Dr. W. Salensky on Hiéickel’s. Gastreea Theory. 23 
In all other animals which pass through the Planula-form 
in their development, the developmental phenomena occur 
nearly in the following order :—After the Planula-stage the 
foundations of various external and internal organs, which 
appear in these animals as typical, persistent, or larval organs, 
are formed, e. g. the limbs, the shell, the velum, &c.; then the 
anterior intestine and the anus are invaginated; and finally 
the intestinal cavity is developed in the interior of the vege- 
tative lamella. This sequence I have endeavoured to represent 
by the already cited three stages in the development of the oyster 
(Pl. V. figs. 1-3). With regard to the invagination of the 
anterior intestine, I have already noted that it is a secondary 
phenomenon which cannot be compared to the so-called inva- 
gination of the exterior lamella of the Ascidia, Amphioxus, Ke. 
—that is to say, to the Gastrula-stage of these animals. The 
middle intestine, which corresponds to the stomachal cavity 
of the Gastrula (of the Coelenterata), is only developed in our 
cases when the typical organs are already formed and the 
middle lamella is differentiated. We may conclude as to the 
existence of the latter from statements derived from very 
thorough investigations (Huaxes, Tubifer, &c.). In these 
cases, therefore, no Gastrula-stage is formed. 
If we take the Vermes first into consideration, we have 
an example in Hwaxes, which has already been repeatedly 
mentioned. As the formation of the intestinal cavity in 
this animal has already been spoken of, I will only men- 
tion here that the invaginations for the mouth and anus 
(anterior and posterior intestine) are produced rather early. 
The Cheiopoda have also been referred to above. We have 
seen that in them also the ciliary bands and tufts are first 
formed, then probably the mouth is invaginated, and finally a 
stomachal cavity is produced in the interior. (See Claparéde 
and Mecznikow (oc. cit.) | 
With regard to the Mollusca, the statements of Lovén, 
Lacaze-Duthiers, Gegenbaur, and myself have already been 
mentioned. Although we have seen that the statements of 
different observers with respect to this type of animals differ 
somewhat from each other, and that the intestine originates 
from the exoderm according to some, and from the entoderm 
according to others, it is nevertheless probable that in most 
animals of this type the sequence of the developmental pheno- 
mena is accordant. If the development takes place in the 
same way as in the oyster, the shell, velum, and buccal inva- 
gination first appear, and it is only afterwards that the intes- 
tinal cavity makes its appearance. The development of the 
Pteropoda, Heteropoda, and Prosobranchiata (Calyptrea, 
