THE SHELL OF MOLLUSCA. 



333 



forms of shell have branched off. Derived from these we find on 

 the one hand those which are more highly developed, and on 

 the other those rudimentary forms which have the character of 



degenerate shells. 



§ 259. 



The simpler shells of the Cephalopoda must also be regarded as 

 rudimentary structures, and not as early forms ; as derived, in fact, 

 from the more complicated and perfect forms, even if their geological 

 succession did not indicate that the shell has undergone gradual 



Fig. 175. Nautilus; median section of a shell, i Funnel, i Tentacles, t; Cephalic 



lobes. Eye. 6 Dorsal lobes of the mantle. II Connections between the shell and 



the mantle, s Part of the shell still connected with the right pallial muscle, a Mantle. 



s Siphon, s' Siphonal canal of the shell (after Owen) . 



reduction. Their structural characters, as well as their relations to 

 the body, that is, to that portion of the dorsal integument which 

 represents the "mantle," are further instances of the arrangements 

 which we have already described. We either meet with straight 

 (in extinct families only) or coiled shells, which are formed by the 

 mantle, and either completely enclose the animal, or are rudimentary 

 and contained within the mantle ; these latter have lost their signifi- 

 cance as shells and only form internal organs of support. 



The well-developed shells of the Cephalopoda, as seen in the 

 fossil Ammonites and Orthoceratites, and in the extant ISTautilus, 



