Dr. W. Salensky on Héckel’s Gastreea 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 
Tlickel 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 Nawplius with the 
larvee of Annelids has been carried out by 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 embryological facts known to me with regard 
to the developmental history of the Arthropoda, and with 
these also the statements of EZ. van Beneden upon the develop- 
ment of the above-mentioned Crustacea, there is no indication 
of the existence of the Gastrula-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 process 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 intestinal cavity is formed. We have noticed 
the same series of developmental phenomena in the Cheetopoda. 
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 Besselst, and 
especially by the figures to the developmental history of An- 
chorella uncinata and Clivella hippoglosst, and also by my own 
researches { upon the development of Spharonella Leuckarti. 
With regard to the higher Crustacea it may also be regarded 
as demonstrated by the investigations of EX. van Beneden and 
Bessels (ibid., Gammarus locusta, where a stage is figured pl. 
i. fig. 6 at the commencement of the formation of the tail) 
by A. Dohrn § (Asellus aquaticus), by Mecznikoff || (Nebalia) , 
and by Bobretzky | (Astacus fluviatilis, Paleemon). 
* Hackel, loc. cit. 
+ Mémoires couronnés de l’Acad. Roy. de Belg. tome xxvi. 
{ Archiv fur Naturg. 1869. 
§ Zeitschr. fiir wiss. Zool. Band xvii. 
|| Zapiski Imperatorskoi Akademii Nauk, 1869. 
{] Zapiski Kievskago Obshchestva Estestvoispitatelei, 1873. 
