GANGLIONIC NERVOUS SYSTEM 



521 



light or chemical stimuli but unable to burrow. If the suboesophageal ganglion is then 

 removed, the worm lies quiet and inert (Loeb, 1894 ; Maxwell, 1897 ; Prosser, 1934). 

 It would thus seem that the cerebral ganglion is primarily a sensory centre exercising 

 an inhibitory control upon the motor centres in the suboesophageal ganglion. One of 

 the main functions of the brain is thus anticipated. In the group of worms, we there- 

 fore see the disappearance of peripheral independence, the 

 establishment of central control and the beginning of cerebral 

 dominance. 



THE NERVOUS SYSTEM OF ARTHROPODS 



THE ARTHROPOD NERVOUS SYSTEM is bllilt On the 



same plan as that of the polychaete worms. In 

 Crustaceans, such as the crayfish {Astacus) and in 

 Insects there is a bi-lobed cerebral ganglion receiving 

 sensory nerves from the eyes and the first two antennae 

 which contain the organs of smell, hearing, taste and 

 equilibration ; this connects by the circum-oesophageal 

 nerve-ring with the fused and ganglionated ventral 

 nerve-cord in which run giant fibres as well as asso- 

 ciated neurones (Fig. 686). In some of the smaller 

 Crustaceans and the Onychophora {Peripafus) the two 

 nerve-cords are widely separated. On the other hand, 

 in many of the higher Insects such as flies (Diptera) 

 several consecutive ganglia of the ventral nerve-cord 

 are fused (Figs. 687-91) ; in crabs (Decapoda), sessile 

 barnacles (Cirripedia), spiders (Araneida) and bugs 

 (Hemiptera) the fusion is complete so that the 

 ventral ganglia form a single mass (Figs. 688, 

 691). Moreover the higher Crustaceans and Insects 

 possess a simple visceral or sympathetic system con- 

 nected with the circum-oesophageal ring, which passes 

 backwards on the alimentary canal. 



The optic lobes and cerebral ganglion of Arthropods 

 are illustrated in Figs. 692 to 696. The cerebral ganglion 

 consists of three fused segments forming one mass : 

 (1) the PROTOCEREBRUM or optic segment forming the 

 greater part of the brain and receiving nerves from the 

 compound eyes and ocelli, (2) the deuterocerebrum 



derived from the antennary segment, and (3) the tritocerebrum from the 

 third segment of the head ^^•hich supplies the region of the mouth. The 

 whole structure contains a peripheral layer of ganglion cells with a central 

 mass of neuropile containing several groups of associative cells forming the 

 CENTRAL body, the PEDUNCULATE BODIES and Other smaller accumulations 

 of cells ; these are comparatively large in social insects and are generally 

 regarded as regulating behaviour. 



Fig. 686.— The Ner- 

 vous System of a 

 Typical Crusta- 

 cean. 



CG, cerebral (supra- 

 oesophageal) ganglia ; 

 OR, circum-CBsoiDha- 

 geal nerve ring ; 8 A, 

 sternal artery running 

 between a separation 

 of the two ventral 

 nerve-cords ; SOG, 



sub-ojsojjhageal gan- 

 glion (consisting of 6 

 pairs of fused ganglia); 

 G, a pair of fused 

 ganglia of the ventral 

 cord (after Thomson). 



