EVOLUTION 



265 



all the neuro-muscular mechanisms in the organism. This 

 is effected by free anastomosis of the nerve-fibres — an 

 arrangement of which the best instance is seen in the jelly- 

 fish (Fig. 41). Here a plexus of nerve-cells and nerve- 

 fibres connects all the sensory cells on the outer surface 

 with all the contractile cells in the interior, the nerve- 

 fibres being continuous throughout. This has been termed 

 the diffuse nervous system. Such an arrangemenb has its 

 advantage and its hmitation. The advantage is that the 

 whole motor apparatus can be immediately brought into 

 action as the result of a stimulus arising at any one spot on 



m 



.SEMiOICC CELL-. 



M'J5CUi-/iR. PK0CE33 



NERyE-CElL 



^MUSCLE CELL 



Fig. 40. — Diagram (after Foster) to show the evolution of the 

 nervous system. 



the epithehum. The hmitation is that the whole muscular 

 mechanism must be brought into play if at all. Owing to 

 the freedom of the nervous connections no contraction of 

 parts of the muscle sheet is possible. But from the nature 

 of the organism this is not necessary, since locomotion, 

 which is the only response possible, can only be brought 

 about by a contraction of the whole swimming-bell. The 

 response, then, in this stage is always crude and maximal. 

 It is possible that the nerve-net system is represented in 

 Auerbach's plexus of the intestine. 



The development of the capacity for graded responses 

 is the fifth and last stage in the evolution of the nervous 

 system. It is associated with the appearance of the 



