10 Dr. W. Salensky on lliickeCs Gastraja Theory. 



" In the stock of the MoUusca the Oastrula seems to be 

 widely prevalent, especially in the classes Conchifera and Gas- 

 teropoda, and probably also in the Spirobranchiata; among the 

 Givsteropoda it was first observed in Limnteus^'*. In proof of 

 this statement lliickel appeals to the memoir by Ray Lan- 

 kester (Ann. & ^lag. Nat. Hist., February 1873, pp. <S6, 87). 

 As regards the observations of Kay Lankcster on Aplasia, 

 which are described in most detail, we cannot see there a Gas- 

 trula-stiigc, as, according to the statements of that natiu-alist, 

 the external organs (mantle &c.) make their appearance very 

 early, and it is not stated when the stomachal cavity appears. 

 And as regards the other Mollusca, Doris, Tethys, Pleurobran- 

 chusj Polycera quadrilineata, and Eolis exigua, which are also 

 brietly referred to by Hay Lankester, it must be admitted that 

 these few words, '' I was able to determine in these that the 

 first step in development, after the formation by cleavage 

 of the mass of embryo-cells or * polyblast,' is the invagina- 

 tion or in-pushing of these cells at one pole, just as Kowa- 

 levsky has drawn it in Amphioxus and P/iaUusia, and as 

 seen also in the heteropod mollusk Atalanta,'' do not prove 

 very much. These statements are supported neither by 

 figures nor by a detailed description of the observed facts. 

 I am far from doubting the coiTCctness of Ray Lankester's 

 statements, and indeed cannot do so, because we already 

 know many cases in which in animals systematically nearly 

 related the invagination (and, indeed, the Gastrula-s,ia.g&) occurs 

 in some and not in others (<?. g. Euaxes and Lumhricus). But 

 for me they have too little force as evidence to enable us to 

 rest the existence of the Gastrula-f>,i&.gQ in the !Mollusca upon 

 them. It is the more necessary to describe such observations 

 in detail, because, with respect to the developmental history of 

 the Mollusca, there exists a mass of statements which are mu- 

 tually very contradictory. With regard to the Lamellibran- 

 chiata the statements of different naturalists are tolerably con- 

 cordant. For the greater part of the observations we are in- 

 debted to the remarkable, although already old, investigations 

 of Lov^n, which give the most complete picture of the deve- 

 lopment of several marine bivalves. From these observations 

 and the figures accompanying them we see that the first stage 

 of the development is an embryo which consists of two layers 

 and has no cavity in its interior, that then various external 

 organs and a buccal invagination are formed, and finally an 

 intestinal cavity is produced in the interior of the entoderm. 

 The phenomena ai-e closely in accordance with what we have 

 already had occasion to mention in other animals. They are 

 • Ilackel, ' Gastraea-Theorie.' 



