CEPHALOPODA. 



427 



are all directed towards the oral extremity (Histioteutliis). Aquiferous pores 

 leading into spaces in the integument, but not communicating with the vascular 

 system, are found in many Dibranchs. 



There is an internal cartilaginous skeleton which serves for the 

 protection of nerve-centres and sense-organs, and for the attachment 

 of muscles. The cartilaginous tissue itself closely resembles the 

 hyaline cartilage of vertebrates, differing in the fact that the cells 

 are connected by their branching processes which traverse the matrix 

 in all directions. In the head there is in Dibranchs a complete 

 cartilaginous investment for the great ganglionic masses and otocysts, 

 which furnishes lateral cup-like expansions for the eyes. In Nautilus 

 there is a corresponding cephalic cartilage on the ventral side only of 

 the nerve-centres, extending also into the tissue of the funnel. 

 Cartilage is present also in other parts of the body, e.g. the branchial 

 cartilage, fin cartilages, nuchal cartilage, dorsal cartilage. 



Nervous system. There is a great concentration of ganglionic 

 matter round the oesophagus, and it is difficult to trace exact 

 homologies with the nerve-centres of other Molluscs. 



In Nautilus there are two ganglionic rings round the oesophagus 

 behind the buccal mass (Fig. 342). They are connected dorsally, 

 above the oesophagus, in a common mass (JT), which may be called 

 the cerebral mass and compared to 

 the cerebral ganglion of other Mol- 

 luscs. The anterior ring innervates 

 the funnel and cephalic lobes, and 

 may be compared to the cerebro- 

 pedal commissure and pedal ganglia 

 of other types ; the posterior ring 

 innervates the mantle and viscera, 

 and may be compared to the cerebro- 

 pleural commissure, pleural ganglia, 

 and visceral commissure of other 

 Molluscs ; it gives off a pair of stout 

 nerves (V) which pass backward 

 (dorsalwards), on either side of the 

 vena cava ; these supply the gills, 

 osphradia, and viscera, but do not 

 form dorsalwards in the visceral sac, visceral ganglia. The cerebral 

 portion supplies the eyes, otocysts, ocular tentacles, lips, etc., and 

 gives oft" on each side two nerves to form stomatogastric commis- 

 sures (XII}, which surround the oesophagus immediately behind 



XII 



Vli. 



FIG. 342. Central nervous system of a 

 female Nautilus from the left side (from 

 Pelseneer after Valenciennes). / ac- 

 cessory pedal ganglion ; II nerve to 

 funnel; /// pedal; IV visceral gan- 

 glion ; V visceral nerve ; VI pallial 

 nerves ; VII tentacular nerves ; VIII 

 olfactory, IX optic nerve ; X cerebral 

 ganglion ; XI otocyst ; XII stomato- 

 gastric commissure (for detail see 

 Fig. 343) ; XIII buccal mass. 



