14 ILLINOIS BIOLOGICAL MONOGRAPHS [118 



an organism with the most avenues along which it may specialize. How- 

 ever, when one is confronted by a nematode which has organs or a system 

 of organs that are structurally very generalized and at the same time 

 other systems are very highly specialized, the question may be asked 

 whether the simple structures have devolved or are hold-overs of the 

 primitive type which existed in the ancestral nematode. There is naturally 

 no adequate nor absolute solution to such a query and if any explanations 

 are offered they can at best be based only on a critical examination of 

 details in numerous free and parasitic species, each detail being selected 

 with careful consideration of its stability in the stress of environmental 

 factors. 



According to Steiner (1919) the type form of nematode body is a spin- 

 dle in which the principal axis is much elongated over the two similar 

 dextro-sinistral and dorso-ventral axes. Any alterations in the relative 

 proportions of these axes of the primitive form will necessarily alter 

 profoundly the general outline of the body: with extreme lengthening, for 

 example, of the principal axis and only a slight shortening of the other 

 axes, or none, it is a very easy transition into such a filariform individual 

 as an adult Dracunculus medinensis Velsch, measuring more than a meter 

 and a half in length. On the other hand, lengthening of the two secondary 

 axes in greater proportion than the principal axis would produce a form of 

 adult such as Heterodera schachtil Schmidt, the common parasitic nematode 

 of the sugar beet, the female of which at maturity becomes a swollen lemon- 

 shaped individual. 



In cross section the primitive nematode is always circular with no 

 suggestions whatever of dorso-ventral or lateral flattening. Neither is 

 there any evidence of metamerism, a fact which is borne out in extant 

 forms in none of which there is the slightest suggestion of septa. In this 

 connection it should be noted that there is also no coelom, the existing body 

 cavity remaining as a derivative of the primary body cavity or blastocoele. 

 Pseudo-segmentation is present in the cephalic bristles, according to Cobb, 

 of about thirty per cent of the free living nematodes and in one form 

 Scaptrella cmcta Cobb, even the mandibles are jointed. However, this 

 condition is limited only to the cephalic appendages and in no case, either 

 in the embryo or adult, has any trace of true segmentation been observed 

 in the body proper. 



The mouth of the primitive form is terminal as in present forms, except 

 a few genera in which it has become secondarily dorsal, notably in the 

 genera of the family Ancylostomidae. Embryologically the mouth is 

 subterminal ventrally and during development it migrates to the terminal 

 position. There is, however, still a difference of opinion among investiga- 

 tors on this point; some believe that the blastopore as a slit-like opening 

 closes completely from behind forward and that the mouth forms inde- 



