16 ILLINOIS BIOLOGICAL MONOGRAPHS [120 



Alterations in hilateralily of primitive type 



Among the legion of nematodes existing now, strict bilaterality in which 

 each half of the individual is a mirror image of the other, does not exist as 

 far as known, at least in the adult forms. Some of the immature forms 

 prior to their last moults more nearly approach bilaterality than do any 

 of the adults for in them the gonads are present only in rudimentary form, 

 lying in the mid ventral line. Changes in bilaterality are very easily brought 

 about by any shifting of the relative proportions of the axes: any lengthen- 

 ing of the principal axis without proportionate concomitant increase of the 

 other two axes would for mechanical reason alone produce a serial ordering 

 of elements which had heretofore lain side by side in the body cavity. 

 Evidence of this fact is to be found in the serially arranged testes of many 

 of the free living nematodes and similarly the caudal glands have become 

 serially ordered in a most striking manner (Fig. 22). The female repro- 

 ductive system exhibits the largest number of variations in arrangement 

 of its parts. The ovaries and uteri are double but the uterine ducts unite 

 so that there is always one vagina and one vulvar opening. In general 

 one ovary is reflected anteriorly and the other occupies the posterior por- 

 tion of the body cavity, or in cases where the vulva lies far anteriorly or 

 far posteriorly either the anterior or posterior ovary may sufiFer partial 

 suppression or become entirely vestigial. 



Such changes as these just mentioned alter the actual bilaterality but 

 do not in any way change the fundamental bilaterality of the organism. 

 Whatever the changes in symmetry which replace or become superimposed 

 upon the bilaterality of the nematode as a whole, they are secondary fea- 

 tures having arisen during the evolution of the primitive form into the 

 present types of great complexity. Strict asymmetry is most noticeably 

 present in the free living nematode, Bunonema inequale Cobb, and in 

 related species which possess on the dextral side a row of immense tubercles 

 giving the individual a curious unbalanced appearance. Such striking 

 asymmetry is not very often seen and in place of it radial symmetry con- 

 structed on plans involving varying numbers of radii is much more general. 

 In order to understand better, perhaps, the advent of this type of symme- 

 try, it is necessary to consider the question of the orientation of the primi- 

 tive nematode with respect to its surroundings. Steiner has discussed this 

 question in considerable detail and his views in main will be outlined in the 

 following few paragraphs. 



Primitive orientation of nematodes 



Whoever has dealt with free living or parasitic nematodes is aware of 

 the fact that they always lie upon either the dextral or sinistral aspect of 

 the body, so that their looping and twisting is in reality confined to the 

 plane of their principal axis. The morphological ventral surface becomes a 



