[Read before the Otago Institute, 5th November, 1912.]

The genus Plagiochaeta was established by Benham in 1892 for a species which was termed P. punctata, and a full account of its anatomy will be found in the journal cited (2). In this article a brief account of the nephridium was given. The worm is meganephric, with alternating nephridiopores, but differs from Maoridrilus in the perichaetine multiplication of the chaetae. It was, however, found later that the worm to which Captain Hutton had given the name Megascolex sylvestris belongs to this same species. In 1902, Benham (3) described four new species which he attributed to Plagiochaeta, and gave a history of the matter, with references to previous literature.

Of these four species, P. lateralis is meganephric, but has the pores in a single series, as in Eodrilus of Michaelsen. P. rossi was described as micronephric, as also were P. ricardi and P. montana; but in the case of the last two Benham, in a footnote to a paper (4) on “Some Edible and other Worms” (1904, p. 229), stated that a re-examination of these had shown him to have been in error; they are really meganephric, though the organ is small in comparison with the size of the worm, and is thrown into a series of coils which simulates the scattered individual nephridia of a micronephric worm.

Dr. Michaelsen (9), who has discussed so fully the systematic position of the New Zealand worms, suggested in 1907 (p. 140), by his tabular arrangement of the “holoandric Acanthodriline” genera, that the meganephric P. lateralis should be placed in a new genus, on the ground that the nephridiopores are in line, instead of being alternate as in P. sylvestris.

In 1909, in his memoir on the earthworms of India and neighbouring lands, he again discussed (10, p. 202) the systematic position of these four worms, and suggested that the three “micronephric” species should be removed from the genus Plagiochaeta and placed in his genus Hoplochaetella; and finally, in 1910, after examining material which Benham had sent him, he recognized the distinctness of lateralis by erecting the genus Perieodrilus for it (11, p. 61). As a result of this examination he confirms Benham's statements for lateralis and rossi. In the case of montanus he was unable, owing to the poor state of the specimen, to satisfy himself as to the true condition of affairs, but was inclined to believe that it is micronephric. As will be seen below, in this he was misled by the difficulty of tracing out the tubules, owing to the presence, as he says, of the numerous cysts of Gregarines.

As to P. ricardi, he had no specimens at his disposal, and accepts Benham's statements as to its being meganephric, and therefore places it in the genus Perieodrilus along with P. lateralis. During the expedition to the subantarctic islands Benham obtained and described (1909, p. 275) a new species, which he places in the genus Plagiochaeta; but according to Michaelsen's definition it should be called Perieodrilus plunketi. At the same time, apart from the condition of the nephridia, it may be noted that it differs from the other species in having a mere vestige of a gizzard, whereas in P. lateralis this organ is well developed. As he has separated Eodrilus from Microscolex (Notiodrilus) owing to the condition of the gizzard, perhaps this worm ought to be placed in a new genus; but for the present it may be left in Perieodrilus, as it is possible that the state of the gizzard may be due to its mode of life—that is, it may be adaptive.

Hoplochaetella Michaelsen.
H. rossi Benham.

There still remained some doubt as to montanus, but, as will be shown below, it is really meganephric, though the nephridia are very small and the tubule clustered so as to simulate a micronephridium.

The three species of earthworm dealt with in this paper have been found only in quite out-of-the-way localities—H. rossi was collected on the western shore of Lake Te Anau; P. montanus in thick forest, 1,100 ft. above sea-level, between that lake and George Sound; while P. ricardi comes from Resolution Island. Those who are at all familiar with the geography of this part of New Zealand will recognize the great expenditure of time and money that would be required to obtain fresh material from such inaccessible places. This must be the excuse for the poor state of preservation of the material, sent as it was in formalin, and for the attempt to work out the finer anatomical points on such material.

Note on Hoplochaetella rossi.

We take this opportunity of giving a figure of the nephridium of this species, as seen under a dissecting microscope; it is clearly micronephric, as Benham has already stated.

Fig. 1.—Hoplochaetella rossi. The micronephridia of one side of a segment just behind the chtellum. n.c. nerve-cord.

There is a single series of small looped tubules running along the inner surface of the body-wall, just behind the septum, and lying wholly anterior to the chaetal row. This series starts from a short distance outside the nerve-cord, and reaches almost to the mid-dorsal line (fig. 1).

In the posterior region of the body the tubules are smaller and more densely packed.

The various loops of tubules are so extended that under a No. 8 dissect-ing-lens of Leitz they appear to be independent of one another. They are variously and irregularly arranged from segment to segment (of. fig.-6). Though usually concentrated near the nerve-cord, there is frequently a second bunch, or it may be only two or three loops, farther away; and in other cases the loops are more evenly distributed along the body-wall: The coil is, however, usually limited to the lower half of the body.

In the segments immediately following the clitellum the nephridial loops are larger and more closely packed (fig. 7).

Fig. 7.—Perieodrilus montanus. The nephridium, distinctly meganephrio, of one side of a segment just behind the clitellum.

A very fine duct, overlooked at first, passes upwards to penetrate the body-wall at about the level of the 12th or 13th chaeta, and there is no bladder.

A funnel was detected, but owing to the poor state of preservation of the tissue and the rather imperfect condition of the sections we are unable to figure it or do more than note its existence.

We have not observed any branching of the narrow duct in the form of processes looping round the other ducts of the fold; nor are there any blood-capillaries having the characteristic arrangement observed in P. ricardi.

The most prominent feature of P. montanus in preserved specimens is the elaborate investment of blood-vessels which ramify over all the folds, forming a closely covering network. This makes the nephridium an extremely beautiful object when viewed through the microscope, as each capillary shines out clearly in the transparent folds, especially when glycerine has been added to the preparation.