NERVOUS SYSTEM AND BEHAVIOUR 433 



tute the entire nervous system, as in coelenterates, or exist conjointly with 

 distributive pathways (balanoglossids), or co-ordinating centres (echino- 

 derms). Typically, the nerve-net displays diffuse conduction, a necessary 

 corollary of its organization, and is also characterized by so-called decre- 

 mental conduction and facilitation. Decremental conduction is a conse- 

 quence of facilitation, and refers to the need for a barrage of impulses in 

 order to force excitation into distant parts of the nerve-net. Facilitation, 

 however, is not an exclusive characteristic of the nerve-net: interneural faci- 

 litation is found in the polychaete nerve cord, and neuromuscular facilita- 

 tion in claw-muscles of decapod Crustacea. 



The nerve-net by itself can initiate intrinsic phasic activity, through 

 functional differentiation of temporary or permanent pacemakers. Through- 

 conduction pathways exist, even in coelenterates, as special condensations 

 of fibre-tracts. Distributive pathways are well marked in balanoglossids, 

 and assume a co-ordinating role in asteroids. Functional polarization is 

 manifest in some regions of the net, in the different levels of facilitation 

 required for transmission in opposite directions. Central and reflex nervous 

 organization has probably developed through condensation of associative, 

 distributive and efferent neurones into centralized ganglia and cords 

 spatially arranged in patterns conformable to overall organization of body 

 form. This has been accompanied by progressive suppression of peripheral 

 nets. Enteric nets, such as those of the Arenicola extrovert, are best re- 

 garded as outliers of the central nervous system, secondarily developed in 

 conjunction with specialized effector structure and habits, and acting as 

 semi-autonomous nervous centres. Nerve-nets of other phyla still await 

 exploration, e.g. polyclads, nemertines, phoronids {see, however, Silen 

 (101)), ascidians, etc.; many features of those already studied would be 

 further clarified by electrical recording. 



Through-conduction Systems 



Nerve-nets and neuropile consist mostly of short nerve fibres, but many 

 animals have, in addition, through-conduction tracts which extend long 

 distances. Often the fibres concerned are conspicuously large and are 

 termed giant axons. Giant-axon systems have evolved independently in 

 many groups, in which they mediate quick responses to harmful stimuli, 

 withdrawal reflexes and the like. 



In the mesenteries of sea-anemones, such as Metridium, there is a well- 

 defined through-conduction system of large fibres extending 7 or 8 mm 

 before forming synapses. The conduction velocity in these tracts is much 

 higher than elsewhere, and there is little synaptic delay. Functionally, the 

 through-conduction system is concerned with transmitting impulses which 

 call forth the protective withdrawal reflex (82). 



Among animals more complex than coelenterates, the following have 

 giant-axon systems: cestodes, nemertines, many polychaetes, decapod 

 cephalopods, many crustaceans, especially decapods, brachiopods, phoro- 

 nids, enteropneusts and lower vertebrates. Several patterns can be recog- 



