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 Turoellaria 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 

 ~Ecliiiioclermata ; 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) \ 



continuous sheath of iiervejbiasua in vesting, th ft body, 

 Xk_a. starfish nr nf art ffchiTjnid (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 



X ig-.fmpegoia.]_jn he- gtgrfishj 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 



Having now sufficient evidence of the truth of the 

 statement that the nervous system is primitively 

 superficial in position ; that is to say, tha.t n.t first the 

 lig siil_by.pidp wif.Vi fcW_apif.]i filial ppllp and 



grp,r|np.11y nrt^ie to IJP jllFiti 



-rj we may return to that -pla \ i f oi'jiurl i sposjiion 



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 Arthropod a and of the Molliisca. Of the 



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



