THE CEPHALOPODA 



313 



all Cephalopods. In Nautilus the stomato-gastric commissure passes 

 under the pharynx immediately behind the radula and bears a 

 buccal ganglion (Fig. 279, VII) on either side. 



In the Dibranchia the nerve-centres are much more completely 

 enclosed in the cartilaginous cephalic capsule than in Nautilus, and 

 consequently many nerves notably the pallia! nerve traverse the 

 cephalic cartilage at their origin. The cerebral centres themselves 

 appear externally to be unpaired, and in the Decapoda they are 

 divided transversely into a small anterior (Fig. 282, III) and a 

 large posterior lobe, the latter being separated by some considerable 

 distance from the former in various Oigopsida, such as Ommato- 

 strephes (Fig. 282, IV), Spirula, etc., but the distance is less in Sepiola 



III 



IV 



and Loligo, and is very small in Sepia. These two lobes are united 

 by a pair of slender connectives, which are sometimes fused together 

 for a certain distance. In the Octopoda, on the other hand, the 

 cerebral mass is apparently single, the two lobes above mentioned 

 being intimately fused together, and their limits are barely indicated 

 by a transverse furrow, behind which is the posterior lobe with six 

 longitudinal furrows on its surface. The cerebral centres of all the 

 Dibranchia give off a large optic nerve on either side, each nerve 

 expanding to form a very large ganglion, whose size is greater than 

 that of the whole mass of the cerebral centres. As in Nautilus, a 

 labial commissure is given off from the anterior part of the cerebral 

 centres (Fig. 279, IX), and the stomato-gastric commissure originates 

 from the labial commissure. Finally, the cerebro- pedal pair of 

 connectives issues from the anterior part of the cerebral mass ; 

 these connectives are simple in the Octopoda (Fig. 281), but in the 



