92 THE EVOLUTION OF MAN 



transmission, such as is abundantly seen in sea-anemones, seems 

 to be completely absent from sponges. When this fact is 

 coupled with the inability of histologists to discover any trace 

 of nervous tissue in sponges, it seems safe to conclude that 

 these animals are devoid of nervous elements and that their 

 muscles are set in operation by direct stimulation rather than 

 by anything that may be described as a nervous impulse. From 

 this standpoint, therefore, sponges may be regarded as 

 animals provided with muscles but devoid of nervous tissue. 

 They possess effectors but no receptors and their condition 

 suggests that in the evolution of the neuromuscular mecha- 

 nism, muscle appeared first and nervous tissue later, develop- 

 ing probably in close proximity to the previously formed 

 muscle as a device for exciting the muscle to action. 



If this conclusion is true, the evolution of the neuromuscular 

 system must have begun with the appearance of muscle as an 

 independent effector, after which receptors, or nervous ele- 

 ments to serve as triggers for setting off the muscles, were 

 added. Finally, between the receptors and the effectors a 

 central nervous organ, or adjuster, grew up as the third ele- 

 ment and thus completed the neuromuscular system as exempli- 

 fied in the higher animals. So important has this central organ 

 become in the life of these animals that we scarcely realize 

 that it is the latest addition to our nervous equipment and yet 

 such seems to be the case. In its first appearance it must have 

 been chiefly an organ of transmission and intercommunication, 

 a specialized outgrowth of the more diffuse nerve-net. Later 

 it doubtless took on the function of modifying the animal's 

 responses in relation to its past and thus became a storehouse 

 of experience and the seat of the intellectual life. As such it 

 has reached its highest point of development in the verte- 

 brates where as the brain of man it is without doubt the most 

 remarkable structure ever evolved. 



