DEVELOPMENTAL HISTORY OF THE MOLLUSCA. 35 
In no case is the orifice of invagination of the invaginate Gastrula known to persist 
under any form. It appears solely to effect the invagination, and when that is effected 
vanishes. 
Enough has been said to show the importance of observations relating to the Gastrula 
phase of development. In the preceding parts of this paper well-marked invaginate 
. Gastrule are described from :— 
1. Pisidium (Lamellibranch). 
. Tergipes (Nudibranch). 
. Polycera (Nudibranch). 
. Limax (Pulmonate). 
. Limneus (Pulmonate). 
fis addition to these cases of the development of invaginate Gastrulw among Mollusca, 
the examination of the very beautiful figures in the papers of Loyin on molluscan 
development leaves no doubt that he has observed invaginate Gastrul@ in the following 
cases, but has not understood their structure :— ‘ 
6. Cardium (Lamellibranch). 
7. Crenella (Lamellibranch). 
Similarly, Kart Voer’s observations on Actwon indicate the same state of things as I 
have pointed out in Polycera; and hence we may add 
8. Actwon (Nudibranch), 
and, finally, from KowaLrysky’s statement, though not accompanied by figure or 
description, 
9. Atalanta (Heteropod). 
(2) The second matter of theoretical interest (namely, the early features in the develop- 
ment of the shell) has not been previously discussed, since the structures described in 
the paper as shell-patch, shell-groove, and shell-plug were unknown. 
If, as seems justifiable, the Cephalopoda are to be regarded as more nearly repre- 
senting the molluscan type than do the other classes, or, in other words, more closely 
resemble the ancestral forms than they do, we might look in the course of the develop- 
ment of the less typical Mollusca for some indication of a representative of the internal 
pen of the higher Cephalopoda. We might expect to find some indication of the 
connexion between this and the calcareous shell of other forms; in fact the original 
shell of all Mollusca should be an internal one, or bear indications of a possible deve- 
lopment into that condition. 
In Pisidium, in Aplysia, and in Neritina I have submitted evidence of the 
existence of a specially differentiated patch of epidermic cells at the aboral pole, which 
develops a deep furrow, groove, or pit in its centre, almost amounting to a sac-like 
cavity opening to the exterior. The first (chitinous) rudiment of the shell appears as a 
disk on the surface of this gland, but also in some cases the cavity or groove is filled by 
a chitinous plug. 
Let the walls of the sac close and the activity of its living cells continue, and we 
F 2 
oe oo bo 
