42 ILLINOIS BIOLOGICAL MONOGRAPHS 1146 



DISCUSSION AND CONCLUSIONS 



Regarding orientation of the primitive nematode with respect to its 

 surroundings, Steiner's view that it maintained a position perpendicular 

 to the substrate and followed the half-sessile mode of life seems to be 

 tenable and is well borne out by many of the free-living forms found on 

 and about marine algae and particularly by those worms possessing eye 

 spots with the lenses vertically oriented. The crawling mode of travel 

 engaged in by many nematodes as they lie upon a lateral surface is a 

 secondarily acquired mode of locomotion. Furthermore, the vertical 

 orientation suggests that possibly the ancestor of the nematode, in view of 

 cilia being present, was a free swimming pelagic elongate animal which, 

 after assuming the tendency to cuticularize, settled down to a half-sessile 

 life. The ancestral mouth in the stage with the ciliated digestive tract 

 was possibly ventral and circular, opening into a ciliated esophagus, only 

 slightly muscular or not at all so, and in all probability the anus of such an 

 individual was terminal as well also as the openings of the excretory 

 system. This view is in accord with single openings of these systems spoken 

 of by Seurat in the definition of the primitive nematode. Such an ancestor 

 might easily be derived from a trochophore form by extensive elongation 

 and a partial migration of the mouth anteriad. Further the symmetry of 

 such an individual would be bilateral which is of course in accord with the 

 fundamental bilaterality of the nematode. 



The limiting descriptions of the structural units, lips, jaws, and cap- 

 sule, proposed by Ward primarily for the parasitic nematodes, are equally 

 applicable to free-living forms, but here there are intergradations from one 

 form to the other so that as a means of grouping the free-living round- 

 worms, these terms are too restrictive and do not permit of placing many 

 intermediate conditions. 



From the foregoing discussion and the data in the preceding sections, 

 the following conclusions may be drawn: 



1. Cilia are present in nematodes in modified forms and as discrete 

 elements structurally identical with those of vibratile ciliated cells. 



2. The nematode ancestor was probably ciliated throughout its 

 digestive tract, and possessed perhaps external cilia, a ventral, 

 simple mouth, and a terminal anus. 



3. Loss of external ciliation was succeeded by a half-sessile life and 

 tendency toward cuticularization. The muscular esophagus arose 

 as a pumping organ. 



