METAZOA 137 



The nerve-net is joined by processes from the bases of the sense 

 cells. Probably at an early stage in the evolution of the Metazoa 

 stimuli were transmitted only by such processes, running directly 

 from the sense cells {receptor cells) to end against the muscle or other 

 cells which are set in action through them {effector cells). This con- 

 dition, however, is now rare, occurring only in the tentacles of some 

 coelenterates : nearly always there are nerve-cells which have left the 

 surface layer, whose processes continue those of the sense cells and so 

 extend and complicate the system of communications. In the primitive 

 condition of the nervous system, as seen, for instance, in Hydra, the 

 nerve-cells have numerous similar branches, forming a network over 

 which messages pass in all directions from any point of stimulation. 

 That there is co-ordination in the action which results is due only to 

 the fact that the messages do not evoke responses equally in all the 

 effector cells. 



The condensation of nerves and a central nervous system out of 

 this network is due to a change in the form and arrangement of the 

 elements of which it is composed. The change, which is already fore- 

 shadowed in certain parts of the bodies of coelenterates (p. 150), 

 consists in processes of the nerve cells elongating in particular 

 directions and thus forming paths of conduction which in the higher 

 triploblastica are isolated by the loss of the rest of the network. As this 

 system is perfected its elements become neurones — cells with one main 

 process, the axon or nerve fibre, along which the impulse passes from 

 the cell body. The axon ends by breaking up into a tuft of branches, the 

 terminal arborization, from which a stimulus is given either to another 

 nerve cell or to an effector cell. Thus instead of spreading in all 

 directions the impulse is conducted to a definite destination: the 

 interference of the environment in the affairs of the organism is 

 regulated. The cell body may be a sense cell in the epithelium, or it 

 may be internal. In the latter case it possesses other processes — the 

 dendrons — which by fine branches — the dendrites — receive stimuli 

 from the axons of other neurones. Two neurones at least are con- 

 cerned in the transmission of an impulse. In the simplest case the 

 axon of a sense cell (or of a cell whose dendriies receive stimuli from 

 a sense cell)^ transmits the impulse to a neurone whose axon conducts 

 it to the effector cell. This process is known as a refiex and the arrange- 

 ment of neurones as a reflex arc. The impulse is passed from the first 

 neurone to the second in an exchange station — the central nervous 

 system. The nerve fibres run to and from this station in bundles which 

 are the nerves. This arrangement is not only, as w^e have seen, more 



^ In the Vertebrata the cell bodies of the afferent fibres from the receptors 

 are moved far inwards to lie in the dorsal root ganglia, receiving impulses by 

 a dendron nerve fibre which is longer than the axon. 



