Diagnosis: Tentacles digitate, 10–12, rarely 13 or 14. Digits two on each side. Cartilaginous ring wanting. Polian vesicles 2–10 or rarely only one. Stone canal usually single. Stocks of the anchors more or less branched, but only finely toothed; arms usually serrate; vertex without knobs. Anchor-plates without handles, with numerous perforations, and with a more or less imperfectly developed bow across the outer surface of the posterior end. The anchor-plates and their perforations have either smooth or dentate margins.

Type Species: Protankyra abyssicola (Theel).

The genus Protankyra is a large and perplexing one. Most of the species are distinguishable on the basis of differences in the shape and size of the anchors and anchor-plates and their ornamentation. Heding (1928) stated ". . . . the ciliated funnels are of the greatest value as specific characters in Protankyra". It is felt, however, that the calcareous deposits should be regarded as of primary importance in diagnosing species of this genus, and such variable structures as the ciliated funnels should be used with caution.

The single New Zealand representative of this genus is quite distinct from the other known Protankyra species.

Protankyra uncinata (Hutton) Plate I, figs. 2–10

• Synapta uncinata Hutton, 1872. p. 16; Theel, 1886. p. 27; Dendy, 1896, p. 25; Farquhar, 1898, p. 325.
• Synapta inaequalis Hutton, 1872, p. 17.
• Protankyra uncinata Mortensen, 1925, p. 367, figs. 48–51; Heding, 1928, p. 252; Dawbin, 1950, p. 40.

Material Examined: VUZ 32, off Petone Beach, 8 fathoms, mud, 3 specimens; VUZ 37, off Shelly Bay, 10–11 fathoms, mud, 8 specimens; VUZ 40, off Ward Island 2–4 fathoms, mud, 2 specimens; VUZ 64, off Point Howard Wharf, 5 fathoms, blue mud, 10 specimens.

Diagnosis: Tentacles 12, with sensory cups and four terminal digits. Colour white transparent to reddish brown. Anchors small (0.3–0.5mm long), usually symmetrical, with unbranched finely toothed stocks. Arms with few serrations or none. Anchor-plates oval or rectangular, smooth, with large smooth polygonal perforations of average diameter 0.025mm. Ciliated funnels slipper-shaped, numerous.

Description: The body is smooth, approximately cylindrical in shape, tapering abruptly to the terminal anus. Many of the specimens are completely contracted and carry a number of transverse wrinkles. In extended specimens the skin is semi-transparent. Length of largest extended specimen 100m; diameter at anterior end, 5mm. In alcohol, the colour varies between white transparent and reddish brown.

The skin is prickly to touch, the prickly sensation being caused by the sharp-pointed anchor arms which project above the level of the body wall. These arms can be seen with the naked eye. When completely contracted, the anterior end of the body folds inward and the tentacles disappear from view. If the animal is viewed end on, the five interradial areas between the radial muscle bands appear as soft fleshy lumps. This method of contraction may be a characteristic of the species. (Plate I, fig. 8.)

Twelve elongate, cylindrical tentacles surround the mouth, which lies in a shallow depression. There is a brown pigment spot at the base between each pair of tentacles. Each tentacle carries a terminal claw composed of two pairs of digits, and the stems carry a number of sensory cups on their inner margins (Plate I, fig. 9). The true function of these cups has apparently not been determined.

The calcareous ring is composed of twelve small, square pieces. The five radial pieces are each perforated for the passage of the radial nerves.

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A short, thin-walled oesophagus leads into a well-defined stomach which has a thick muscular wall (Plate I, fig. 7). The intestine takes a short loop and terminates in an undifferentiated cloaca.

There are 4–5 Polian vesicles, two or three of which may be longer than the rest (Plate I, fig. 7). They are tubular, and their length varies between 3mm and 25mm. The single stone canal leaves the water-vascular ring on the dorsal side, and runs anteriorly in the dorsal mesentery as a short loosely coiled tube 0.2mm in diameter, terminating in a small madreporite of an irregular shape.

The genital tubules are moniliform and sparsely branched, arranged in two bunches, extending for about half of the length of the body cavity in mature specimens. The common page 6genital duct lies in the dorsal mesentery, and opens to the exterior as a minute genital pore in the mid-dorsal interradius, immediately posterior to the ring of tentacles (Plate I, fig. 7). Longitudinal muscles are well developed, and transverse muscles are represented as bunches of fine fibres.

Calcareous deposits of four types were found:

1. Anchors: The anchors are approximately symmetrical, although some asymmetrical examples were found, especially near the posterior extremity of the body (Plate I, fig. 3). The total length of each anchor varies between 0.3mm and 0.5mm. The arms have 3–7 irregular serrations on their upper edges, or none at all, but smooth anchor arms are not common. The stock is unbranched, and carries a number of fine teeth, irregularly arranged (Plate I, fig. 4).

2. Anchor-plates: The anchor-plates lie in association with anchors in the skin. They are rectangular to oval in shape, with an average length of 0.3mm, and have many large and small polygonal to circular perforations (Plate I, fig. 6). The narrower end of the anchor-plate is reflected to form a bridge for the support of the anchor stock. The anchor is supported in such a way that the arms can project above the level of the skin. Anchors develop before anchor-plates, and stages in the development of anchor-plates (Plate I, fig. 5) are commonly found near the extreme posterior end of the body.

3. Tentacle Deposits: Curved and dumbbell shaped rods are present in great numbers in the tentacles. The curved rods are C- or bracket-shaped, 0.03–0.06mm long, and they are confined to the tentacle digits, where they form an investing layer (Plate I, fig. 2). Dumbbell shaped rods invest the tentacle stems. Thy are 0.02–0.05mm in length (Plate I, fig. 10).

Distribution: Protankyra uncinata is now known from Auckland, Colville Channel (Mortensen, 1925), and Wellington Harbour, and may yet prove to have a wider distribution pattern. This species is restricted to the New Zealand region.

Ecology: Mortensen's (1925) specimens were taken from muddy or sandy bottoms at depths ranging between 5 and 35 fathoms. The present collection contains specimens taken from a muddy bottom at depths ranging between 2 and 11 fathoms. This species is then a member of our sub-littoral fauna, and may eventually come to be known from other sheltered muddy localities.

Discussion: Mortensen (1925) figured small irregular perforated buttons which he found ". . . . in the anterior end . . . more or less sparsely". These were not found in the specimens on hand. He also commented on the small size of the eggs in this species (approximately 0.1mm in diameter), and suggested that Protankyra uncinata may possess a typical auricularia larva.

Hutton (1872) gave a very inadequate description of his two new species Synapta uncinata and Synapta inaequalis. As a consequence Clark (1907) declared that the two species were absolutely unrecognisable on the basis of Hutton's descriptions. Mortensen (1925) established the validity of P. uncinata and reduced P. inaequalis to synonymy with that species. P. inaequalis was based on a fragment of skin containing asymmetrical anchors, but Mortensen (1925) suggested that this was probably an individual variant of P. uncinata.