520 



THE EYE IN EVOLUTION 



they unite as the first ventral ganghon and then run to the posterior 

 extremity of the body to form a double but compact united ventral nerve- 

 cord. The segments are short and the segmental paired ganglia which 

 connect with the subepidermal nerve plexus are almost confluent ; and down 

 the nerve-cord there run three dorsal and two ventral giant fibres which 

 transmit impulses down the entire length of the worm, mediating rapid 



end-to-end " startle " reactions (Stough, 

 1926-30 ; Smallwood and Holmes, 1927 ; 

 Bullock, 1945).^ The peripheral nerve- 

 plexus is largely a sensory relay, and 

 although occasional connections ^ between 

 sense organs directly to the underlying 

 muscle may persist, they are unimportant 

 in behaviour over which the central nerve- 

 cord has taken complete control (Janzen, 

 1931 ; Coonfield, 1932 ; Prosser, 1935 ; 

 and others). 



In the polychaete worms, the segmen- 

 tation becomes more obvious : the well- 

 formed cerebral ganglion, the oesophageal 

 ring and the commencement of the ventral 

 ganglionated cord of Nereis are seen in 

 Fig. 685. The bi-lobed cerebral ganglion, 

 which resembles structurally the cerebral 

 ganglion of Arthropods, receives nerves 

 from the tentacles and palpi as well as 

 the short, thick of)tic nerves from the four 

 simple eyes which seem almost to be sit- 

 ting upon it. 



Fig. 685. — The Nervous System of 



THE POLYCH.'ETE WoRM, NeJUIS. 



CO, cerebral (supra-oesophageal) 

 ganglion, in close association with 

 which are the 4 eyes — a paired 

 anterior, E^, and posterior, E"^. The 

 infra- oesophageal ganglion, G, marks 

 the beginning of the ganglionated nerve 

 cord, N, connected to the cerebral 

 ganglion by circuni-oesophageal con- 

 nectives, C (after Q^iatrefages). 



The progress of cephalic dominance in the 

 segmented worms is interesting. Normally, the 

 earthworm is negatively phototactic to light, 

 but after removal of the cerebral ganglion the 

 direction of the response is reversed ; if the ventral cord is sectioned the anterior 

 part of the animal turns away from the light, the j^osterior towards it (Hess, 1924 ; 

 Nomura, 1926-27 ; Prosser, 1934 ; Howell, 1939). The negative responses are thus 

 controlled by the brain, the positive by the ventral cord. The activity of the cerebral 

 ganglion therefore normally dominates that of the lower ganglia, the responses of which 

 it normally opposes. After the cerebral ganglion is removed from the earthworm, the 

 animal remains active, eats, burrows and copulates, the reactions, however, being per- 

 formed some 10 or 15 times more slowly (30 as compared with 2 minutes) ; a similar 

 decrease in responses is induced by subnormal temperatures or depressive drugs. The 

 same operation in Nereis, on the other hand, leaves it overactive in its responses to 



^ Th( -need of travel in the giant fibres is 17 to 45 m./sec, whereas that in the small fibres 

 of the eov : • 0-025 m./sec. (Bovard, 1918 ; Eccles et al., 1933 ; Bullock, 1945). 

 « Not i ,11(1 in the Polychrptes (Just, 1924). 



