Dr. G. Biitschli on the Gastreea- Theory. 381 
although upon other grounds, decide in favour of the primi- 
tiveness of the first form. We come to this decision the more 
readily because we have already indicated that the so-called 
parenchymula appears to us, as a primitive form, to offer great 
difficulties as regards its explanation. 
We must not close this discussion without pointing out 
that, in considering these circumstances, we have purposely laid 
no stress upon the ontogeny of the Sponges. In this respect 
my ideas are directly the reverse of those of Metschnikoff, to 
whom the ontogeny of the Sponges appears to be especially 
favourable to his parenchymula theory. As [ am of opinion 
that the group of the Sponges is completely shut off from 
the rest of the Metazoa, proceeding quite independently from 
the section Choanoflagellata (Savile Kent), it appears to me 
incorrect to bring this group into consideration in the expla- 
nation of the phylogenesis of the other Metazoa. 
Finally there still remains for discussion one circumstance 
which may perhaps give essential support to our hypothesis, 
namely, the quite recent discovery of an organism which in 
many respects fulfils the requirements which we must lay 
upon the hypothetical, tabulitorm, bilamellar primitive stage, 
our placula. 
This organism is the singular marine Trichoplax adherens 
lately described by F. E. Schulze*. Although the life- 
history and especially the reproduction of this form are not 
thoroughly elucidated, it nevertheless appears certain to me, 
as well as to Schulze, that it is a mature, fully-developed form, 
and not a larva. This Zrichoplax, then, would in every re- 
spect form a representative of our placula, if it had not already 
made an advance towards a higher development, inasmuch as 
it forms, not a bilamellar, but a trilamellar plate. Between 
the entoderm, occupying the lower surface, which resembles 
a cylinder-epithelium, and the thin flat-celled ectoderm cover- 
ing the upper surface, there is interposed a connective-like 
layer, no doubt proceeding from the entoderm, and which is 
comparable to amesoderm. For my part, I regard the com- 
parability of the tissue-layers of this Zrichoplax with the 
germinal layers of the Metazoa, already indicated by F. E. 
Schulze, as exceedingly probable, except as regards a direct 
homology of the intermediate so-called mesoderm, which is 
rather to be esteemed an independent analogous formation. 
However, I regard it as very probable that Zrichoplax 
adherens forms one of those transitional forms towards the 
higher Metazoa, constructed in accordance with the gastrula- 
* Zool. Anzeiger, Jahrg. vi. no. 132 (1883), p. 92, 
