145] COMPARATIVE STUDIES ON NEMATODES— HETBERINGTON 41 



temperature conditions or to the absence of calcium salts from the isotonic 

 saline used as an examination fluid — a point not fully appreciated at the 

 time of observation. 



In sections (Fig. 45) stained with Dobell's iron hematein one may dis- 

 tinguish the long cilia, an indistinct, rather fused row of basal granules 

 and the fibrils extending into the cell body. Usually the middle portion of 

 the cell is very granular and at times alveolar in appearance so the fibrils 

 disappear but sometimes reappear in the region of the nucleus. Such cells 

 are structurally identical with other ciliated cells and differ physiologically 

 only in the lack of motile cilia. 



Significance of Ciliation 



From a morphological point of view this loss of motion and graded 

 fusion of cilia indicate a retrogression and an atrophy because the divers 

 parts constituting the vibratile apparatus become less and less evident 

 until they are finally obliterated; but from a physiological point of view, 

 this regression, when it is a case of differentiation, is a step in advance, for 

 there results the formation of new organs with new functions. In the case 

 of nematodes the possession of cilia, though they be immotile, is best 

 construed as a hang-over from a more primitive condition of active cilia- 

 tion. This being the case, the way is open for phylogenetic speculations 

 and a still closer possible relationship can exist then between the nematodes 

 and the rotifers and gastrotricha to which the roundworms at present seem 

 most closely related. 



With a ciliated alimentary tract throughout, as rotifers have at present, 

 the ancestral nematode would necessarily have had no need of a muscular 

 sucking esophagus or lips or pharynges. A simple circular mouth in that 

 case would be the most logical form, such as many of the rotifers possess. 

 Perhaps then the very structureless mouth region of some of the simpler 

 marine nematodes mentioned in the fore part of this paper shows the most 

 primitive form of oral structure, i.e. circular mouth, no lips, indistinct 

 papillae and no pharynx, in contradistinction to the three lipped form de- 

 scribed by Seuraf as the most primitive condition of oral structure. Loss 

 of ciliation may have arisen by the propensity for cuticularization and by 

 some other unknown change or cause, the simple ciliate esophagus became 

 a muscular sucking organ to carry on the process of acquiring food after 

 ciliary motility had given place to non-motility as evinced by the "bor- 

 dures en brosse" of species today. 



