Art. XX.—The Nephridia of Perieodrilus ricardi and of P. montanus
[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.
Consequently the seven species which have been attributed to the genus Plagiochaeta will be distributed among three genera, belonging to two subfamilies of the family Megascolecidae.
P. sylvestris Hutton.
P. lineatus Hutton.
P. lateralis Benham.
P. ricardi Benham.
P. montanus Benham.
P. plunketi Benham.
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.
Perieodrilus (Plagiochaeta) ricardi.
This worm was originally stated to be micronephric (3, p. 287); but, as has been pointed out in the introductory note, this is not the case, for, though the appearance of the nephridia suggests separate tufts of tubules arising independently from the body-wall, these tufts, are all connected, and constitute the looping of one long bundle of tubules. The worm, therefore, though the nephridium is diffuse, is meganephric; but compared with the very great size of the worm it is exceedingly small, is concentrated near the nerve-cord, extends for about one-fifth of the semicircumference of the body-wall, and takes up but little space in the body-cavity. A specimen measuring 13 mm. in diameter, when laid open, has its body-wall 40mm. wide; the nephridium commences at a distance of 5mm. from the nerve-cord, and the coils of the organ occupy only a length of 4 mm.; the duct passes outwards to the pore in a straight line.
It shows no trace of the distinct regions presented by such worms as Lumbricus, Maoridrilus, &c. It consists of one long tubule of nearly uniform diameter, which is looped or folded vertically upon itself, the separate folds being connected by horizontal tubules lying on the body-wall near the base of the septum. The ventral ends of the upright folds and the horizontal connections are bound together by blood-vessels and connective tissue, and we may call this part of the nephridium the basal portion.
The various folds have a diameter of about the same size as that of the anterior or posterior folds of Maoridrilus, and the canals contained therein are similar to those in the latter worm. The specimens of P. ricardi which were examined were all preserved, and the regions of the excretory tube could not be clearly seen in all parts of the nephridium, nor could they be traced individally from funnel to pore. But the tubule is differentiated into the narrow tube (a.o and a.i, fig. 2), the ciliated middle tube (b), (“bridge” of Maoridrilus), and the wide tube (c), as usual in earthworms. The tubes follow the course of each fold, bending back on themselves at the apex or spur. There are several of these single folds in each nephridium, and this fact, together with the similarity in structure between the excretory canals of Perieodrilus and Maoridrilus or other meganephric genera, leads one to think that the nephridium of P. ricardi represents a transition state between a meganephric and micronephric condition. The one large nephridium is still single, but has become diffuse in form, and has lost the specialized regions of its original form, though it has not yet broken up into separate tufts of micronephric tubules.
The outer and inner narrow tubes wind in and out of each other at the apex of a fold in a manner which recalls the dorsal lobe of Maoridrilus; the wide tube is not affected by these windings, but retains its direct course round the fold. The narrow canals pass above and below it as they loop round each other, and there is sometimes a loop round the wide canal. Over a great part of the nephridium the inner narrow canal (fig. 2, a.i) sends out processes (a.br) which loop round the other canals on the convex side of the fold and return to the one from which they are given off. This branching does not form a network such as occurs in the excretory tubules
of Microchaeta (1), where the narrow duct, indeed, is made up of a network of anastomosing canals; but each branch is separate, and forms a single loop round the ducts which lie between the narrow canal and the outer side of the tubule. Sometimes, however, the branch divides into two before it bends round; but, as a rule, there is no connection between the individual loops (cf. also Eisen's figure of A. marmoratus).
Bach such excretory loop is accompanied by a fine capillary blood-vessel, and when seen in optical section the vessels form a perfectly regular trelliswork radiating from the inner to the outer side of the fold (fig. 3), and the appearance presented when the nephridium is mounted in glycerine is remarkable, for the blood-vessels shine out very clearly and form a striking characteristic of the nephridium.
But for this regular trelliswork of blood-vessels the looping of the excretory canal might be easily overlooked—at any rate, in preserved specimens—for the canals are so transparent that they are seen with great difficulty except in that part of a fold where the ciliated regions occur; but the canals are here easily distinguishable, and the direction of the current may be deduced from the direction of the cilia. A more minute examination of the blood-vessels proves, however, that each capillary accompanies a loop from the narrow duct, and that both have a semicircular course, passing outwards to the edge of the fold and back again to the duct round the intervening tubes.
Fig. 3.—Perieodrilus ricardi. Sketch of the appearance presented by the blood-capillaries as they follow the branches of the narrow canal.
These capillary blood-vessels are exceedingly fine, and their actual connection with the main blood-supply cannot usually be seen. It was observed, however, in a few cases, and these were sufficient to set at rest any doubts of a possible misinterpretation of the appearance presented.
It is worth noting the behaviour of similar outgrowths from the narrow canal in the various genera which possess a branching system of tubules.
In Microchaeta rappi (1), the nephridium of which, excluding the well-developed bladder, is not unlike that of P. ricardi, there occurs in each looping fold of the rosette an anastomosing system of ducts which ramify over and around the wider canals of the fold. In this species there is no narrow canal sending outwards regular processes from its lumen; the branching has become quite irregular, and the duct itself has become lost in its branches.
In Moniligaster grandis (7) the narrow canal is not branched in such a complicated manner; it is still distinguishable in most regions, but in the distal portion of the glandular lobe transverse connections appear between the two parallel regions of the narrow tube, though in some parts the outgrowths of ductules are irregular.
