4 Dr. W. Salensky on Haéickel’s Gastreea Theory. 
(hydroid polyps, sponges), even in their developed state, di- 
verge very little from the Gastrea form. But even in this 
animal type the Ctenophora are distinguished from the other 
Ceelenterata by some very important embryological pheno- 
mena, since in them, according to the well-known investiga- 
tions of Kowalevsky*, the gastrovascular system is first pro- 
duced from the exoderm in the form of a solid cellular cord or 
of a cylinder, which only acquires a cavity at a later period, 
after the meridional rings are indicated. The very important 
objection which this case offers to the Gastrwa theory consists 
in the fact that it cannot be referred to either of the two modes 
of development of the Gastrula indicated by Hiickel, and that 
here no Gastrula-stage exists. 
Vermes.—‘ In the stock of the Vermes the Gastrula (the 
so-called ‘ infusoriiform embryo ’) occurs sometimes in exactly 
the same, sometimes in a more or less modified form in the 
Platyelmintha (Turbellaria and Trematoda), in the Nematelmia 
(Nematoda, Sagitta), and in the Bryozoa, Gephyrea, and An- 
nelida (Phoronis, Huaxes, Lumbricus, Cheetopoda).”’ 
From the fact that embryos of very different organization 
are comprised under the so-called ‘ infusoriiform embryos,” 
we may assert & prior? that these embryos are like the Gas- 
trula mm some cases and different from it in others. Such 
differences often occur between the embryos of one and the 
same class of Vermes, as, for example, between the various 
Trematoda. In some of the digeneous T'rematoda, the mouth 
and intestine have been demonstrated in the embryonic state ; 
in others (and indeed in the majority) they have not. In the 
subsequent stages of development, as is well-known, the redia 
are distinguished from the sporocysts by these characters [. 
The development of the monogeneous 'l'rematoda is so little 
known that we are not at present in a position to say, from the 
ascertained facts, whether or not a stage resembling the Gas- 
trea occurs in these animals. The most complete investiga- 
tions in this direction, namely those of E. van Beneden f, 
Zeller §, and Willemoes-Suhm ||, furnish so little information 
as to the embryonic history of these Trematoda, that we only 
learn from them the fact that the animals on escaping from 
the egg already possess all their organs (except the sexual 
organs). 
* Mém. de l’Acad. Imp. de St. Pétersb. tom. x. 
+ Leuckart, ‘Die menschlichen Parasiten,’ Bd. i. p. 491. 
{ “Recherches sur la composition et la signification de l’ceuf,” Mémoires 
couronnés de l’Acad. Roy. de Belg. tom. xxxiv. 
§ Ibidem. 
|| Zeitschrift fir wiss. Zoologie, Bd. xxii. 
