ON THE INVAGINATE PLANULA. 159 
On the INVAGINATE PLaNuLA, or DrieLopiastic PHASE of 
PaLupINA vivipara. By E. Ray Lanxester, M.A. 
Ar the end of last summer I had an opportunity of ex- 
amining some embryoes of Paludina, in the laboratory of 
Exeter College, Oxford. 
' The formation of the primitive cell lining of the alimentary 
canal, or ‘‘endoderm,” by invagination of the wall of a 
multicellular hollow sac, is particularly well seen in this 
mollusc. Leydig, who has given an account of the de- 
velopment of Paludina (‘ Zeitschrift fir Wiss. Zool.,’ Bd. 
II, 1850) observed the commencing invagination, and figured 
it, but he regarded it as the mouth. In this he was mis- 
taken, since the mouth developes subsequently at the pole of 
the embryo opposite to the orifice of invagination. This 
orifice—which in the other mollusca in which I have 
observed it (viz. Pisidium, Limnzeus, Limax, certain Nu- 
dibranchs, &c.) closes up at a very early period—does not 
(I am inclined to think) close up in Paludina, but remains 
open as the anus, and is fringed with cilia. 
The woodcuts give two stages of this development, in 
which though they are merely outline diagrams, the sharp- 
ness of the cell-layers is not exaggerated. Paludina is 
remarkable for the clearness with which the cells of the 
primitive ectoderm and endoderm are presented in their 
earliest condition. 
Fig. 1 shows the stage in which there are but two layers 
of cells—the “ diploblastic Planula ” (see my remarks in 
* Ann. and Mag. Nat. Hist.,’ May, 1873) ; and, moreover, it 
shows this stage with a well-marked “orifice of invagina- 
tion” (0 7) an orifice which I have sometimes (e. g. in my 
