Family Oxtrichidae
Genus Uroleptus Ehrenberg, 1834
Uroleptus setiformis sp. nov. Plate 1, Fig. 7

Specimens (Fig. 7) are subfusiform, the anterior extremity rounded, the posterior portion drawn out into a caudal process of approximately one-fifth the body-length and curved to the right. A narrow crescentic lip projects from the flattened ventral surface and lies about the anterior extremity. The right margin is evenly and strongly convex from the anterior extremity to the base of the caudal process. The central portion of the left margin is also convex, but to a lesser degree than on the right; anteriorly it is concave, which produces the effect of a sinistral flexion of the anterior one-fifth to one-quarter of the body. The dorsal surface is strongly convex, and, except for the extremities, where pronounced flattening occurs, is almost as deep as wide. Specimens range in length from 165 to 204 mu, and in width from 45 to 60 mu.

There are two rows of ventral, a single row of marginal, and three frontal cirri, and, as well, one row of fine setiform cilia laterally and a median longitudinal row dorsally. The three frontals are set in line obliquely across the anterior extremity, the left-hand cirrus occurring at the anterior end of the right peristomal margin. Two rows of ventrals begin towards the right anterior extremity and extend page 13 posteriorly to the tip of the caudal process; the left row is headed by the right frontal cirrus, and contains the larger cirri, especially anteriorly. The right marginals are larger and more conspicuous than the left, not flattened, and project anteriorly and posteriorly, the intervening ones being up to one-quarter of the body-width in from the margin. The left marginals commence level with the apex of the peristome, and in this region are short and fine. Posteriorly they lengthen, become more nearly equivalent with those on the right, and project beyond the margin. The caudal process is markedly setose, the cirri of the ventral and marginal rows projecting at varying angles. The dorsal and marginal setiform cilia are short and fine, and extend at right-angles to the body surface. The marginal setae arise from a slightly dorsal position, are often irregularly spaced posteriorly, or may be absent from portions of the caudal process. The dorsal setae correspond in length with, but are more regularly spaced than, the marginal setae.

A V-shaped peristome occurs anteroventrally on the left, occupying one-quarter to one-third the body-length, and having its right margin curved to the left anteriorly. Along the left margin is an adoral zone of membranelles which is continuous with the frontal zone. The latter consists of long, strongly beating cirri, arising from about the anterior body margin dorsal to the ventral lip. A wide, conspicuous, undulating membrane, its cilia readily seen in live organisms, lies on the right peristomal margin. Following the cytostome, at the apex of the peristome, is a short cytopharynx which curves to the left. The undulating membrane continues on to the right cytopharyngeal wall, but the adoral membranelles do not go beyond the cytostome.

A single contractile vacuole is on the left, between the peristomal apex and body margin within the anterior one-third of the body. It causes a conspicuous bulge in the margin during diastole. A short collecting canal extends anteriorly, and a longer one posteriorly to beyond the middle of the body.

Several features, similar to those found in other species of Uroleptus, include the two oval macronuclei, each with a comparatively large, closely associated, compact micronucleus; the cytopyge opening dorsally on the left near the base of the caudal process; the coarsely granular, greyish-brown endoplasm often containing fat globules and ingested diatoms.

Specimens were collected from two localities near Wellington, a semi-permanent pond at Maori Bank, Upper Hurt, and a tarn on Bull Mound (4,000 feet) in the Tararua Range.

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Among other species of Uroleptus, U. piscis Ehrenberg, is 600 to 800 mu long and has a very long caudal process; U. mobilis Engelmann, 300 to 400 mu long, is very narrow, has an excessively short peristome, and a moniliform macronucleus; U. musculus Muller, though approximately the same size, is proportionately wider than local specimens, is short and stumpy, and has a conical caudal process. U. longicaudatus Stokes is approximately eight times as long as wide, and U. sphagni Stokes is clavate to broadly obovate and about three times as long as wide. U. rattulus Stein has a very long caudal process, and U. violaceaus Stein, apart from being a very brilliant violet colour, possesses a truncated posterior extremity. A recently described species, U. halseyi Calkins (1929), is long and slender, with a moniliform macronucleus. Consideration of such features will avoid confusing these previously described species with the present specimens of Uroleptus. Two further species, U. limnetis Stokes and U. dispar Stokes, resemble local material, but differ in details. The distribution of the frontal cirri in U. limnetis and local specimens is similar, but U. limnetis possesses long, numerous dorsal setae and is thus effectively differentiated. In U. dispar the two right frontal cirri are side by side, almost parallel to the anterior body margin, but the third, left cirrus is more posterior, close to the right side of the peristome field, and not at the extremity of the right margin as in the present material. A distinctive feature is the presence of anterior and posterior collecting canals in local material. These, to my knowledge, have not been previously described in any species of Uroleptus.

While there is no doubt that only an undulating membrane is present on the right peristomal border in the present material, earlier descriptions of organelles in the peristome are variable. Both Calkins (1929) and Engelmann (quoted by Calkins) describe only undulating membranes; Stokes (1888), however, describes that, for U. dispar, the right peristomal margin is ciliated and "apparently has an undulating membrane"; for U. limnetis, that the undulating membrane is absent, but cilia are present; and for U. longicaudatus, that only an undulating membrane is present. In the related genus Stylonychia, the organelles on the right of the peristome were also variously described until Lund (1941) showed that an undulating membrane, together with endoral fibrils, were the only structures present. It is not necessary that the structures along the right peristomal margin should be identical in both genera, but it is strange that, for a conservative organelle such as the peristome, there is so much variation in Uroleptus, and it seems probable that further investigation will show that only an undulating membrane is present.

The combination of well-defined characters—the distribution of the frontal cirri and the form and distribution of the marginal and dorsal setiform cilia, the form and structures of the peristome, the shape and proportions of the body, and the presence of collecting canals—are such as to warrant a new species for the present material.