THE COMPARATIVE ANATOMY OF THE SUBULUMD HEAD (NEMATODA): WITH A CONSIDERATION OF ITS SYSTEMATIC IMPORTANCE

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Abstract
SUMMARY: The structure of the head in the nematode family Subuluridae has been studied comparatively and the results obtained used as the basis of a new classification. The three structures at the base of the buccal cavity are shown to be not teeth, as they are commonly described, but modifications of the anterior end of the pharynx. These modifications are of two basic types: one which is lobed and spirally arranged and one which bears an elaborate arrangement of tooth‐like structures and flanges on the inner surface. It is shown that at least three parts of the head are of value in systematic analysis: the shape of the mouth opening, the cross‐sectional shape of the buccal cavity and the structure of the pharyngeal portions. The various structures are analysed throughout the family, and are shown to vary along three evolutionary lines which show a high degree of parallel evolution leading to complex mouth openings with lip‐lobes in all three. It is suggested that this trend towards a complex mouth is associated with increasing efficiency of the mouth as a holding organ. It is shown that the so‐called ‘double’ buccal cavity, commonly reported from Subulura species, has always been reported because of a misinterpretation of the pharyngeal portions, due to the reduction of their musculature leading to fully cutieular portions. The nine genera of which the family is composed, two of which–Prima‐subulura and Tar subulura–are new, have been grouped into three subfamilies, of which one–Allodapinae–is new, on the basis of the analysis of the head structure, in conjunction with other previously better known characters. The genera are grouped as follows: Subulurinae with three genera, Subulura, Oxynema and Labiobulura; Allodapinae subfamily nov. with two, Allodapa and Aulonocephalus; Parasubulurinae with four, Parasubulura, Primasubulura, Tarsubulura and Leipoanema. As far as possible nominal species have been referred to these various genera on the basis of published descriptions as well as a study of specimens. Since most of the rearrangements suggested in this paper are based on the structure of the head, which has been almost completely ignored heretofore, many nominal species cannot be referred to a genus and a list of thirty such names is given. These names are considered to represent species incertae sedis.