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Stentor rubra sp. nov. Plate 2, Figs. 10, 11

Individuals are coloured a diffuse rose-pink and when fully extended range between 250 and 448 mu long, with the disc between 70 and 100 mu wide. Thus, for a species of Stentor, the specimens are small. The body is narrowly and almost evenly conical, the disc slightly wider than the subsequent portion of the body; occasionally there is a very slight distension in the anterior one-third or one-quarter of the body. The proportion, disc-width to body-length, is 1:4.5 or 1.5. The disc, in extended specimens, is subcircular, slightly convex, but not protruding, and is posterior to the anterior edge of the body, which extends as a thin, deep, rim-like margin. The ventral margin is flattened, with, at its mid-length, a small V-shaped notch (Fig. 10), the margin on the left of which is lower than on the right. To the left of the median longitudinal plane, the disc dips sharply along an arcuate line to form a deep, U-shaped vestibular portion leading to the cytostome and cytopharynx. The membranelles of the adoral wreath are relatively longer than in S. loricata and arise from the base of the rim on its inner side. To the right of the ventral notch the wreath is narrowly incomplete, while on the left, the membranelles continue in a right-hand spiral into the vestibule-like portion and then into the cytopharynx. The body and disc are clothed by short fine cilia, which arise in numerous, very narrow, longitudinal grooves in the pellicle. No groups of setiform cilia occur.

The cytopharynx, a short narrow tube, curves posterodorsally; its cilia run in a right-hand spiral to its innermost portion, which appears to be permanently open into the endoplasm. Food vacuoles forming at this point are usually large and ovoid. No studies of the relationship of disc myonemes and cytopharyngeal structures were carried out.

Band-like, comparatively wide, regularly spaced, colourless myonemes run the length of the body (Fig. 10), but are indistinct and difficult to see on the disc. They produce a high degree of contraction, the body becoming squat and ovoidal and the disc margin widely, deeply, and regularly crenulated. On the rest of the body the pellicle is but slightly ridged between the myonemes, with correspondingly shallow grooves overlying them. In free-swimming forms (Fig. 11) contraction is not so great; the body is pyriform, with the disc retracted to between one-third and one-half the body width and not protruding beyond the general body contours. Following a short period as a free-swimming form, individuals "rub" the attachment area against debris, or place and withdraw it several times, such behaviour preceding reattachment. Attachment, effected by the extremity expanding and flattening page 11 over the substrate, is followed by a lengthy period of semi-extension, the more expanded form being but slowly assumed.

The contractile vacuole is to the left of the cytopharynx (Figs. 10 and 11), and a collecting canal extends into the posterior half of the body. At the end of diastole the subtriangular form changes to spherical. Systole is slow; the vacuolar membrane flattens on the right, then moves slowly towards the left, thus expelling the contents. A comparatively immense vacuole which distends the anterior region of the body forms slowly prior to defaecation; in one specimen it measured 30 mu, in another, 24 mu, in diameter. It discharges through a temporarily large, ovoidal opening on the dorsal surface, posterior to the disc margin.

Specimens were collected from an open-air aquarium on the roof of the Biology Block, Victoria University College, where they occurred attached to flocculated masses of a unicellular alga. They were not numerous, and attempts at subculturing were only, partially successful.

To my knowledge there is one other pink Stentor described—S. igneus Ehrenberg. Pritchard (1841) translates Ehrenberg as saying that S. igneus is "the fire-coloured Stentor," and later adds, "the skin is bright yellow or vermilion." Kent (1880–1882) states that the body possesses "a rich layer of Chlorophyll-granules," but that "the more superficial, transparent layer . . . contains a finely granular pigment of an intense scarlet hue." He also gives as his opinion that an inadequately described and figured, bright rose-coloured species, S. roseus De Fromental, is apparently referable to S. igneus. De Fromental's work is not available to me; Kent, however, expresses considerable doubt as to the soundness of his observations and the completeness of his diagnosis, so that it appears justifiable to accept Kent's view that S. igneus was the species described. Kahl's (1932) paper is unobtainable, but Kudo reproduces his figure of S. igneus and says it is rose-coloured or colourless. Roux (1901) shows precisely the colour of the present material. Although colour is a variable characteristic among most of the coloured ciliates, to judge from these accounts, there is a very wide degree of variation indeed in S. igneus.

Ehrenberg and Kent report S. igneus as being one seventy-second of an inch long (353 mu approximately), Roux as 200 to 400 mu long, and these measurements fall close to or within data for local specimens. The ratio of disc-width to body-length differs, however, from author to author. Kent reports the peristome as "equal to about one-half the entire length," and he figures a short, widely flared page 12 specimen. Kahl's figure indicates a disc-width of about one-third the body-length and actually narrower than the immediately posterior region of the body, which is bulbous over the anterior one-third to one-half of the body, a feature not referred to by other writers. Roux describes S. igneus as generally less conical and elongated than other species of Stentor and as broadly rounded and little contracted posteriorly; the disc-width, as illustrated, is a little less than half the body-length. The present material is then slenderer and differently proportioned. Groups of setiform cilia and a wide non-ciliated region of the ventral notch are further differences which Roux describes. He states that S. igneus is usually free-swimming; his figure suggests this, and, if so, the evenly curved body margins and wider disc bear little resemblance to the pyriform, free-swimming Wellington specimens (Fig. 11).

All writers describe a spherical or subspherical macronucleus for S. igneus, and this and similar size ranges are the only common features covering the descriptions available. Other characteristics described vary so grossly that it seems probable that more than one form of Stentor has been included within the species. The present specimens, which are slender, narrowly conical, coloured rose-pink, seldom free-swimming, and which have a distinct, rim-like margin to the disc, thus differ from any one of the descriptions for S. igneus.