Diagnosis: Tentacles with two pairs of digits. Caudal appendage usually long and slender. Deposits not tables but cups (buttons), perforated plates or irregular rods (Heding, 1931, in part).

Type Species: Paracaudina chilensis (Muller).

Heding (1931) diagnosed this genus and listed seven species, one of which (pigmentosa Perrier) was included with some doubt. Clark (1935) agreed with Heding's new genus, but did not agree with the part of Heding's diagnosis which stated "retractor muscles more or less well developed". After examination of many specimens of Paracaudina, Clark found no true retractor muscles.

Paracaudina chilensis (Muller) Plate IV

• Molpadia chilensis Muller, 1850, p. 139; 1854, Pl. VI, fig. 14, Pl. IX, fig. 1.
• Molpadia coriacea Hutton, 1872, p. 17; Hutton, 1878, p. 307.
• Caudina meridionalis Bell, 1883, p. 58. Pl. XV, fig. 1.
• Caudina coriacea Theel, 1886, p. 47, Pl. III, fig. 4; Dendy, 1896, p. 28, Pl. 3, figs. 9–18; Dendy, 1897, p. 456, Pl. 29; Farquhar, 1898, p. 324; Ludwig, 1898, p. 63; Dendy and Hindle, 1907, p. 108; Mortensen, 1925, p. 363, figs. 46–47.
• Caudina pulchella Perrier, 1905, p. 117, Pl. V, figs. 14–17.
• Caudina coriacea var. brevicauda Perrier, 1905, p. 121.
• Caudina chilensis H. L. Clark, 1907, p. 175; Benham, 1909. p. 28; Hozawa, 1928, p. 363; Ohshima, 1929, p. 39.
• Pseudocaudina coriacea Heding, 1931. p. 283.
• Paracaudina coriacea Heding, 1932, p. 455; Heding, 1933, p. 127, Pl. IV, figs. 8–13, Pl. VII, figs. 6–7, Pl. VIII, fig. 4; Dawbin, 1950, p. 39, Pl. 1, fig. 5, Pl. 2, fig. 16.
• Paracaudina chilensis var. coriacea H. L. Clark, 1935, p. 267.
• Paracaudina chilensis forms coriacea Deichmann, 1960.

Material Examined: VUZ 15, Palliser Bay, 100–150 fathoms, mud, 2 specimens; VUZ 87, South of Cape Palliser, 400 fathoms, mud, 2 specimens; VUZ 101, off Palliser Bay, 550 fathoms, mud, 135 specimens.

New Zealand Oceanographic Institute, Wellington: Stn. B 8, Hawkes Bay, 39° 06′ S., 177° 23′ E., 15.5 fathoms, 26/8/56, fine grey sand, 326 specimens.

Diagnosis: Body cylindrical, attenuated posteriorly into a long tail. Colour in alcohol white; old specimens frequently light brown. Calcareous deposits in the form of thick, solid crossed cups with small perforations, the marginal projections when present being low and rounded. The cups, especially in young specimens, approximately octagonal in shape, while the points of the octagon may be obscured in old specimens. Diameter of cups 0.06–0.1mm.

Description: These are caudinids whose tail length is about 40% of the total body length. Total length of largest specimen 115.0mm, tail 44.0mm, diameter of body at widest point 37.0mm. Total length of a juvenile specimen 15.0mm, tail, 6.0mm, diameter at widest point 6.0mm.

In all specimens the body is cylindrical, tapering abruptly to form a long tail. The body wall is thin and firm, and is marked by numerous transverse striations. In juveniles the body wall is semi-transparent, and through it the gut can be seen as a dark coloured mass. Colour in life and in alcohol varies from white to light yellow or brown. Anal papillae are present. The mouth is circular, lying in the middle of a circular oral disc. Tentacles 15, usually retracted.

