154 ERNST HAECREL. 



Gastraea-theory builds up a new system on the basis of phy- 

 logenesis, the fundamental principles of which, as regards 

 classification, are the homologies of the germ -lamellae, and 

 the primordial rudiment of an intestine ; and, secondarily, 

 the diiferentialism of the tranverse axis, and of the coelom. 



But the Gastraea-theory may attain to great importance 

 by this fundamental remodelling of the system of zoology, as 

 it is the first attempt to lead to a casual knowledge of the 

 most important morphological relations, and the principal 

 typical differences in the structure of animals, as well as to 

 discover the historical sequence in the origin of the animal 

 organization. Inheritance and adaptability here appear in 

 tbeir full light as modifying agents, and as the only two for- 

 mative factors of the organic relations of form. Inheritance 

 and adaptability are the only " two mechanical causes," with 

 the help of which the Gastraea-theory explains the origin of 

 the leading natural groups of the animal kingdom, and the 

 characteristic relations of their organizations. 



3. The Phylogenetic Signification of the two 

 Primary Germ-lamella. 



The individual developmental form of the animal kingdom, 

 by the general distribution of Avhich the Gastraea-theory 

 next supports itself, is the Gastrula (PL VII, figs. 1 — 8). In 

 the ' Biology of the Calcareous Sponges ' I have applied this 

 name to that very early stage of development in which the 

 embryonic animal-body exhibits the simplest conceivable 

 form of entity : a uniaxial segmented hollow body without 

 appendages, the simple cavity (primitive intestine) opens on 

 one pole of the axis by an orifice (primitive mouth) , and the 

 body-wall consists of two cellular membranes or lamellae ; 

 entoderm or gastral-lamella, and exoderm or dermal-laraella.i 



The gastrula is the most important and suggestive em- 

 bryonic form in the animal kingdom. The extreme sig- 

 nificance which I attach to it is, firstly, supported by its 

 recurrence in animals of the most different groups, from the 

 sponges to the vertebrata in the same characteristic form and 

 arrangement ; and, secondly, because the morphological and 



* About the right comprehension of the individuality of the entity (as of 

 the morphou, or of the morphological individual of the third order), compare 

 my 'Biology of the Calcareous Sponges,' p. 113 ; about the right notion of 

 the gastrula, compare 1. c, p. 333. Our gastrula is identical in many 

 respects with the embryoaic animal form, which was formerly called 

 planula; but in many other respects the so-called " Planula " is a very 

 differently constituted body. 



