80 ILLINOIS BIOLOGICAL MONOGRAPHS [298 



live under such similar conditions as the Didymozoonidae and the Colly- 

 riclidae could by the effect of habitat alone become so totally different. 



That the habitat in which the worm lives is a factor in the modification 

 cannot be disputed but is certainly only one factor which contributes to 

 the modification. What makes up the aggregate of factors in a given case 

 could be only a matter of conjecture. Recent experimental investigation, 

 however, has shown that factors influencing modifications of an individual 

 or race may be both external and internal and that the change produced 

 may be gradual or occur as a mutation in which case little or no trace may 

 be left to depict the ancestral route. While the writer is inclined to look 

 upon the loss of sucking musculature as a gradual change brought about 

 by a group of factors among which the habitat seems to have played an 

 important part, there is every reason to believe that sudden radical re- 

 arrangements have taken place and these are no doubt responsible for many 

 of the variant forms. While there is good reason to believe that the Mono- 

 stomata have arisen from highly divergent groups the evidence seems as 

 yet about equally divided and the final decision must necessarily rest on 

 further studies of the anatomy and life history of members of this and 

 other groups. 



The systematic arrangement used in the earlier part of this paper is 

 essentially that employed by Luhe (1909), Kossack (1911) and Ward 

 (1918), which for the above stated reason it seemed best to preserve at 

 least for the present. It is hoped that interest in this aberrant group will 

 increase and that careful studies will result either in the preservation of 

 these forms as a natural taxonomic group or in their separation and subse- 

 quent distribution among other well organized units to which they are truly 

 related. 



