4 Dr. W. Salciisky o/i Ifackers CKastiJva Theory. 



(liydroid ])olvp.>5, sponges), even in their developed state, di- 

 verge very little from the Gastra'a t'orni. But even in this 

 animal type the Ctenophora are distinguishocl from the other 

 Coclenterata by some very important embryological ])lieno- 

 nicna, since in them, according to the well-known investiga- 

 tions of Kowalevsky*, the gastro vascular 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 imj)ortant 

 objection which this case otlers to the Ciastra'n theory consists 

 in the fact that it cannot be referred to either of the two modes 

 of develo])ment of the Gastrida indicated by Hiickel, and that 

 here no Gastrula-stage exists. 



Vermes. — " In the stock of the Vermes the GastruJa (the 

 so-called ' infusoriiform embryo ') occurs sometimes in exactly 

 the same, sometimes in a more or less modified form in the 

 Platyelmintha (TiirbcUaria and Trematoda), in tlie ^sematclmia 

 (Nematoda, Sagifta), and in the Bryozoa, Gejihyrea, and An- 

 nelida {Phoronis^ Eua.ces, L2i}iibriciiSj Ch^etopoda)." 



From the fact that embryos of very different organization 

 are comprised under the so-called " infusoriiform embryos," 

 we may assert h jrriori that these embryos are like the G(is- 

 trula in 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 Trematoda, 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 f- 

 The development of the monogeneous Trematoda is so little 

 known that we are not at present in a position to say, from the 

 ascertained facts, Avhether or not a stage resembling the Gas- 

 trcea occurs in these animals. The most complete investiga- 

 tions in this direction, namely those of E. van Beneden |, 

 Zeller §, and Willemoes-Suhm il, furnish so little information 

 as to the embryonic history of these Trematoda, that we only 

 learn from them the fact that the animals on escajnng from 

 the Q^^ already possess all their organs (except the sexual 

 organs). 



» M^m. de I'Acad. Imp. de St. Pdtersb. torn. x. 

 t Leuckart, * Die menschlicben Parasiten,' lid. i. p. 401. 

 \ " Kecherches sur la composition etla si^niitication de I'ceuf," M^oioireu 

 couninnes de I'Acad. Roy. de lielg. torn, .\xxiv. 

 § Ibidem. 

 II Zeitschrift fiir wi.ss. Zoologie, Bd. .xxii. 



