DEVELOPMENTAL HISTORY OF THE MOLLUSCA. 7 
a 
were it then to continue its chitinous secretion, we should have produced an internal 
chitinous rod like the pen of Loligo. A reference to what I have said on the deve- 
lopment of the pen in another communication will show that I have not yet got the 
detailed demonstration of the mode of development of the “ pen” of the Decapodous 
Cephalopods, which is required to substantiate the supposed relationship now suggested *. 
Fig. 51 shows another embryo with the foot-surface turned to the right instead of 
the left. The focus is somewhat deeper, showing, instead of the ciliated surface and 
commencing mantle-flap, the rudimentary alimentary canal a/ andrp. The pharynx 
(ph) is seen lined with cilia; it now is about the stage of development at which it 
opens into the gastric chamber, a/. The groove of the shell-gland is well seen (sh) in 
this figure. 
Fig. 31a. Mouth-region (0) of an embryo at a somewhat earlier period, more highly 
magnified. Surface-view. 
Plate 3. figs. 32, 33, represent two views of an.embryo less fully formed than that 
of fig. 30, but still showing the shell-gland remarkably well (sh). The embryo is also 
remarkable for the distinctness with which it exhibits the condition of the alimentary 
canal with its two central lobes or gastric chambers, and fore and aft the pharynx and 
the rectal peduncle. 
Though the foot is developed to so slight an extent in this specimen and the body- 
walls generally were so thin and transparent as to suggest some abnormality of deye- 
lopment, yet at the point marked f in the figure, slow movements of contraction and 
expansion were going on. From this phase onward, in fact, the foot exhibits muscular 
movements. 
Plate 3. figs. 34, 35, 36 go together, giving different views of three embryos of very 
nearly the same stage of development—that is to say, a little in advance of the embryo 
of fig. 31. The foot has now grown out as a very prominent conical mass, and being 
covered with vibratile cilia and capable of considerable alteration of form, becomes the 
chief locomotive organ. ‘The embryos in this condition move about freely in the brood- 
pouch, and feed on the material supplied to them from its walls. The pharynx (ph) 
now actively functions, expanding widely and bringing in material to its cavity by 
means of its ciliate lining, then contracting sharply, and passing on its contents to the 
left gastric chamber. Hence the food passes by a slow circular movement into the 
adjacent right gastric chamber, and thence to the rectum. As yet, however, there is 
* Dec. 1874.—This evidence I subsequently obtained in the spring of 1874 at Naples. The pen-sac of 
Loligo does develop as an open pit, which becomes closed in, and it corresponds in position with the shell- 
gland, the existence of which I have now demonstrated in Pisidiwm, Aplysia, Pleurobranchidium, Neritina, 
Limneus, and Paludina. M. Hermann For has, subsequently to the publication of my first observations on 
this matter (which were made in 1871 and 1872), observed the structure which I term the “shell-gland” in 
certain Pteropod embryos. Although there is a correspondence between the pen-sac of Cephalopods and the 
shell-gland of other mollusks, I haye, in the Quart. Journ. Microse. Science, Oct. 1874, adduced reasons 
(based on paleontological facts) for considering them not to be identical structures. See also the same Journal 
for January 1875. 
