86 THE EVOLUTION OF MAN 



upon this plan. The so-called brain of this animal is a small 

 mass of nervous material lodged in the dorsal part of its 

 anterior end. Connected with this brain is a long segmental 

 chain of ganglionic enlargements extending along the ventral 

 midline of the worm. These parts collectively constitute the 

 central nervous apparatus, or adjuster, for which the skin is 

 the chief receptor and the muscles the main effectors. In 

 neuronic composition the three types of elements already de- 

 scribed are abundantly represented. Afferent neurones reach 

 from the skin of the worm to its central organs from which 

 efferent neurones pass out to its muscles. Internuncial neurones 

 are also present, but in relatively small numbers as compared 

 with the conditions found, for instance, in the vertebrates. 

 Here many parts, like the cerebral hemispheres, to take only 

 one example, are made exclusively of internuncial neurones 

 whereas in the earthworm there is probably not a single im- 

 portant nerve center that is not entered by sensory neurones 

 or which does not give rise directly to efferent neurones. 

 With this difference, however, the neuromuscular mechanism 

 of the earthworm is based on the same principles of construc- 

 tion as those met with in the higher vertebrates including man 

 himself. 



THE NERVOUS ORGANIZATION OF SEA-ANEMONES 



To gain some idea of the evolutionary steps by which such 

 a nervous system as that just described has been arrived at, it 

 is plainly necessary to examine the types of nervous organiza- 

 tion found among the lowest of the multicellular animals. As 

 a good example of these lowly organisms the sea-anemones 

 may be selected. Sea-anemones are sac-like animals attached 

 to rocks or stones, and provided with a single aperture which 

 leads from the exterior into their large central cavity in which 

 digestion goes on and from which the undigested residue is 



