191 1.] THE PRmiTIVE NERVOUS SYSTEM. 219 



sory cells in the integument of the earthworm. These organs have 

 for their function the reception of the external stimuli and the pro- 

 duction of the sensory impulses. The receptors are connected by 

 nerve-fibers with the central nervous organ or adjustor composed of 

 the central ends of the sensory and the motor neurones and of the 

 association neurones. Here the impulses arriving from the re- 

 ceptors are directed toward the appropriate groups of muscles by 

 which the animal may respond to the stimulus and, if the animal is 

 highly organized, impressions are made upon the adjustor which, as 

 memories, may become more or less permanent parts of the animal's 

 nervous equipment. Finally the adjustors are connected by nerve- 

 fibers with the third set of elements, the effectors, which as muscles, 

 electric organs, glands, etc., enable the animal to react on the en- 

 vironment. Thus three physiological categories are to be distin- 

 guished which in the order of their sequence in action are sense 

 organs or receptors, central nervous organs or adjustors, and muscle, 

 etc., or effectors. 



It is to be noted in passing, that the physiological scheme just 

 outlined includes a wider range of parts than is generally admitted 

 under the head of the nervous system. The additional parts are the 

 effectors, which, as will be shown later, form as truly a part of the 

 whole system as do the sense organs or the central nervous organs. 

 Since the term nervous system does not ordinarily include the effec- 

 tors, it is perhaps best to designate the whole chain of related parts, 

 receptors, adjustors, and effectors, as the neuromuscular mechanism 

 and in dealing with the origin of the nervous system, it will be found 

 important to keep this relation in mind, for in such an inquiry, the 

 real question that must be confronted is the origin of the neuromus- 

 cular mechanism rather than that of the nervous system alone. 



The type of neuromuscular mechanism described in the preced- 

 ing paragraphs in which a group of receptors is connected with a 

 well centralised adjustor which in turn controls a complex system of 

 effectors, is found only in the more dift'erentiated metazoans. Cer- 

 tainly in the simple metazoans, like the jellyfishes, corals, sea-ane- 

 mones, etc., only the slightest evidence of this type of nervous orga- 

 nization can be discovered. Nevertheless these animals possess a 

 neuromuscular mechanism but on so simple a plan that investigators 



