REPORT ON THE NEMERTEA. 123 



All this has already been stated by Balfour in clearer terms in his Comparative 

 Embryology (vol. ii. pp. 308, 311), where he describes the gradual steps by which a 

 radiate medusa-like animal may have passed into a bilateral worm-like form, with two 

 longitudinal nerve-stems, which are regarded by Balfour as the stretched nerve-ring of 

 the Medusa. 



I fully endorse these views ; only, with respect to the nervous system, I hold it to be 

 safer to leave out of comparison the already speciahsed nerve-ring of the Medusa, and 

 rather to go back to a Ccelenterate nervous system as primitive as that of the Actinia, 

 where the plexus, both of the epiblast and the hypoblast, with an increase in density in 

 the region of the mouth and the tentacles, may be said to be the fair representative of 

 one of the lowest starting points. In this the plexiform arrangement predominates. 



Now we find in all the lower invertebrates various though distinct nerve tracts 

 that are being specialised in this plexiform nerve-tissue according to the modes of motion 

 of the animal, and according to the general shape of the body. 



Thus in the Medusae, which move about in the water by annular contractions of the 

 lower portion of the bell-shaped body, one of the nerve-rings already alluded to was 

 demonstrated by the Hertwigs to innervate the musculature by which this is brought 

 about. 



In the Ctenophora the nerve-system is less satisfactorily known, but still Lang does 

 not hesitate to bring them into genetic relationship with the Polyclada (XVIIl). Among 

 the latter, Gunda, with its two longitudinal lateral stems, may be looked upon as 

 an extreme term in this series. 



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 co-existent with or to have preceded the nerve-ring. 

 The longitudinal 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 

 preserved 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 longitudinal 

 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 movements 

 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 gradually coalesced ventrally, as 

 was first suggested by Gegenbaur. Again, in Nematodes diff"erently situated longitudinal 



