370 



11. T. GUNTHER. 



highly suggestive of it. The uniform sheathing of ganglionic 

 cells^ indicating lack of concenti'ation^ which gives peculiar 

 interest to primitive nerve-cords, as in the Chitonid^, is not 

 to be found in Chastognatha, whose active movements demand 

 a more perfect apparatus for their co-ordination, and in 



CH/£T0GNATHA 



Vise. 



2. Nautilus. 



,OpL. 



-Olf. 



Ped. 



Vise. 



Tkxt-fig. 6. — Nervous systems. Biicc. Buccal f;anc:lioii ; Ph. 

 Piiaryiigeal cjanglioii ; Ped. Pedal ganglion ; Vise. Visceral or 

 ventral e:an2;lion of Cliaetosrnatha. 



which a more centralised nervous system has accordingly 

 been evolved. 



The cerebral ganglion, consisting of a median group of 

 nerve-cells and of two lateral ones joined by commissural 

 fibres, gives off posteriorly a pair of rhinophoral and a pair 

 of optic nerves to the olfactory organs and eyes respectively, 



