10 Dr. W. Salensky on Héickel’s Gastreea Theory. 
“Tn the stock of the Mollusca the Gastrula seems to be 
widely prevalent, especially in the classes Conchifera and Gas- 
teropoda, and probably also in the Spirebranchiata; among the 
Gasteropoda it was first observed in Limneus”*, In proof of 
this statement Hickel appeals to the memoir by Ray Lan- 
kester (Ann. & Mag. Nat. Hist., February 1873, pp. 86, 87). 
As regards the observations of Ray Lankester on Aplysia, 
which are described in most detail, we cannot see there a Glas- 
trula-stage, as, according to the statements of that naturalist, 
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, Plewrobran- 
chus, Polycera quadrilineata, and Holis exigua, which are also 
briefly referred to by Ray 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 imvagina- 
tion or in-pushing of these cells at one pole, just as Kowa- 
levsky has drawn it in Amphiowus and Phallusia, 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. 
Tam far from doubting the correctness 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-stage) occurs 
in some and not in others (e. g. Huaxes and Lumbricus). But 
for me they have too little force as evidence to enable us to 
rest the existence of the Gastrula-stage 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 Lovén, 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 are closely in accordance with what we have 
already had occasion to mention in other animals. They are 
* Hickel, ‘Gastreea-Theorie.’ 
