

XI 



PHYLUM MOLLUSCA 



301 



backwards, that of Nautilus forwards. Moreover the shell 

 of Spirula is an internal structure, being almost completely 

 covered by the mantle. 



In the other Dibranchiata the shell may consist of three 

 parts a horny pen or pro-ostracum, a calcareous guard, and 

 a part termed the phragmocone. The last, which alone 

 represents the shell of Spirula y has the form of a cone 

 divided internally by a series of septa perforated by a 

 siphuncle. These parts are most com- 

 pletely developed in the extinct genus 

 Belemnites, in which the shell consists of 

 a straight, conical, chambered phragmo- 

 cone, with a siphuncle, enclosed in a cal- 

 careous sheath, the guard, produced into 

 a horny or calcareous plate, the pro- 

 ostracum. In the Cuttlefish (Sepia) the 

 shell is a leaf-like body, with a rounded 

 and comparatively broad oral end, and a 

 narrower aboral end provided "with a sharp 

 projecting spine. The main mass of the 

 shell consists of numerous closely arranged 

 thin laminae of calcareous composition, 

 between which are interspaces containing 

 gas. The spine-like projecting point re- 

 presents the guard, and the main substance 

 of the shell is to be looked upon as the 

 pro-os tracum and phragmocone, the septa 

 of the latter being represented by the calcareous lamellae. 

 In the Squid (Loligo) the shell (Fig. 175, ) is long, 

 narrow, and completely horny ; it corresponds to the pro- 

 ostracum, the phragmocone being entirely absent. 



In Octopus the shell is represented only by a pair of 

 rudiments with which muscles are connected. In Argonauta 



FJG. 174. Shell of 

 Sepia cultrata, 

 posterior view. 

 Reduced. 



