Dr. W. Salensky on Héckel’s Gastrea Theory. 13 
production of the intestine at a much later stage, when several 
germ-lamelle already exist, and the embryo already possesses 
the characteristic organs of its type, or at least their founda- 
tions. Why are we in these last cases to assume the Gastrula- 
stage, when we can discover no traces of any thing of the kind ? 
This could aid us in the comprehension of the developmental 
processes only if we could derive these instances of the later 
formation of the intestinal cavity through a series of transitions 
from the stage which possessed a primitive intestine and had 
two germ-lamelle—that is tosay, from the Gastrula. But we 
can trace this gradual differentiation on/y in the animals which 
pass through a true Grastrula-stage (e. g. Amphioxus, the Asci- 
dia, &c.). In most others we cannot bring the embryonal pro- 
cesses into connexion with the Gastrula, we cannot regard 
them as dependent upon the G'astrea (in many Vermes, Mol- 
lusea, Arthropoda, and most Vertebrata). This shows at once 
that the Gastrula-stage is proper only to a few animals, and 
does not occur in the others; and these other animals pass 
through their embryonal development, their subsequent dif- 
ferentiation of the imtestine, in a somewhat different manner 
from the former. Can such a form be regarded as the stock- 
form of all the Metazoa? At least we have no facts in proof 
of this assertion. 
On theoretical grounds we cannot expect to find the Gas- 
trula-stage universally diffused :—in the first place because the 
intestinal cavity is developed in different animals at different 
periods of their development; but this intestine is the same 
as the intestine of those animals which havea Gastrula-stage, 
and yet it is not bound to adefinite stage, 7. e. to definite tem- 
porary conditions of the embryo (as, for example, the existence 
of two primary germ-lamelle). Secondly, we cannot expect 
the Gastrula-stage to be universally diffused, because there 
are animals which never arrive at the development of an in- 
testinal cavity. I do not refer to the parasites which have 
lost their intestinal cavity in consequence of retrogressive me- 
tamorphosis, although this loss cannot be regarded as ontoge- 
netically proved in all parasites (e. g. in the Cestodea). I refer 
to the accelous Turbellaria, which live under the same condi- 
tions as the Rhabdoccela and Dendroccela, which move in the 
same manner as these and yet possess no intestine. Ulianin 
has with perfect justice separated them from the others as 
Aceela*. Instead of the intestine these Turbellaria have a 
* Such as Convoluta, Schizoprora, Nadina, &c. See O. Schmidt, ‘‘ Un- 
tersuchungen iiber Turbellarien von Corfu und Cephalonien” (Zeitschr. 
fiir wiss. Zool. Bd. xi.) ; Claparéde, ‘ Beobachtungen iiber Anatomie und 
Entw. wirbelloser Thiere ;’ and especially Ulianin, ‘Turbellaria of the 
Black Sea’ (in Russian). 
