CONCLUSION 



205 



migration of the central organ also takes place, but in 

 this group it is accomplished, not by a process of delam- 

 ination, but by an involution of a portion of the skin 

 itself. The infolded layer of ectoderm thus produced 

 assumes the form of a tube and the walls of this tube by 

 differentiation give rise to the spinal cord and brain, the 

 two chief organs in the central nervous system of these 

 animals. 



The concentration of the diffuse nervous activities 

 of the lower animals from the skin in general to a par- 



FIG. 49. Transverse section of the ventral nerve-chain of the earthworm Allolobophora 

 showing this chain as separated from the superficial ectoderm of the worm but still retaining 

 on its ventral or more superficial side the ganglion cells g, and on its deeper side the nerve 

 fibers /. (After Hatschek, 1888.) 



ticular part of this layer and the separation of this part 

 from the rest of the skin, together with its migration 

 into a deeper position in the animal, are rearrangements 

 in which are retained a good deal of the original distribu- 

 tion of the elements as seen in the more primitive sys- 

 tems. Thus in the ccelenterates the elements of the re- 

 ceptor-effector system consist of cell-bodies in the super- 

 ficial part of the epithelium, followed by transmitting 

 fibers, in the midregion, and finally completed' by muscle 

 fibers in the deepest part (Fig. 3). When this system 

 concentrates and breaks away from the skin, as it does in 

 the annelids and the higher invertebrates, it is not sur- 



