338 ' THE POPULAR SCIENCE MONTHLY 



THE ORIGIN OF THE NERVOUS SYSTEM AND ITS 

 APPROPRIATION OF EFFECTORS 



By G. H. PARKER 



PKOFESSOH OF ZOOLOGY, HAEVAED UNIVEESITY 



IV. The Appropriation of Effectors 



IN the preceding articles in this series the origin and development of 

 the neuromuscular mechanism has been broadly sketched in a suc- 

 cession of representative stages. The first stage was that of the inde- 

 pendent effector, the muscle which was brought into action by the direct 

 influence of environmental changes as seen in the pore sphincters of 

 sponges. The second stage was that of the combined receptor and 

 effector in which the receptors, in the form of diffuse sensory epithelia 

 or specialized sense-organs, served as delicate triggers to set the muscles 

 in action and thereby render the effectors responsive to a wider range 

 of stimuli than they would be under independent stimulation. Finally, 

 the third stage is seen in the complete neuromuscular mechanism in 

 which a central nervous organ or adjustor has developed between the 

 receptors and the effectors. This adjustor serves as a switchboard for 

 nervous transmission and a repository for the effects of nervous 

 activities. 



This line of progressive differentiation from the muscle to the com- 

 plete nervous system is complicated by the fact that in the more com- 

 plete examples of the third stage the nervous system is found connected 

 not only with such effectors as muscles, but with electric organs, chro- 

 matophores, glands, luminous organs, etc. If the history of the growth 

 of the neuromuscular mechanism as it has been sketched in these articles 

 is a correct one, the effectors just named must be regarded in the light 

 of relatively recent acquisitions and in my opinion they illustrate an 

 invasion and appropriation on the part of the nervous system of terri- 

 tory that was not originally under its control. This principle of appro- 

 priation results not only in the acquisition of totally new forms of 

 effectors such as glands, etc., but also in gaining control over inde- 

 pendently and newly developed muscles. Examples of this kind will be 

 taken up first in discussing this question of nervous appropriation. 



The differentiation of the central nervous organs is in large part a 

 process that goes on hand in hand with the differentiation of the 

 muscles. This is well seen not only in the higher invertebrates, but also 

 in the vertebrates. The differentiation of a single muscle into a group 

 of muscles and the consequent and corresponding changes in the nerv- 



