higher groups of the Vertebrata, but has not yet been found to occur in fishes. There are in all fifty-five known species, of which twenty-two occur in the Mammalia.
The genus is, however, a very old one, and contains a number of species that depart very widely from the type as defined by the type-species P. clausa. Further, the characters by which many of the species in the genus have been defined are really of generic value, so that, in the absence of the original material, there is little beyond a measurement or two upon which the specificity of several species remains. This is unfortunately true of the description of Physaloptera caucasica with which the present species required to be most carefully compared.
The generic characters that appear to be constant for the species of the genus Physaloptera s. str. may be summarised as follows:—
The general form resembles markedly that of the genus Ascaris, but in the male the cuticle at the posterior end of the body is flattened out to form a somewhat heart-shaped "bursa." In both sexes the cuticle overreaches the mouth structures, forming a sort of cuticular sleeve or collar; it is, moreover, unmarked by the regular transverse striations seen in other forms, although, of course, in contracted specimens, certain transverse groovings appear adventitiously.
The mouth is surrounded by two large fleshy lips, situated laterally. Each lip bears two large submedian flattened papillae, and is surmounted by a cuticular prong called the external tooth. Beneath this, in the surface in contact with the opposing lip, is a cuticular fold that shows varying form with the different species, and is known as the inner tooth.
The bursa in the male presents a series of papillae which fall into two classes, pedunculated and sessile. Typically,
