294 CEPHALOPODA. 



to consider it as fully homologous with this latter structure. The most 

 natural course is to seek for a confirmation of this view in the ontogeny of 

 those Cephalopods which are provided with an external shell. Since the 

 development of the shell is unknown both in Nautilus and Spirilla, em- 

 bryos being unobtainable at present, we might turn to the only Cephalopod 

 with external shell which is more accessible, viz., Argonauta, if the conditions 

 in this case were not essentially modified. 



It has already been shown (p. 266) that a shell-gland does indeed appear in 

 the embryo of Argonauta, but that it disappears later and does not give rise 

 to the shell of the adult. The latter is not formed within the egg-shell, as 

 was assumed by a few of the older authors (Poli, Delle Chiaje, No. 9) but 

 arises later, as was observed by Mrs. Power, Adams and Kolliker (Nos. 1 

 and 24). The statements made as to the origin of the shell are somewhat 

 peculiar and obscure. According to the almost universal view, the shell is 

 secreted by the expanded surfaces of the dorsal arms which cover the shell 

 when fully formed. This view, which at first sight is rather improbable, is 

 rendered still more so by the fact that the regeneration of parts of the shell 

 which are lost is said to take place from within. The mantle might also be 

 regarded as a source of the shell, but it is not closely connected with the 

 latter and so this view also has no support. 



The disappearance of the shell-gland in the embryo of Argonauta shows 

 that the adult shell, in this case, is not a structure which can be homologised 

 with the shells of other Cephalopods. If, as is stated, the shell-gland actually 

 flattens out, the shell may have arisen from a part of the mantle which 

 originally corresponded to the shell-gland. The position of the animal with 

 respect to the shell is the same as in the Ammonites and Nautilus, while, in 

 Spirula, on the contrary, the orientation is different, the concave and not the 

 convex side of the shell here corresponding to the ventral side. 



An attempt has recently been made to derive the Argonaut shell directly 

 from that of Scaphites, the external form in the two cases having a certain 

 similarity (Steinmann, No. 43). We are unable to accept such a view be- 

 cause we do not regard the Argonaut shell as directly homologous with the 

 Ammonite shell, apart from the fact that a long period elapsed between the 

 disappearance of Scaphites and the appearance of Argonauta, the latter, more- 

 over, belongs to the order Octopoda and is thus closely related to the other 

 living Dibranchia. 



If the shell of Argonauta is to be derived from that of Scaphites, a com- 

 paratively quick disappearance of the chambering of the shell without 

 essential modification of the external form must be assumed. The chamber- 

 ing of the shell, however, and the manner in which the animal is connected 

 with it reappears in so marked a manner in all Cephalopoda (in which the 

 shell is well preserved) that we are not warranted in assuming that Argonauta 

 relinquished the chambering shell and received sea-water into the living 

 chamber, a change which would involve a complete alteration of the manner 

 of life of the animal. 



From what is known of the modifications undergone by the Cephalopod 

 shell, it always appears to take place in the same way as in the forms with 

 internal shells. Although, in them also, the significance of the shell is 

 essentially modified, the chambering is retained (Belemnites) and disappears 



