524 



THE INVERTEBRATA 



two suborders, whose members respectively possess, like Sepia, ten 

 arms (Decapoda), or, like Octopus, only eight (Octopoda). In no 

 member of either division is there any known form in which the shell 

 is external ; in all cases the shell is more or less rudimentary or, in the 

 case of the Octopoda, entirely absent. There is a well-known and 

 extremely numerous fossil group, the Belemnitidae (Fig. 385 B), in 

 which impressions of the entire creature show^ the internal shell, the 

 ink sac, and the ten arms beset with hooks. The shell consists of a 

 chambered phragmocone, protected by a thickened guard, and with an 



Fig. 385 . Series of Cephalopoda to illustrate the evolution of the internal shell. 

 After Naef. A, Orthoceras, Palaeozoic. B,Belemmtes, Mesozoic. C, Spiridi- 

 rostra, Tertiary. C, Spinila and D, Sepia, living. (D', enlargement of 

 posterior end of D.) The reflection of the mantle over the shell is indicated 

 by a dotted line. This is incomplete in Orthoceras, but the shell is com- 

 pletely internal in the rest. cr^. guard ; phr. phragmocone ; prst. proostracum ; 

 Sep. septa; sip. siphuncle. 



anterior plate, the proostracum. It may well have been derived from 

 a nautiloid form like Orthoceras (Fig. 385 A), as may be seen in the 

 accompanying series of diagrams, in which the soft parts are of 

 course partly conjectural. In a rare living form, Spirula (Fig. 385 C), 

 the chambered shell is reduced, but not quite so much as is the case in 

 the belemnites. It is coiled and there is no guard or proostracum. 

 Both are, however, present in the related fossil Spirulirostra (Fig. 

 385 C). Finally, in Sepia (Fig. 385 D) the guard is represented by the 

 minute rostrum, and according to one interpretation, one side of the 

 phragmocone has expanded to cover the surface of the proostracum, 



