400 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



end of the lateral trunks. Connected, finally, with 

 the two chief nerve trunks is a network of nervous 

 cells and fibres, which lies just below the dermis, and 

 forms a continuous layer over the whole of the worm. 



In the Turfoellaria we find also that the nervous 

 system is superficial in position, and that the nerve 

 fibres so branch as to be distributed widely over the 

 surface of the body. 



A similarly primitive condition obtains in the 

 I :< h i iKHh'iiii:! I :i the epidermis consists not only of 

 supporting: cells, but of others which are sensory? 

 and have their basal ends continued into nerve fibrils, 

 which ordinarily run parallel to the surface of the 

 body ; with these fibrils small ganglion cells are con- 

 nected (Hamann) ; as a result of this, we have a 

 continuous sheath of nerve tissue investing the body 

 of a starfish or of an Echinoid (Fig. 171). In the 

 Ophiuroid and the Holothurian, the superficial nerve 

 plexuses have as yet been detected only on the tube 

 feet. By far the greater part of the nervous system 

 is superficial in the starfish, for the nervous band 

 that runs down the groove of every arm is placed just 

 below the investing epithelium ; and, in addition to 

 this, the more primitive histological condition is still 

 retained, for the ganglia are scattered among the nerve 

 fibres, and not collected into separate masses. 



Having now sufficient evidence of the truth of the 

 statement that the nervous system is primitively 

 superficial in position ; that is to say, that at first the 

 nerve cells lie side by side with the epithelial cells, and 

 that they gradually come to lie just below the epithe- 

 lial layer, we may return to that plexiform disposition 

 of fibres which precedes the arrangement in definite 

 strands or cords. Evidence as to this is afforded by 

 the most primitive members now existing, both of 

 the Arthropoda and of the Mollusca. Of the 

 former, Peripatus is a striking example (Fig. 172). 



