6oo THE INVERTEBRATA 



extremely numerous fossil group, the Belemnoidea (Fig. 410 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 

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

 a nautiloid form like Orthoceras (Fig. 410 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. 410 C), 



''U:,:::;-___^ ^P' sip. 



A^ 



phr. sip. 



y 



prst. ^^w 



'"c 



Fig. 410. Series of Cephalopoda to illustrate the evolution of the internal 

 shell. After Naef. A, Orthoceras, Palaeozoic. B, Belemnites, Mesozoic. 

 C, Spirulirostra, Tertiary. C, Spirula and D, Sepia, living. (D', enlargement 

 of posterior end of D.) I'he reflection of the mantle over the shell is indicated 

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

 internal in the rest. gd. guard; phr. phragmocone ; prst. proostracum; 

 sep. septa; sip. siphuncle. 



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. 

 410 C). Finally, in Sepia (Fig. 410 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, 

 the septa forming the oblique calcareous partitions of the cuttle bone, 

 while the other side forms a minute lip in which the septa are crowded 

 together (Fig. 410 D'). The siphuncle (p. 602) is a short wide funnel 

 in between the two sides. 



