74 
Salensky’s hypothesis seems to be almost identical with Ray - 
Lankester’s so-called “ plannla theory,” published the year- 
before (On the Primitive Cell-layers of the Embryo as a Basis 
of Genealogical Classification of Animals. Ann. and Mag. 
Rat. Hist., May, 1873 : and, Rotes on Embryology and Classi- 
fication, for the nse of Students. London : 1877), the essential; 
difference between which and Haeckel’s gastrula theory is 
the conception of the gastrula as a secondary modification of 
the planula. The fact that in IJnio and in the oyster a plan- 
ula is formed by the modification of a gastrula, would seem,, 
so far as it goes, to be as adverse to Lankester’s hypothesis 
as it is to Salensky’s. Yon Jhering’s view, that the primitive 
embryonic form among the Mollusca is not a gastrula, but a 
“ leposphaera,” does not seem to involve any new conception, 
for according to his definition (TJeber die Ontogenie von Cy. 
clas, und die Homologie die keimblatter bei den Mollusken, 
von Dr. Hermann von Jhering. Zeitschr. f. Miss.. Zool. xxiv. 
Marz. 1876, pp. 414-438), his leposphaera is the same as 
Lankester’s and Salensky’s planula. “ Die Leposphaera wird 
also aus zwei concentrischen Zellschichten gebildet von denem 
die aussere oder das primare Ectoderm die inn ere oder das pri- 
mare Endoderm umgiebt, wfie die Schalle einer Russ den Kern 
einschliesst. Der bleibende Mund entsteht im Ectoderm der 
Leposphaera, der Oesophagus entweder vom Munde aus, wfie 
bei den Gasteropoden, oder vom primare Endoderm aus, wie 
bei den Lamellibranchien,” p. 429. 
The facts in the development of the oyster are thus seen to 
be opposed to the only consistent and probable hypothesis 
which has ever been proposed in the place of the gastrula 
theory ; the hypothesis that the planula is the primitive em- 
bryonic form, of which the gastrula is a specialized modifica- 
tion ; but it must not be concluded that since the embryology 
of the Mollusca is opposed to the alternative hypothesis it 
therefore tends to support the hypothesis that the gastrula 
stage has a phylogenetic significance, and shows the descent 
of the Mollusca from an ancestral gastraea. 
The most satisfactory and trustworthy papers which we 
possess upon the development of the Mollusca show that 
