RELATION OF THE NEMERTEA TO THE VERTEBRATA. 609 



Another series may indeed be supposed to have derived 

 longitudinal stems from a ring which became extended to form 

 lateral cords, as the animal passed from the radial to the 

 bilateral symmetry, in the way suggested by Balfour. Still, 

 even in this case, a nerve-plexus may be expected to be coex- 

 istent with or to have preceded the nerve-ring. The longitu- 

 dinal stems originating from the anterior thickenings of the 

 plexus that innervate the sense organs and the tip of the head 

 (specially sensitive in connection with the forwardly directed 

 movements of the body), would all the more probably be pre- 

 served and increase in development, as during this forward 

 movement they form a right and a left centre for the reception 

 of outward stimuli. In the same way those of the radially 

 arranged stems of the Polyclada that are parallel to the longi- 

 tudinal body-axis, and mark out right and left, are more 

 strongly developed than the others, presumably on account of 

 their importance in connection with the well-directed move- 

 ments of the body in response to external agents. 



In the ancestral MoUusca I think we may assume with 

 great probability the presence of four longitudinal stems 

 — two latero-dorsal and two latero-ventral ones; in the ances- 

 tral forms of Annelids and Arthropods two, which have gra- 

 dually coalesced ventrally, as was first suggested by Gegenbaur. 

 Again, in Nematodes differently situated longitudinal stems in 

 what was originally a uniform plexus are preserved ; whereas 

 in ancestral Neraertea two lateral longitudinal trunks in the 

 plexus were undoubtedly characteristic features. 



That one medio-dorsal stem in this plexus, in which all the 

 impressions made by outward agencies on both halves of the 

 body might be concentrated, and from whence the correspond- 

 ing movements might be regulated, will more fully answer the 

 purpose than two lateral stems, however they may be united 

 by an intervening plexus, is a priori probable, and would 

 explain the first impulse towards the formation of such a 

 longitudinal concentration in the uniform plexus. 



And when once such a dorso-raedian stem is present, in 

 addition to two lateral ones, a struggle for supremacy, presided 



