132 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



lirostra (fig. 56, B), Belemnites (fig. 56, c), and 

 Conoteuthis (fig. 56, D), a progressive simplifi- 

 cation of the shell may be observed, the latter 

 becoming less and less coiled, until finally the 

 original shell is transformed into a straight 

 chambered portion surmounted by a stiliform 

 process. Degeneration is principally exhibited in 



FIG. 55. Spirula 



prototypes. 



a, body of the animal; 

 p, mantle fold ; c, 

 shell, partly internal, 

 partly external. 

 (After Owen.) 



FIG. 56 Shel's of various Cephalopoda. 

 A, Spirula; B, Spirulirostra ; C, Belemnites; D, Cono- 

 teuthis; E, Ommatostrephes; F, Loligopsis. (After 

 Boas.) 



the segmented part, which becomes more and more 

 reduced. In Ommatostrephes (fig. 56, E), and in 

 all existing species (such as Loligopsis, fig. 56, F), 

 with the exception of those just mentioned above, 

 degeneration has become complete. The shell no 

 longer contains a cavity for the reception of the 

 animal, and the phenomenon already mentioned 

 with regard to Spirula the development of the 



