204 THE ELEMENTARY NERVOUS SYSTEM 



between receptors and effectors represents the final step 

 in the growth of this group of organs. As just indicated, 

 the novel" element in this particular combination is the 

 central organ or adjuster, and this arises in the region 

 between the receptor and the effector and out of that 

 material which in the elementary system constitutes the 

 nerve-net. Its growth is associated with an inward mi- 

 gration whereby it retreats from the surface of the ani- 

 mal to a deeper situation and comes thus to gain what is 

 significant of its growing importance, a certain degree of 

 protection. In the coelenterates the nervous elements are 

 mostly contained in the epithelial layers of the body and 

 especially in the external epithelium, the ectoderm. In 

 the worms, where the body has gained greatly in thickness 

 and solidity as compared with the coelenterates, this in- 

 ward migration is clearly seen. These animals no longer 

 possess the diffuse system of the lower forms, but have a 

 definitely centralized band of tissue extending the length 

 of the body. The relation of this band to the ectoderm is 

 well seen in a series of annelids. In Sigalion, a marine 

 worm, this band, which consists of a brain and a ventral 

 nerve-chain, is still a part of the superficial ectoderm 

 (Fig. 48). In Nereis, by a process of delamination, the 

 ventral chain has freed itself from the skin, though the 

 brain is still a part of that layer. In the earthworm this 

 process has been carried to its completion and the whole 

 nervous band has separated from the ectoderm and mi- 

 grated into the deeper parts of the animal (Fig. 49). This 

 condition of a deep-seated central nervous organ is char- 

 acteristic of the higher invertebrates such as the arthro- 

 pods, mollusks, and so forth, and, as their embryology 

 shows, it was brought about by a process analogous to 

 that seen in the worms. In the vertebrates this inward 



