Date of this Version
Journal of Parasitology (September 1914) 1(1): 22-30.
Contributions from the Zoological Laboratory of the University of Illinois, under the direction of Henry B. Ward, number 31.
One of the most difficult problems in zoologic science is the classification of round worms. Authors and lecturers, after a carefully outlined and definitely arranged discussion of trematodes and cestodes, are compelled to consider nematodes in a somewhat desultory and inaccurate fashion. Two reasons for this may be given, the greater being the apparent lack of a basis for determining the phylogeny of the major groups, a condition with which we are not at present concerned. The other obstacle is the difficulty of differentiating between species and uncertainty as to the value of different kinds of taxonomic characters. The multitude of synonyms for many of our common insects is a sore point among entomologists, but nemat-helminthologists have great difficulty in deciding that any particular name should be relegated to the synonymy.
In the absence of definite structural differences, Dujardin in 1846 found himself compelled to give a few measurements of the length, breadth, tail, etc., of the species which he described. This method was further applied by Eberth in Germany and Bastian in England, followed by Bütschli and others. Finally, in 1890, N. A. Cobb arranged a "nematode formula" which he has applied in all his subsequent work. This formula shows two kinds of measurements: first, the length of the worm in millimeters; second, the percentage of that length which is represented by the distance from the anterior end of the worm to (a) the base of the pharynx, (b) the nerve ring, (c) the cardiac constriction, (d) the vulva, and (e) the anus; and also the width of the body at each of these points. He uses the formulae of different species, both in descriptions and in keys for identification.
Cobb has described something over one hundred species of free-living round worms of the family Anguillulidae and has always worked out and stated the formula. As there are at present no other scientists making a specialty of this family, it would be unfair to emphasize the fact that, in the quarter century since the description of this formula, it has been used only by its originator. There are, however, many helminthologists concerned with parasitic Nematoda where the obstacles of classification are equally great. Some of these have seen the possibilities of such a formula, but a real doubt as to its value has prevented them from adopting it. Until the following questions are answered, one must feel that energy and time invested in descriptions of this nature are not well employed:
1. Can the formula be applied at all to the majority of parasitic species?
2. Is the camera-lucida method of measurement sufficiently accurate for such a purpose?
3. Are the relative proportions of the different organs constant within a single species?