Dr. W. iSalensky on Hackers Gaatraea Tlieory. 23 



In all other aniroalB which pai^s through the Pianula-fona 

 in their development, the developmental phenomena occur 

 nearly" in the following order : — After the P/anuIa-st&ge the 

 foundations of various external and internal organs, which 

 appear in these animals as tvpical, persbtent, or larval organs, 

 are formed, e. ff. 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 

 (rl. V. figs. l-3j. With regard to the invagination of the 

 anterior intestine, I have already noted that it Ls a secondary 

 phenomenon which cannot be compared to the so-called inva- 

 gination of the exterior lamella of the Asc'i^SijAm/jhioxm, &c. 

 — 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 {Euaxes^ Tubifex^ &c.). In these 

 cases, therefore, no Gastrula-sta.ge is formed. 



K we take the Vermes first into consideration, we have 

 an example in Euaxes, 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 ChaetojxKia 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 ClaparMe 

 and Mecznikow loc. cit.) 



With regard to the Mollusca, the statements of Lov^n, 

 Lacaze-Duthiers, Gegenbaur, and myself have already been 

 mentioned. Although we have seen that the statements of 

 diflferent observers with res]>ect to this type of animals difier 

 somewhat from each other, and that the intestine originatea 

 from the exoderm according to some, and from the entoderm 

 according to others, it Ls 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 {Calyptrcea^ 



