Diagnosis: Tentacles 10. Digits 2–6 on each side. Polian vesicle single. Calcareous ring of 10 pieces, the radial not perforated. Calcareous deposits sigmoid hooks, scattered, or arranged into groups, and wheels, scattered, never grouped into papillae (Clark, 1907).

Type Species: Trochodota purpurea (Lesson).

This well-defined genus contains in excess of 15 species at the present time. The species are separated on the basis of differences in average wheel size, in the size and arrangement of sigmoid rods, and the size and shape of the tentacle deposits. Some of the known species are closely related to each other and may yet prove to be synonyms.

The greatest concentration of species lies in the Indo-West Pacific region. Two species are known from New Zealand, and both are members of the Cook Strait Holothurian fauna.

Key to the New Zealand Species of Trochodota Ludwig

1	(2)	Skin smooth, not papillate, with numerous scattered sigmata and wheels	T. dunedinensis (Parker)
2	(1)	Skin distinctly papillate. Sigmata arranged into groups in the papillae. Wheels numerous or scarce	T. dendyi Mortensen

Trochodota dunedinensis (Parker) Plate I, fig. 1

• Chiridota dunedinensis Parker, 1881, p. 418; Theel, 1886, p. 34; Dendy, 1896. p. 26. Pl. 3, figs. 1–8; Farquhar, 1898, p. 323.
• Trochodota dunedinensis Ludwig, 1898, p. 87; Perrier, 1905, p. 123; Clark. 1907, p. 124; Clark. 1921, p. 166; Mortensen, 1925, p. 376, figs. 59b, 60b. 61; John, 1939, p. 315; Dawbin. 1950, p. 40, fig. 19.
• Chiridota geminifera Dendy and Hindle, 1907, p. 112, Pl. 14, fig. 30.
• Chiridota benhami Dendy, 1909, p. 151, Pl. 16, fig. 3a-1.
• Trochodota benhami Clark, 1921. p. 166.
• Non. Trochodota dunedinensis Allan, 1911, p. 325 (= Trochodota alleni Joshua); Ohshima, 1914, p. 478 (= Trochodota diasema Clark).
• Chiridota australiana Theel, 1886, p. 16.

Material Examined: Island Bay. intertidal rock pool, 1 specimen, collected by G. W. Gibbs, 11/7/1960.

Diagnosis: Colour in life reddish-brown, darker near the posterior and anterior extremities of the body. Body elongate, cylindrical, smooth, without papillae. Deposits wheels and sigmoid hooks, scattered in the skin. Radials and interradials irregular in shape, notched anteriorly and posteriorly.

Description: These are small holothurians, rarely more than 50mm in length when fully extended. The single specimen in the collection is 35mm in total length; the diameter at the anterior end is 3mm. Colour in life reddish-brown, deepening to a dark brown at the anterior and posterior extremities of the body; the tentacles are transparent, with numerous dark brown spots. Colour in alcohol, yellowish-white and semi-transparent.

The calcareous ring is composed of 12 unsymmetrical pieces. Each piece is narrow, about 1mm in length, with an anterior and a posterior notch. The radials are not perforated and are virtually indistinguishable from the interradials.

The oesophagus is short and thinwalled and enters the intestine which describes an S-shaped loop and runs to the anus. The single Polian vesicle arises from the ventral side of the water-vascular ring. The stone canal is very small and convuluted, lying in the dorsal mesentery, terminating in a minute nodular madreporite.

Gonads consist of a few long and slender tubes which extend almost to the posterior end of the body cavity.

Calcareous deposits of three kinds were found:

1. Wheels: The dorsal side of the body contains numerous six-spoked wheels in the skin, whose diameters range between 0.06mm and 0.16mm, the average diameter being 0.1mm.

2. Sigmoid Hooks: Sigmoid hooks, with an average length of 0.1mm are scattered among the wheels on the dorsal surface of the body. They lie transverse to the longitudinal axis of the body.

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3. Tentacle Rods: The tentacles contain large numbers of small rods 0.02–0.06mm in length. They are irregular in shape and frequently have enlarged extremities (Plate I fig. 1). The rods are scattered in the stems of the tentacles, but in the digits they are arranged in narrow double rows.

Behaviour of a Living Specimen: A living specimen of T. dunedinensis was placed in a dish of seawater, the bottom of which was partly covered by a thick layer of sandy mud. The general behaviour of the specimen was observed for some time.

1. Feeding: The tentacles were pushed into the mud and sand in turn. Small particles of the substrate adhered to the outer surfaces of the tentacles which apparently were covered by a sticky secretion. No particles of sand adhered to the inner surfaces of the tentacles. The tentacles were then rapidly wiped across the mouth one at a time, or pushed into the mouth, and the adhering particles were removed and ingested. While feeding the specimen was fully extended, and waves of contraction passed along the body from time to time. The feeding process took place almost continuously during the time of observation.

2. Defaecation: The anal aperture of the body was completely closed, and the posterior half of the body contracted. Then the anus opened suddenly and egesta emerged in small lumps. No vermiform "casts" were seen. Defaecation took place at irregular intervals.

3. Locomotion: The specimen pulled itself around the walls of its dish by means of its sticky tentacles. There was no tendency to burrow away from strong light, although the animal was photosensitive.

Distribution: Parker (1881) described the type specimen from Otago Harbour. Since that time specimens of this species have been found in many parts of the South Island of New Zealand, the Cook Strait region, Stewart Island, and the Auckland and Campbell Islands (Mortensen, 1925).

Ecology: T. dunedinensis appears to favour comparatively sheltered sandy to muddy localities where it conceals itself under stones or by burrowing.

Discussion: Mortensen (1925) stated that the oral disc is "distinctly oblique in dorso-ventral section" and "the calcareous ring is . . . parallel to the oral disc". The calcareous ring is no doubt asymmetrical owing to its oblique position.

There appears to be some variation in the course of the intestine in this species. Dendy's T. benhami possessed an S-shaped intestine. However Mortensen (1925) noted that in some of the specimens at his disposal the intestine ran straight to the anus, while in others it looped twice, or described an S-shaped path to the anus. He also showed that the other characters for T. benhami given by Dendy (1909) fell within the range of variation of T. dunedinensis.

Dendy (1896) noted that the sexes are separate in this species, and John (1939) observed that the females are viviparous.

Trochodota dunedinensis has certain characters in common with other species of the same genus. Clark (1921) pointed out that it is almost impossible to make an accurate key to the species of the genus without re-examining many of the species. To the writer's knowledge, this has not been done as yet.

Trochodota dendyi Mortensen

Trochodota dendyi Mortensen, 1925, p. 381, figs. 62–63a; Dawbin, 1950, p. 40.

Diagnosis: Colour white or faint purple. Skin papillate, each papilla containing 3–6 sigmoid hooks. Wheels numerous or absent. Tentacle deposits with bifurcating ends.

Distribution: The type specimen was from Plimmerton. The species is also known from Waikeke (Auckland Harbour), and Paterson Inlet (Stewart Island) (Mortensen, 1925). Restricted to the New Zealand region.

Ecology: Known only from the intertidal zone.

Discussion: Mortensen examined nine specimens of this interesting species and after giving a very careful description, he stated that the species is nearly related to Trochodota japonica (v. Marenzeller) but differs in colour, number of sigmoid hooks in each papilla, and in the shape of the tentacle deposits.