The calcareous ring comprises ten pieces, five radials and five interradials. The radials each have a bifurcated posterior projection and three anterior projections. The interradials each have no posterior projection and one anterior projection. Many workers (Hozawa, 1928; Heding, 1933; Clark, 1935) have described the calcareous ring in detail.

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A short thinwalled oesophagus meets the intestine which takes a large loop (Plate IV, fig. 1), and joins the cloaca, which is attached to the body wall by fine muscle strands which also fill the cavity in the tail. In the juvenile the intestine fills most of the body cavity, and the cloaca has a small number of muscle strands.

The gonad consists of two bunches of irregularly branching vesicular caeca (Plate IV, fig. 1). The bunches join to form the common genital duct which runs anteriorly in the dorsal mesentery, and opens to the exterior as a minute genital pore. The juveniles show no trace of gonads.

A single bulbous and elongate Polian vesicle, up to 10mm in length arises from the right or left ventral side of the water vascular ring. The vesicle has a small patch of dark brown pigment at its distal extremity. A short coiling stone canal lies in the dorsal mesentery, terminating in a madreporite which takes the form of a depressed sphere (Plate IV, fig. 5). The madreporite has a sculptured surface, due to the presence of an investing layer of irregular intertwining deposits (Plate IV, fig. 7). In the juvenile the madreporite has a diameter of about 0.2mm, and the deposits are essentially the same as those in the adult, but they do not constitute a full enveloping network.

Respiratory trees are composed of two main trunks which carry numerous small side branches (Plate IV, fig. 1). The right tree is considerably longer than the left, and extends to the level of the calcareous ring, to which it is attached, while the left tree extends about half way along the body cavity. In most specimens the left tree gives rise to lesser trunks which tangle about the intestine, and lie in association with the rete mirabile.

The longitudinal muscles take the form of five double bands (Plate IV, fig. 1). There are no retractor muscles. In juvenile specimens, the muscle bands are very thin and straplike, and between each member of a pair of double bands the radial longitudinal nerve can be clearly seen as a thin white line. Transverse muscles are visible as fine lines (Plate IV, fig. 1).

Calcareous deposits: In adult specimens the deposits are "crossed cups" of varying shapes. These are crowded together in vast numbers in the skin, from the extreme anterior end of the body to the end of the tail. These cups are 0.06–0. lmm across. They each consist of a "cross" which overlies a "square" (Plate IV, fig. 2). The "square" has a single large perforation which is usually rectangular, with a tendency to become circular. The margin of the "crossed cup" is approximately octagonal in shape, but in adult specimens this shape is often obscured, as the points of the octagon become rounded off. Marginal projections are usually present as low rounded knobs. In the tail the cups are slightly more irregular in shape than those in the rest of the body wall.

The cups in the juvenile are more angular in outline (Plate IV, fig. 2) than those in the adult (Plate IV, fig. 3), and the typical "crossed cup" structure is more readily observed. Half-grown specimens show a mixture of angular and rounded deposits.

Stages in the development of "crossed cups" are readily observed in juveniles. The "cross" is the first to form (Plate IV, fig. 6a). It is a simple four-armed cross, each arm measuring about 0.008mm in length. A perforated square then develops on the cross (Plate IV, fig. 6b). The cross is invariably the starting point in the development of crossed cups. The extremities of the cross and the corners of the square then begin to expand laterally, and these lateral expansions eventually meet to form the "young" deposit which has smooth rounded edges (Plate IV, fig. 6e). The "young" deposits then assume the "classical" form (Plate IV, fig. 6f), with eight sharp projections regularly spaced around the margin, and a few small spines (3–8) on the cross side. In most cases the cross faces the outside of the body, and the short sharp spines project slightly above the level of the skin.

The anal papilla deposits of the juvenile are similar to those in the adult. They arc small, irregular spicules, and take the form of branched rods or perforated plates (Plate IV, fig. 4).

