germ-lamellae or 

 principal layers of tissues : — 1. Cuticular nervous layer. 2. 

 Cuticular muscular layer. 3. Coelom layer, or lymph 

 layer, vascular layer in a modified sense. 4. Intestinal 

 fibrous layer. 5. Intestinal glandular layer. 



An arrangement of these five principal groups of the animal 

 kingdom, with their known and generally accepted " types," 

 would yield the following results : 



1 Ablasteria ... 1 Protozoa . Protozoa Protozoa. 



2Diblasteria. . . 2 ( ^^^^^^.^^^^ • 7. , , 



(_ SpongiEe . > Zoophyta '^ 



3 Triblasteria . . 3 Acalephae . j i 



4 Tetrablasteria . . 4 Acoelomi . 7 Tr I 



rCoelomati . j Vermes j-Metazoa. 

 J Mollusca . ' 



5 Pentablasteria . 5 -i Ecbiuodermata 



J 



] Arthropoda ^ Tjpozoa 

 (^Vertebrata . J 



However attractive it may appear to us from a phylogenetic 

 point of view, to employ the number and differentiation of the 

 germ-lamellse in this manner as a basis for the classification 

 of the animal kingdom, yet on a closer examination important 

 obstacles present themselves, which do not justify the strict 

 carrying out of this principle of division. Independently of 

 the fact that we do not yet know the ontogenesis of many 

 animals (especially of the lower orders) at all sufficiently, 

 there are intermediate transitional forms between the five 



