Dr. W. Salensky on Hackers Gastraea Theory. 9 



very easy to reduce the earliest developmental forms of the 

 Nauplius (as the common stock-form of the Crustacea) and of 

 many other Tracheata to the Gastrula "*. In this connexion 

 Iliickel refers to the ontogenetic works of E. van Beneden and 

 Bessels and to the writings of Weissmann. The comparison 

 of the earlier developmental stages of the NaiipUns with the 

 larvte of Annelids has been carried out hy E. van Beneden in 

 his investigations of the development of Anchorella^ Bran- 

 chiella^ and Hessia. Unfortunately I have been unable to pro- 

 cure this work, which is known to me only by Nitsche's re- 

 ports. From the embryologieal facts known to me Avith regard 

 to the developmental history of the Arthropoda, and with 

 these also the statements of E. van Beneden upon the develop- 

 ment of the above-mentioned Crustacea, there is no indication 

 of the existence of the Gasfrula-stage in the ontogeny of these 

 animals. The developmental processes of the lower Crustacea, 

 and, indeed, of the Arthropoda in general, agree with those of 

 the Annelida in this respect, that the first stage after the com- 

 pletion of the ])rocess of segmentation in the representatives of 

 these two animal types constitutes a body which consists of 

 two layers but possesses no cavity in its interior. The sub- 

 sequent phenomena are as follows : in the Annelida, as in 

 the Crustacea, the organs of motion appear on the surface of an 

 embryo thus constructed — in the former the rows of cilia, in 

 the latter the limbs ; then the mouth and anus are invaginated, 

 and finally the iiitestinal cavitj'" is formed. We have noticed 

 the same series of developmental phenomena in the Chaetopoda. 

 Exactly the same series has been demonstrated in the various 

 Arthropoda ; and this may be proved especially by the investi- 

 gations which go in some detail into the history of the forma- 

 tion of the internal organs. With respect to the Nauplius it is 

 proved by the researches of E. van Beneden and Besselsf, and 

 especially by the figures to the developmental history of An- 

 chorella uncinata and Clivella hippor/Iossi, and also by my own 

 researches \ upon the development of SjyJueronella LeucJcarti. 

 With regard to the higher Cnistacea it may also be regarded 

 as demonstrated by the investigations of E. van Beneden and 

 Bessels (ibid., Gammarus locusfa, where a stage is figured pi. 

 ii. fig. 6 at tlie commencement of the formation of the tail), 

 by A. Dohrn ^{AseUus aquati'cus), by Mecznikoff J [Nebalia)^ 

 and by Bobretzky^ {Astacus jluviatilis, Palcemon). 



* Ilackel, loc. cit. 



t Memoires cuiironn^s de I'Acad. Roy. de Belg. tome xx^^. 



t Archiv fiir Naturg. le6U. 



§ Zk^itscbr. fiir wnss. Zool. liand xvii. 



II Zipiski Iiup;'rator.skoi Akademii Nauk, 1869. 



f Zapiski Kievskajro Obshcbestva Estestroispitatelei, 1^7-"3. 