Ecology: Paracaudina chilensis has been taken in the New Zealand region from muddy and sandy localities to depths of at least 550 fathoms. Some specimens have been recorded from fish stomachs, but it is not known how extensively the species is used as food by bottom-feeding fish.

Discussion: Dendy (1897) gave a very thorough account of the structure and disposition of the anal papillae in this species.

Clark (1907) placed eight species of Caudina Stimpson into the single species Caudina chilensis (Muller), as many of the original species descriptions had page 21been inadequate, based as they were on such characters as size of the specimens, colour, texture of the body wall, all of which are known to be subject to much individual variation. In this synonymy Clark included Hutton's (1872) species coriacea from New Zealand and australis (Semper) from Australia.

Mortensen (1925) criticised Clark's synonymy and declared that C. coriacea from New Zealand, C. australis from Australia and C. chilensis from Chile were different species, and he used apparent differences in spiculation and calcareous rings as his evidence. At the present time, C. australis is still regarded as a distinct species. But the history of C. coriacea is rather more complex.

Hozawa (1928) regarded C. chilensis and C. coriacea as the same species using Clark (1907) as his authority. He may not have seen Mortensen's (1925) paper. Ohshima (1929) agreed with Clark (1907) and Hozawa (1928), and criticised the work of Mortensen, stating that his figures were inadequate. Heding (1932) accepted Mortensen's view and included C. coriacea as a separate species in his new genus Paracaudina, together with C. chilensis. Later Heding (1933) vigorously opposed Ohshima's (1929) opinion, and used the same characters as Mortensen (1925) for distinguishing the species coriacea and chilensis, but on a much more elaborate scale. He used characters such as body form, "retractor muscles", genital papillae, and presence or absence of "Cuvierian organs" as additional evidence. Thus Paracaudina chilensis was re-established as a separate species, but Clark (1935) "re-entered the lists" in his own words, after examining a great number of specimens of Paracaudina. His paper shows that he disagreed with Mortensen (1925) and Heding (1933). He discarded body form, "retractor muscles", "Cuvierian organs", genital papillae and the calcareous ring as bases for classification, and stated that the spicules were the only safe criterion for separation at the species level. As a result of his thorough studies Clark compiled a key to the species in genus Paracaudina, and named the New Zealand form Paracaudina chilensis var. coriacea, adding that Deichmann was in agreement with him. Deichmann (1960) suggested that the New Zealand form be named P. chilensis forma coriacea as, in the words of W. K. Fisher, "it does not protest too much".

I have examined only the New Zealand specimens of the genus Paracaudina and they display some considerable variation in their calcareous deposits. Comparison of these deposits with those figured by Hozawa (1928) and Heding (1933) has served to convince me that they resemble each other in so many features, and show such diversity of form, that the subdivision of the species chilensis into subspecies or even "forms" is unwarranted. Clark (1935) himself stated that if he had a specimen from Chile mixed with specimens from another area he would not be able to identify the Chile specimen with certainty.

Thus the suggestion lies at hand that P. chilensis is a circum-Pacific species, having possibly the Indo-West-Pacific region as its centre of distribution. Near the centre of distribution, the genus Paracaudina gave rise to tetrapora and australis, now in Australia, and to chilensis, which spread north to Japan, and to California and Florida via the Aleutian Islands, and south to New Zealand, leaving a remnant in North-west Australia. The Chilean representatives may have reached South America via New Zealand. Fell (1953) states that it is quite likely that New Zealand supplied contributions to the fauna of southern South America. He does not propose an Antarctic shoreline as does Deichmann (Clark, 1935), but indicates that the west to east circum-polar current may be responsible for this New Zealand affinity in certain elements of the South American fauna. The gap in the distribution of P. chilensis lies between California in the north and Chile in the south. The gap may possibly be due to unfavourable environmental conditions, or the species may still be undiscovered there. As P. chilensis is eurybathic to a certain degree, there should be few depth barriers to dispersal.