524 THE EYE IN EVOLUTION 



there are three — the lamina ganglionaris (or first optic ganghon), the 

 EXTERNAL MEDULLA (or second optic gangHoii) and the internal medulla 

 (or third optic ganghon) which is frequently divided into two or more parts. 

 In some Decapods there are two, while in others, as the crayfish, Cambarus, 

 there are four, a terminal medulla (or fourth optic ganglion) lying 

 proximal to the third. The fibres from the visual cells of the compound 

 eye enter the first optic ganglion directly ; between the ganglia there are 

 two well-marked decussations of fibres, and from the proximal ganglion the 

 afferent fibres enter the cerebral ganglion by several tracts to terminate in 

 the primary optic association areas, particularly the pedunculate body, and 

 to decussate over to the opposite side. Removal of the cellular portion of 

 the pedunculate body abolishes certain responses to light (Bethe, 1897). 



From the ocelli (when they are present) the visual fibres end in a 

 ganglion just proximal to the eye wherein a second neurone enters the 

 protocerebrum and after making connections with the fibres from the optic 

 lobes, seeks the visual centres (Fig. 696)^. 



From the optic centres fibres pass downwards through the circum- 

 oesophageal commissures into the thoracic cord. These fibres have been 

 divided into two systems by Satija (1957) (Fig. 696) : several ipsilateral 

 fibre-tracts pass downwards from each optic ganglion into the commissure 

 on the same side while a single large fibre, also arising from each optic 

 ganglion, crosses in the midline to enter the contralateral commissure. On 

 visual stimulation action potentials have been recorded along their route 

 (Parry, 1947 ; Burtt and Catton, 1952-54) and they presumably link up 

 the visual stimuli with the reflexes mediated by the nerve cord. 



It is interesting that the brain of Insects is large in those with the more complex 

 behaviour ; thus that of Dytiscus is 1/400 of the body- volume, of the bee 1/174 

 (Wigglesworth, 1953). Moreover, the size of the visual centres varies similarly with 

 the degree of developinent of the eyes. In Arachnids and Myriapods with simple 

 eyes the visual centres are some 0-3 to 2-8% of the size of the brain ; in Crustaceans 

 and Insects with rudimentary compound eyes, it is 3 to 10% ; in those with elaborate 

 compound eyes, up to 80% (Hanstrom, 1928). 



It is noteworthy that synchronized spontaneous rhythms resembling those of the 

 vertebrate brain have been found in the ganglia of Arthropods and Molluscs, indicating 

 a considerable degree of coordination and a high level of excitability in the constituent 

 neurones. 2 This type of activity, it will be remembered, is characteristic of integrative 

 centres and absent in those with purely distributive and sensory functions. 



In function, the cerebral ganglion of Arthropods plays a decisive role 

 in the animal's conduct. Apart from its essential purpose as a receiving 



^ For the structure of the nervous system of Arthropods, see Cajal (1918), Snodgrass 

 (1926), Hanstrom (1928-35), Ehnbom (1948) ; for the action-potentials in the optic ganglia 

 on stimulation by light, see Adrian (1937) in the water-beetle, Dytiscus : Crescitelli and Jahn 

 (1942), Bernhard (1942), Burtt and Catton (1956), in the grasshopper, Chortippus ; Antrum 

 (1950), Burtt and Catton (1956), in the blowfly, CaUiphora ; and Burtt and Catton (1954-56), 

 in the locust, Locusta niigratoria, and the larva of the dragon-fly, Aeschna. 



^ The ' ter-beetle, Dytiscus — Adrian (1937); the grasshopper, Chortippus — Crescitelli 

 and Jahn ;•_'); the slug, ^r('oZima.r— Bullock (1945); the blowfly, CaZ?i>/iom—Burkhardt 

 (1954) ; the jst, Locusta mi gr a toria— Burtt and Catton (1956). 



