INTERPRETATION OF THE SHELL IN RECENT CEl'HALOPODS. 293 



The reduction of the shell goes still farther in other recent Cephalopods ; 

 the terminal cone, in older specimens of Dosidicus, is found to be solid, 

 whereas, in the younger animals, it was hollow (Stbensteup). In some 

 ichiidcu the hollow cone is still present at the end of the shell, in others 

 it lias disappeared and, in its place, i here is mere solid swelling. 

 Finally, a simple horny plate develops, as in Loligo. The 

 hollow cone does not even appear ontogenetically, so far as we 

 know at present. 



The above comparison seems to show with certainty that the 

 shell in the Cephalopoda is internal, the manner in which it 

 arose from an external shell being still exemplified in a living 

 form, Spirilla. In Nautilus, only a small part of the mantle 

 covers the external shell, but the process of circumcrescence 

 oi the shell went farther, until the shell became covered, though 

 incompletely, by the mantle, as in Spirula. During this process, 

 the size of the shell as compared with that of the animal became 

 reduced in most cases ; at the same time, it changed its position 

 and gradually degenerated, since it no longer functioned in the 

 same way. Externally, new calcareous layers became added 

 to the primitive shell, for we find that only the inner part of y,. 

 the shell of Belemnites or Sepia corresponds to the shell of 

 Nautilus, the rostrum and its continuation as a covering of the 

 proostracum are secondary structures, no doubt secreted by the 

 mantle-sac which surrounded the shell. We are here again 

 brought to the question of chief interest in connection with this 

 subject, viz., the manner of formation of the shell. 



We saw above that the shell is formed in an ectodermal 

 depression, the shell-gland. It would be well to discover 

 whether this shell-gland is homologous with the organ of the 

 same name in the Lamellibranchia and Gastropoda or not. 

 This question has been raised before now by Ray Lankester 

 (No. 28) who maintained the negative because he believed that 

 the shell of Sepia corresponds to the Belemnite shell and conse- 

 quently must be formed in a mantle-sac and not in the primitive 

 shell-gland. Ray Lankester was obliged to take up this 

 position decisively since he considered the Sepia shell as 

 homologous with the external part only of the shell of 

 Belemnites and left the phragmocone out of account. 



In deciding the question as to the significance of the shell- 

 gland in the Cephalopoda, we are inclined from the first to 

 ascribe to this organ which appears so early, in consequence 

 of its position and development, the same significance as is 

 possessed by the shell-gland in the other Mollusca, and thus % 



Fig. 110. — Posterior part of tin- shell .>t Ommastrephes from the Indian Ocean, seen 

 from the posterior surface (original). /■, conical appendage at the dorsal end; s, 

 plate of the shell which narrows dorsally, again broadening oul ;i- the conical 

 appendage ; /. the strong hornj ledges between which tin- shell consists merely of a 

 thin membrane strengthened by ribs. 



