518 



THE EYE IN EVOLUTION 



rating the centralization of the nervous system. In this way the enormous 

 economic benefit of central exchanges became possible, at first with one or 

 more trunk-pathways running from the dorsal head-ganglion down the length 

 of the body, and then as segmentation of the body progressed, with the 

 interposition of ganglia in the central chain, each ganglion gathering up and 

 issuing incoming sensory and outgoing motor nerves to its owti particular 

 segment. Within and between the segmental ganglia the incoming and 

 outgoing nerves combined with associated neurones in the central system 

 itself to form a complicated interconnecting network, the neuropile, on 

 which reflex activity could be built ; by means of these neurones which have 

 no direct connection with the exterior, in association with the giant fibres of 

 the trunk-pathways, the activities of the whole organism are coordinated, a 

 foretaste of the infinitely complex system which finally constitutes the 

 cerebral cortex of the Primates. In this way peripheral control through the 

 subepidermal nerve-net gave place to central control through reflex path- 

 ways and the way was prepared for the dominance of cephalic sense-organs 

 and nerve-centres, an arrangement seen in the nervous systems of worms, 

 Arthropods and Molluscs. 



THE NERVOUS SYSTEM OF WORMS 



The initial stage in the development of the ganglionic nervous system 

 is thus the appearance of a single cephalic ganglion from which issues a 

 number of nerve-trunks which break up into the peripheral nerve-net. This 



Figs. 681 and 682. — The Nervous System of Unsegmented Worms. 



Oc— 1 



_ PAIRED 



l^^^ 5-'--" LATER AL 

 NERVES 



Fig. 681. — The nervous system of 

 a primitive turbellarian worm. 



Consisting of a cerebral ganglion, 

 CG, Nvith several nerve trunks and a 

 subepi-.-.rmal nerve-net. 



Fig. 682. — The nervous system of 

 a higher type of turbellarian 

 worm. 



The fused cerebral ganglion, CG, 

 with two closely associated ocelli, Oc, 

 and paired nerve-trunks (after Hat- 

 schek and Stempel). 



