GANGLIONIC NERVOUS SYSTEM 527 



locust is stimulated by light, for example, impulses have been recorded as 

 far caudally as the last thoracic ganglion (Burtt and Catton, 1952). Never- 

 theless, these ganglia do not act merely as automatic relay-stations. As we 

 have already seen ^ disturbances of the normal mechanism of locomotion by 

 the amputation of a limb are largely corrected by suitable alterations in the 

 reflex progression (Bet he, 1930 ; v. Hoist, 1935 ; ten Gate, 1936 ; and 

 others). Moreover, within the nerve-cord, the available connections are 

 multiple and after experimental interference it has been showTi that the 

 choice of a particular pathway depends on such factors as the strength of 

 stimulus and the ease of transmission (Prosser, 1935, in the cra;yfish). In 

 view of the complexity of the instinctive behaviour, particularly of Insects, 

 as exemplified in the complicated social behaviour of the ant or the dance 

 of the honey-bee by which it indicates to its fellows the location of a honey- 

 store,^ and in view of their limited but very definite capacity to modify 

 their behaviour by learning and conditioning, it would seem that the gan- 

 glionic organization of Arthropods with its cerebral dominance has reached 

 a very high level indeed of integration. It must be remembered, however, 

 that despite their complexity and seemingly intelligent basis, these complex 

 patterns of behaviour are all innate and their performance depends on the 

 development of the appropriate parts of the nervous system or, in the case 

 of sexual instincts, on the development of hormones at a somewhat later 

 stage in life. Even although their behaviovu" does seem often elaborate 

 and sometimes full of intelligence, however, individual adjustment to any 

 peculiar circumstances is relatively unknown in their totalitarian lives ; 

 individuality and personality cannot be attained below the level of a cen- 

 tralized brain. 



THE NERVOUS SYSTEM OF MOLLUSCS 



In MOLLUSCS which are unsegmented and without appendages, the 

 nervous system appears to be different but nevertheless is basically similar 

 to that of worms and Arthropods. In its essentials it consists of paired 

 dorsal cephalic ganglia which receive sensory fibres from the eyes and other 

 sense-organs ; these ganglia are connected by a short circum-cesophageal 

 nerve-ring with paired pleural and pedal ganglia. Tj^ically, as an offshoot 

 from this bunched-up ganglionated ring m the head-region, a stomato- 

 gastric loop from the cerebral ganglia runs below the gullet bearing two 

 buccal ganglia, and a visceral loop provided "wdth visceral ganglia is given 

 off from the pleurals (Fig. 697). In some types the ganglia m the cephalic 

 ring are separate (Gastropods), in others they are so closely associated that 

 some appear to be fused (cerebro -pleural in Lamellibranchs), while in Cephalo- 

 pods the fusion is almost complete. In these last the three pairs of ganglia are 

 crowded into the head region around the oesophagus so closely that their 



1 p. 59. * p. 70. 



