516 THE EYE IN EVOLUTION 



complete independence is retained is in the proboscis, which, if nervously- 

 isolated, amputates itself and shows independent food-seeking reactions 

 (Kepner and Rich, 1918). In Annelids and Molluscs, however, the nerve-net 

 serves only as a relay system over a local area without independent activity, 

 dealing with messages from the nerve-cord or ganglia. In general, the peri- 

 pheral system is the more important in sluggish animals but as rapidity of 

 response and general activity increase, the central mechanism takes over an 

 increasing share of control. 



Even in Ccelenterates, however, some early signs of specialization are seen within 

 the nerve-net. In some medusae and sea-anemones, through-tracts of long continuous 

 fibres form nerve-trunks for rapid conduction,^ and in sea-anemones a difference in 

 the response between the fi'ee and the central end of a cut tentacle indicates a primitive 

 type of polarity (Parker, 1917 ; Pantin, 1935), Moreover, the possibility of the 

 existence of crude reflex arcs is indicated by the recij^rocal contraction of circular and 

 radial muscles (Bozler, 1926). 



TRUNK-PATHWAYS 



Although a hint of preferential conduction appears in Coelenterates, the 

 advantage is obvious of short-circuiting the diffuse and indiscriminate 

 conduction in a nerve-net through trunk -pathways composed of long giant 

 fibres by which the transmission of vital messages between important points 

 is rapid and direct ; this is first achieved in Echinoderms in which radial 

 symmetry has been attained. In the starfish, for example, there is a diffuse 

 nerve-net, but from the sensory organs — the important olfactory and 

 statolith organs and the yet unimportant eyes — situated at the tips of each 

 of the five arms where the animal first contacts the dangers or opportunities 

 of its environment, there arises a large nerve-trunk which runs down to the 

 centre of the body where the five trunks combine to form a ring encircling 

 the oral aperture (Fig. 166). This central nerve-ring with its five radiating 

 nerve -trunks acts as the main directive system without which the animal 

 shows sluggish and poor coordination in such activities as righting move- 

 ments (Cole, 1913) ; at the same time these main pathways are linked 

 closely with the peripheral net, which even in isolation can effect a certain 

 amount of coordination, particularly by local reflexes between neighbouring 

 spines (Langeloh, 1937 ; Smith, 1937-50 ; Kinosita, 1941). At this stage 

 central control is neither fixed nor complete but there is a plastic reciprocity 

 between it and the still important peripheral system. It is probably for 

 this reason that the starfish, although showing considerable complexity in 

 behaviour in such reactions as feeding, righting itself, or escaping from 

 restricted confines, yet shows no ability to profit by experience by adopting 

 persistent modifications in its conduct (Jennings, 1907). Indeed, the animal 

 may pull itself apart by the antagonistic activity of its o^\^l tube-feet 



1 Call j's, in which the velocity of the contractile wave is 1-2 m./isec, compared with 

 0-15 m./set the nerve-net (Pantin, 1935). In the nerves of the cat it is 119 m./sec. 



