Dr. W. Saleiisky on f lackers Gastraia Theory. 13 



production of the intestine at a miicii later stage, when several 

 germ-lamella3 already exist, and the embr)'o already possesses 

 the characteristic organs of its type, or at least their founda- 

 tions. Why are we in these last cases to assume the Gastrida- 

 stage, when we can discover no traces of any thing of the kind ? 

 This could aid us in the comprehension of the develo])mental 

 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-lamella; — that is to say, from the GaMruJa. But we 

 can trace this gradual difterentiation only in the animals which 

 pass through a true Gastnila-stage {e. g. Amjjhioxus^ 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 Gastrcea (in many Vermes, Mol- 

 lusca, 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 intestine, 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- 

 <rM?a-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 Avhich have a Gasfrula-stage, 

 and yet it is not bound to a definite stage, t. e. to definite tem- 

 porary conditions of the embryo (as, for example, the existence 

 of two primary germ-lam ellaj). Secondly, we cannot expect 

 the Gastrnla-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 acoelous Turbellaria, which live under the same condi- 

 tions as the EhabdoccBla 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 

 Acoela *. Instead of the intestine these Turbellaria have a 



* Such as Convoluta, Schizoprora, Nadina, &:c. See 0. Schmidt, " Un- 

 tersuchung-en iiber Turbellarien von Corfu und Cephalonien " (Zeitschr. 

 fiir wiss. Zool. Bd. xi.) ; Claparede, * Beobachtung-en iiber Anatomie und 

 Entw, wirbello.«er Thiere ; ' and especially Ulianin, ' Turbellaria of the 

 Black Sea' (in Russian). 



