Detailed accounts of the anatomy and histology of U. chilensis have been given by Seitz (1907) and Embleton (1900), and of the anatomy of U. caupo by Fisher (1946). The general morphology of these species resembles that of U. novae zealandiae, but there are some important differences in detail.
U. novae-zealandiae (Fig. 1) is cylindrical, sausage-shaped, the relative length and diameter depending on the amount of contraction on preservation. Only one of the specimens available for study has been preserved in a relaxed condition, the others showing varying degrees of contraction. The posterior end is rounded, the anterior narrower, terminating in a short scoop-shaped proboscis. In the contracted specimens this is folded into a tube, the inner surface of which is thrown into longitudinal folds continuous with those of the mouth. In one specimen 230 mm long the proboscis is 10 mm. The species approaches U. caupo and U. chilensis in size. Fisher and MacGinitie (1928) give the average size of U. caupo as 150–180 mm, although their largest specimen measured 545 mm when fully relaxed in anaesthesia and 375 mm when preserved. Wesenberg-Lund (1955) records specimens of U. chilensis from South America up to 255 mm long. The single well preserved extended specimen is 230 mm long and 28 mm across the broadest region of the body. One specimen from a dogfish stomach measured nearly 300 mm. The lengths of the other specimens from the vicinity of Banks Peninsula were 165 mm, 124 mm, 110 mm, 103 mm, 82 mm and 72 mm. The last three specimens were immature juveniles.
In the larger specimens the two anterior setae (Fig. 8) are situated 3–4 mm back of the groove leading into the mouth. They are 10 mm to 13 mm long, iridescent, bluish yellow, brownish at the tip. The exerted portion is flattened, strongly curved and directed backwards. Internally they are enclosed in a setal sac (Fig. 4) projecting well into the body cavity and usually have a short substitute seta alongside. They are united by a strong interbasal muscle and numerous variable muscles radiate from their inner ends. The anal setae (Figs. 6, 7) ten or eleven in number are cylindrical, tapering, terminating in a sharp curved tip. The dorsal setae are
Fig. 6.—Posterior end of the body showing the eccentric anus and the circle of setae incomplete mid-ventrally.
Fig. 7.—Anal setae with an enlarged view of the tip.
Fig. 8.—Two views of the anterior setae.
longer than the ventral. As in U. caupo a midventral seta is absent, and the anus is eccentric to the circle of setae (centre of anus about 6 mm from the dorsal setae and 4 mm from the ventral).
The surface of the body is traversed by fine irregular channels giving a rugose appearance. This is more pronounced at the anterior and posterior ends. The anterior end to about 10 mm behind the anterior setae is deeply cut by channels into irregular rectangular or circular areas. This region passes abruptly into the region of the slime glands which is distinguished by rather fine close circular channels.
Nephrida. Two of the three adult specimens examined from the type locality resemble U. uncinatus in having two pairs of nephridia. These vary in size according to the degree of distension with eggs or sperm. In one specimen 165 mm long the posterior nephridial tubes were 160 mm in length and up to 6 mm in diameter. The nephrostome is on the anterior side of the base of the tube and has long, grooved, spirally coiled lips. The nephridial tubes of the immature specimens were spherical with a cone shaped tip. One of the adult specimens had a third posterior nephridium on the right side, as has one of the juvenile specimens from Pegasus Bay, off Banks Peninsula. In another of these specimens one of the nephridia has a double nephrostome with two pairs of spiral lips. It appears that the primitive number of nephridia in the genus Urechis is three pairs. Fisher (1946) states that in U. caupo rarely one of the anterior pair of nephridia may be missing. U. uncinatus has two pairs of nephridia and in U. novae-zealandiae one or both members of the posterior pair are usually absent. Of 10 specimens dissected 8 had two pairs of nephridia and two had a third right posterior nephridium.
Alimentary Canal. The general arrangement of the alimentary tract is closely similar in all four species. The proportions of the different regions differ, although reliable comparisons are difficult owing to the contraction that occurs during preservation. In Fig. 3 the greater part of the siphonal and part of the post-siphonal
gut have been removed. It can be seen that the foregut is considerably longer than the body. In this specimen, which was 165 mm long, the pharynx is 15 mm long, the oesophagus 135 mm, the crop 90 mm, the gizzard 115 mm and the stomach 100 mm; total 455 mm. Fisher (1946) gives the following figures for a well expanded preserved specimen of U. caupo, 300 mm long; pharynx 30 mm, oesophagus 40 mm, crop 85 mm, gizzard 85 mm, stomach 50 mm; total 310 mm. Proportionately the foregut of U. novae-zealandiae is considerably longer, particularly the oesophagus and stomach portions.
As in U. caupo the pharyngeal lining is thrown into longitudinal folds (Fig. 4) which begin on the ventral side of the proboscis and run into the pharynx, decreasing in size. In the oesophagus these longitudinal ridges are deeply cross cut by narrow channels dividing them into rings of oblong verrucae which are visible externally. The oesophagus commences a few mm behind the last of the dorso-ventral mesenteries of the pharynx.
The crop is subtended by a strong mesentery not attached to the body wall. The gizzard is a slender threadlike tube with a conspicuous ringed appearance externally. The stomach has the mucosa thrown into transversely elongated compressed verrucae giving an irregularly ringed appearance externally. It ends abruptly at the beginning of the midgut. As in U. caupo the siphon commences 2–3 mm from the end of the stomach. In U. chilensis, according to Fisher (1946) the siphon commences 9 mm from the end of the stomach.
The course of the intestine is shown in Fig. 2. It can be seen that the siphonal part of the intestine is long and has three anterior and three posterior bends. Throughout its length it is attached by muscular mesenteries to the body wall.
The post-siphonal “small intestine” is about ⅕ the length of the pre-siphonal. In the specimen for which the measurements of the foregut were given above the post-siphonal intestine was 240 mm long. Proportionately this is considerably longer than that of U. caupo. It is anchored to the body wall by heavier mesenteries than those of the pre-siphonal portion.
The respiratory hindgut in all the specimens dissected except one of the juveniles is a straight, thick-walled, flattened tube. In this juvenile specimen it is a thin walled, semi-transparent, distended bladder indicating that it is capable of considerable dilation. It is firmly attached along its entire length to the left side of the nerve cord. The attachment of the anterior end of the respiratory hindgut is shown in Fig. 5. Fisher (1946) has figured the attachments of the other three species of Urechis. The method of attachment in U. novae-zealandiae more closely resembles that of U. caupo than of the other two species.
The cloaca is an elongated tube, deeply furrowed in its posterior half. The two large anal vesicles open into this posterior region. Their length in a specimen 165 mm long is about 40 mm.
Lectotype. Canterbury Museum collection.
Type Locality. Pegasus Bay, off Brighton Beach.
Distribution. The species is apparently confined to New Zealand. Localities recorded in this paper range from Auckland to Stewart Island. It is probably widespread on suitable bottoms, but is seldom dredged owing to its deep burrowing habits.