306 Mr. V. A. Bather on Shell-growth 



cells are connected with secretion is inevitable ; in fact the 

 distal layer seems to be nothing more than chitin in course of 

 deposition ; as its connexion with the cell is lessened it splits 

 into membranes. The suggestion of Appellof that the upper 

 portions of the [rapidly growing] cells coalesce and are throvvn 

 off as a chitinous membrane seems to explain their peculiar 

 character, and agrees with what we know of chitin-formation 

 elsewhere. 



Therefore the external evidence of the soft parts corroborates 

 the internal evidence of the shell. The intussusception- hypo- 

 thesis does not explain the growth of the shell in Sepia any 

 more than in Nautilus. 



IV. Proposed Explanation. 



Any theory must of course explain the facts for all the 

 specialized forms, e. g. Nautilus, Ammonites, Belemnites, 

 Sepia, Spirula, Loligo. But, since ontogeny is parallel to 

 phylogeny, we shall expect our theory to be borne out by the 

 facts of paljEontology, and possibly to illustrate the methods 

 by which the evolution of the group was accomplished. The 

 latter point I deal with more fully elsewhere (14). 



In the shells of all Cephalopoda chitinous membranes are 

 first secreted ; these form a framework in which lime can be 

 deposited, and on their arrangement depends the structure of 

 the shell. Tlie membranes of the outer portion must be dis- 

 tinguished from those of tlie inner portion; the lime also 

 differs in physical character according to the portion in which 

 it is deposited (13). The membranes of the inner portion are 

 secreted by the visceral hump, and in them is formed the 

 True Shell (shell of Nautilus and OrtJioceras ; conch of Ammo- 

 nites ; phragmocone of Belemnites ; middle plate, inner plate, 

 and lamellge of Sepia ; conch of Spirula ; gladius of Loligo, 

 in part). Kiefstahl postulates for Sepia, as F. Miiller has 

 done for Lamellibranchs (7), a primal mass of homogeneous 

 chitin, from which, as it grows by intussusception, these 

 membranes are repeatedly detached. There is none such ; 

 but on the surface of the cells that coat the visceral hump a 

 layer of chitin is, by concrescence of their distal portions, 

 continually formed, and from it the membranes are, as it 

 were, exfoliated. Secretion and exfoliation, beginning in the 

 anterior region of the shell-wall, proceed backwards to the 

 suture ; thence, centripetaliy over the septum, to the poste- 

 rior margin of the septal neck ; a membrane of the septum 

 is therefore one with a membrane of the shell-wall, and each 

 complete membrane is typically shaped like a funnel. Lime, 



