ON THE DEVELOPMENT OF THE POND-SNAIL. 373 



shell is small as compared with Spirulirostra. It appears 

 to be largest in Huxley's genus ILiphoteuthis. Hence, in the 

 series Spirula, Spirulirostra, Xiphoteuthis, Belem?iifes, 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 

 sac, 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 Belemnitidse 

 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 pro- 

 bable that the living Dibranchiata are modified descendants 

 of the mesozoic Belemnitidse. 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 sac and perform their part, producing the 

 chitino-calcareous pen of the living Dibrauch, 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 sac 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 sac thus formed in Loligo must 

 be held to represent the sac formed by the primaeval upgrowth 

 of mantle-folds over the young nautiloid shell of its Belem- 

 nitoid ancestors, and has accordingly no general significance 

 for the whole niolluscan group, but is a special organ belong- 

 ing 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 repre- 

 sent 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 



