350 



INVSRTEBRATE MORPHOLOGY. 



The nervous system of the Cephalopods shows a Mgh 

 degree of concentration, the various ganglia being more or less 

 fused with one another to form a mass surrounding the oeso- 

 phagus just behind the pharyngeal mass. In Nautilus this 

 mass takes the form of two rings surrounding the oesophagus, 

 united in front but widely divergent behind, and in which the 

 various ganglia are but indistinctly indicated, the condition 

 which occurs in Chiton in this respect being recalled. That 

 portion of the ring which lies in front of the oesophagus rep- 

 resents the cerebral ganglia; the 

 lateral portions of the more ventral 

 of the two rings found on the pos- 

 terior surface are the pedal ganglia, 

 giving rise to the nerves to the pedal 

 lobes and the infundibulum ; while 

 the more dorsal posterior ring rep- 

 resents the combined visceral, pa- 

 rietal, and pleural ganglia. In other 

 forms the ganglia become more 

 perfectly marked off and at the same 

 time more concentrated. A cerebral 

 ganglion (Fig. 155, c) is always dis- 

 tinguishable, and with it are con- 

 nected pleuro-parieto-visceral {pi 

 and v) and pedal (p and p') ganglia ; 

 the latter, however, are usually divi- 

 ded into two portions — a more ven- 

 tral mass {p') which sends branches 

 to the armlike prolongations of the pro- and mesopodium and 

 which is hence termed the brachial ganglion, and a more dorsal 

 one {p) which supplies the infundibulum and is known as the 

 pedal ganglion proper. A study of a number of different 

 forms shows clearly that the brachial ganglion is merely a 

 separated portion of the pedal, and that the arms are to be con- 

 sidered portions of the foot and are not cephalic appendages. 

 At the sides of the cerebral ganglia there are to be found a 

 pair of large ganglia (op) which stand in relation to the eye and 

 are termed the optic ganglia ; they are undoubtedly spe- 

 cializations of the cerebral ganglia, owing their separate exist- 



PP' B 



I^G. 155. — Nekvous Ganglia 



oir {A) Loligo and {B) Octopus 

 (after Pelskneer). 



b = buccal ganglion. 



= cerebral ganglion. 



p = pedal ganglion. 

 p' = brachial ganglion. 

 op = optic ganglion. 



V = pleuro - parieto - visceral 

 ganglion. 



