A new genus, Kuhnia n.g., is created for the mazocraeidid monogenetic trematodes from the gills of mackerel. The type species, K. scombri (Kuhn), is redescribed (also K. minor (Goto), from the first specimens to be found outside Japanese waters), special attention being paid to the cuticularized parts, and the clamp skeleton is newly interpreted.

An alternation of sex phases has been found in Monogenea, which is well marked in K. scombri, resulting in changes in appearance which are described.

The comparison of heterauxesis curves constructed from the summarized data from six sets of dimensions shows that retardations in the rates of growth may be correlated with the onset of maximal reproductive activity, and accelerations with the relief from reproductive drain. Localized growth influence (similar shaped curves, with low post-larval coefficients of growth) has been found in the haptoral region, which is in marked contrast to the growth picture of the cuticularized oral suckers and the width of the body—the latter having both the highest and most variable growth coefficients between the six arbitrary length groups compared. Thus, neither absolute measurements nor ratios are, by themselves, of any value in specific diagnosis.

Graphical representation of allomorphosis for the genus Kuhnia affords a synoptic view of the changes brought about by the different types of differential growth of similar organs in each species throughout post-larval life. The dissimilar growth coefficients for certain dimensions are utilized in specific diagnosis. The relative diagnostic value of different entities is examined: the eggs of K. scombri are shown to be highly variable in size and shape.

The new facts obtained from the application of this metrical treatment, and its further application as a test for conspecificity of specimens outside the known size range, are discussed.

The systematics of the family Mazocraeidae are reviewed, and emended diagnoses of three species of Kuhnia are given in tabular form.