MOLLUSCA. 253 



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 Belemnitidie with the living Dibranchiata. The booklets 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 Belemnitidae. 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 

 primaeval up-growth 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 Dibran- 

 chiate stem, similar to but not necessarily genetically connected with the 

 mantle-fold in which the shell of the adult Aplysia and its congeners is con- 

 cealed. 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." 



The funnel. The general development of the funnel has 

 already been sufficiently indicated. The folds of which it is 

 formed are composed both of epiblast and mesoblast. The 

 mesoblast of the anterior part of each half of the funnel would 

 appear to give rise to a muscle passing from the cartilage of the 

 neck to the funnel proper. The posterior parts gradually 

 approximate, but meet in the first instance ventrally. The two 



