154 NERVOUS CORRELATION 



ceptor which is stimulated and the effector which ordi- 

 narily comes into action as a result of its stimulation 

 is known as a reflex path. We can imagine an animal 

 which has anywhere from twenty to one hundred recep- 

 tors, and from half-a-dozen to forty effectors, exhibiting 

 a great variety of simple reflex actions, as one receptor or 

 another is stimulated and so one or another effector is 

 thrown into operation. 



Nervous Organization in Jointed Animals. — In the 

 paragraph above a situation is pictured as existing in a 

 fairly simple metazoan whose neryous system consists of 

 a single ganglion with outlying nerve-cells connecting it 

 with the receptors on the one hand, and with the effectors 

 on the other. Complexity of bodily structure comes 

 about in the animal kingdom in various ways, but the 

 one which is characteristic of most of the higher animals 

 is the device of jointing, that is, of having the body made 

 up of a sequence of joints or segments. In its simplest 

 form such a jointed body has each division exactly like 

 every other, and such a body can be represented as in 

 Fig. 43, which is simply Fig. 42 repeated over and over. 

 In an animal so constructed each joint has all the 

 organs necessary for the maintenance of life, and such a 

 mechanism could be thought of as a colony of relatively 

 independent organisms. Needless to say, practically all 

 of the jointed animals go beyond this simplicity, and there 

 is specialization among the joints (in respect to most 

 organs') so that some joints carry on one bodily function 

 and some another. So far as the nervous organization 

 is concerned, however, the specialization is in many cases 

 not very marked. As Fig. 43 shows, the structure in 

 such organisms consists essentially of a reduplication 

 of the arrangements already described for a simple 

 organism, that is to say, each segment has its 

 receptors, its effectors, its ganglion, with sensory 

 nerves passing from the receptors to the ganglion and 

 motor nerves from the ganglion to the effectors. In order 



