ep. epiblast; y. food-yolk; m. mesoblast; m´. cellular yolk membrane; shs. shell-gland.
The edges of the mantle now begin to project, especially on the posterior side ([fig. 127]), and within the cavity formed by this projecting lip there are placed the anus (an), gills, etc. The projecting lip of the mantle is formed both of epiblast and mesoblast. The whole of the anterior side of the mantle is filled by the elongated shell-sack (cch), within which the shell or pen soon becomes secreted.
There are certain difficulties in comparing the shell-gland of the Cephalopoda with that of other Mollusca which will best be rendered clear by the following quotation from Lankester[105]:
“The position and mode of development of the shell-gland of the Cephalopoda exactly agree with that of the shell-gland as seen in the other Molluscan embryos figured in this paper. We are therefore fairly entitled to conclude from the embryological evidence that the pen-sack of Cephalopoda is identical with the shell-gland of other Mollusca.
“But here—forming an interesting example of the interaction of the various sources of evidence in genealogical biology—palæontology crosses the path of embryology. I think it is certain that if we possessed no fossil remains of Cephalopoda the conclusion that the pen-sack is a special development of the shell-gland would have to be accepted.
“But the consideration of the nature of the shell of the Belemnites and its relation to the pen of living Cuttle-fish brings a new light to bear on the matter. Reserving anything like a decided opinion as to the question in hand, I may briefly state the hypothesis suggested by the facts ascertained as to the Belemnitidæ. The complete shell of a Belemnite is essentially a straightened nautilus-shell (therefore an external shell inherited from a nautilus-like ancestor), which, like the nautiloid shell of Spirula, has become enclosed by growths of the mantle, and unlike the shell of Spirula, has received large additions of calcareous matter from those enclosing overgrowths. On the lower surface of the enclosed nautilus-shell of the Belemnite—the phragmacone—a series of layers of calcareous matter have been thrown down forming the guard; above, the shell has been continued into the extensive chamber formed by the folds of the mantle, so as to form the flattened pen-like pro-ostracum of Huxley.
“Whether in the Belemnites the folds of the mantle which thus covered in and added to the original chambered shell, were completely closed so as to form a sack or remained partially open with contiguous flaps must be doubtful.
“In Spirula we have an originally external shell enclosed but not added to by the enclosing mantle sack.
“In Spirulirostra, a tertiary fossil, we have a shell very similar to that of Spirula, with a small guard of laminated structure developed as in the Belemnite (see the figures in Bronn Classen u. Ordnungen des Thierreichs).
“In the Belemnites the original nautiloid shell is small as compared with Spirulirostra. It appears to be largest in Huxley’s genus Xiphoteuthis. Hence in the series Spirula, Spirulirostra, Xiphoteuthis, Belemnites, we have evidence of the enclosure of an external shell by growths from the mantle (as in Aplysia), of the addition to that shell of calcareous matter from the walls of its enclosing sack, and of the gradual change of the relative proportions of the original nucleus (the nautiloid phragmacone) and its superadded pro-ostracal and rostral elements tending to the disappearance of the nucleus (the original external shell). If this view be correct as to the nature of these shells, it is clear that the shell-gland and its plug has nothing to do with them. The shell-gland must have preceded the original nautiloid shell, and must be looked for in such a relation whenever the embryology of the pearly Nautilus can be studied. Now, everything points to the close agreement of the Belemnitidæ with the living Dibranchiata. The hooklets on the arms, the ink bag, the horny jaws, and general form of the body, leave no room for doubt on that point; it is more than probable that the living Dibranchiata are modified descendants of the mesozoic Belemnitidæ. If this be so, the pens of Loligo and Sepia must be traced to the more complex shell of the Belemnite. This is not difficult if we suppose the originally external shell the phragmacone, around which as a nucleus the guard and pro-ostracum were developed, to have finally disappeared. The enclosing folds of the mantle remain as a sack and perform their part, producing the chitino-calcareous pen of the living Dibranch, in which parts can be recognised as corresponding to the pro-ostracum, and probably also to the guard of the Belemnite. If this be the case, if the pen of Sepia and Loligo correspond to the entire Belemnite shell minus the phragmacone-nucleus, it is clear that the sack which develops so early in Loligo and which appears to correspond to the shell-gland of the other Molluscs cannot be held to do so. The sack thus formed in Loligo must be held to represent the sack formed by the primæval upgrowth of mantle folds over the young nautiloid shell of its Belemnitoid ancestors, and has accordingly no general significance for the whole Molluscan group, but is a special organ belonging only to the Dibranchiate stem, similar to—but not necessarily genetically connected with—the mantle fold in which the shell of the adult Aplysia and its congeners is concealed. The pen, then, of Cephalopods would not represent the plug of the shell-gland. In regard to this view of the case, it may be remarked that I have found no trace in the embryonic history of the living Dibranchiata of a structure representing the phragmacone; and further, it is possible, though little importance can be attached to this suggestion, that the Dibranchiate pen-sack, as seen in its earliest stage in the embryo Loligo, etc., is fused with the surviving remnants of an embryonic shell-gland. When the embryology of Nautilus pompilius is worked out, we shall probably know with some certainty the fate of the Molluscan shell-gland in the group of the Cephalopoda.”