In Argilophilus, as described by Eisen (8), we find a condition which resembles more nearly that of P. ricardi. Here, however, the branching is limited to a definite portion of the narrow tube—viz., along the whole length of the anterior fold—in accordance with the perfectly definite form of the nephridium, while in Perieodrilus it appears to occur in many regions and in any.
Fig. 4, a transverse section of a fold containing three canals, shows the branching of the inner canal (a.i.) to form a loop round the thick-walled wide canal (o). It appears from this that the loop is not perfectly straight, but has a slightly wavy outline, so that it is cut across in parts of its course (a.br).
The narrow canal has the same histological structure as that of Maoridrilus, &c.—a thin wall surrounding a very wide lumen: i.e., a cell so largely perforated that but a small part of its outer wall remains, and this part carries the nucleus. The small amount of protoplasm forming the wall of the lumen is granular.
The wide canal (fig. 4, c) has a very characteristic appearance when seen cut across. The lumen is intracellular, but it is comparatively very small, being reduced to a mere slit between the thick surrounding walls. The granular protoplasm is radially striated, the striations appearing very distinctly, so that the protoplasm seems to be arranged in lines radiating outwards from the lumen to the outer wall of the duct. Similar striations have been recorded in the nephridium of Lumbricus and some other earthworms.
There is no muscular bladder or terminal vesicle of any kind: the nephridium ends in a long narrow outlet-duct which opens to the exterior at some distance from the main portion. At first sight the duct appears to be threefold, suggesting that the nephridium is formed of three closely opposed nephridia whose ducts lie parallel to each other and open by a common pore; but towards the distal end of the duct one of the three canals turns back upon itself, forming a loop, and passes back again into the nephridium, while the now single outlet-duct passes through the bodywall to the nephridiopore.
The length of the outlet-duct is remarkable, and the appearance presented in the opened worm recalls immediately the figures given by Bourne (6) of the nephridia in the embryo of Mahbenus, where the long narrow excretory duct grows out from the neck of the embryonal nephridium and passes for some distance along the body-wall before it communicates with the exterior. The absence of any muscular sac in Perieodrilus marks this genus off from Maoridrilus, in which the terminal vesicle is exceedingly well developed.
The nephridiopores are not arranged, as in Maoridrilus, in a dorsal and ventral series; they show no alternation, but open laterally, or slightly nearer to the dorsal surface than to the ventral, at some point between the 14th and 18th chaetae. The chaetae are numerous, and form a continuous circle round the body. Between these small limits, however, there is no definite position for the nephridiopore. It opens to the exterior either at the level of a chaeta or at any point between two chaetae. From the other end of the nephridial coil the short nephrostomial duct passes from the nephridium through the septum and ends in the preceding segment in an extraordinarily minute funnel—minute, that is to say, in comparison with the size of the worm, which is very large, and even with the size of the nephridium, which is small for such a large worm. Seen in section, the funnel lies right against the muscular layer of the body-wall, and not far removed from the nerve-cord. It is much smaller and less well developed
than that of more typically meganephric worms, but there is only one in each segment, which fact is a further indication of the meganephric condition of the worm. Unfortunately, the preservation of the worm was not sufficiently good to allow of the study of detailed histology: what could be observed of the nephro-stome is given in fig. 5, taken from a series of sections cut transversely and stained in borax carmine. The funnel is seen to consist of a central cell and marginals. It is cupshaped, and not unlike the distal portion of the funnel of Maoridrilus, except that the latter is larger and its marginals are more distinctly seen. There appear to be no cilia on the central cell of Perieodrilus. The marginals are bent inwards from the back of the cup to form a hood over the cavity of the funnel—-fig. 5 (b)—so that a section from side to side near the back of the funnel does not show an opening to the coelom.
There is no doubt as to the presence of a central cell, the nucleus of which may be distinctly seen (n.c.) in fig. 5 (a).
The nephridium of this species is similar to that of P. ricardi. It has the same diffuse form, on account of which the worm was originally considered to be micronephric (see figs. 6, 7). P. montanus is smaller than P. ricardi, and the size of the nephridial tube corresponds with the smaller size of the worm, the tubules being much finer and more delicate in the former species. A worm 12 mm. in diameter, the body-wall when laid open and flattened out measures 30 mm. across, the nephridial coil is 5 mm. in length and commences 2 mm. from the nerve-cord.
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.
List of papers referred to in the text.
Benham. (Microchaeta.) Quart. Journ. Micro. Sci., vol. 26, 1886, p. 267.
Benham. “Notes on Two Acanthodriloid Earthworms from New Zealand.” Quart. Journ. Micro. Sci., vol. 33, 1892, p. 294.
Benham. “On the Old and some New Species of Earthworms belonging to the Genus Plagiochaeta.” Trans. N.Z. Inst, vol. 35, 1902, p. 277.
Benham. “On some Edible and other New Species of Earthworms from the North Island of New Zealand.” Proc. Zool. Soc., 1904, vol. 2, p. 220.
Benham. “Report on Oligochaeta of the Subantarctic Islands of New Zealand.” Subant. Islds. N.Z., 1909, p. 251.
Bourne. (Mahbenus.) Quart. Journ. Micro. Sci., vol. 36, 1894, p. 11.
Bourne. (Moniligaster.) Quart. Journ. Micro. Sci., vol. 36, 1894, p. 307.
Eisen. “Californian Eudrilidae.” Mem. Cal. Acad. Sci., vol. 2, 1894.
Michaelsen. “Die Fauna Sudwest Australiens: Oligochaeta.” 1907.
Michaelsen. Oligochaeta from India, Nepal, Ceylon, &c. Mem. Ind. Mus., vol. 1, 1909.
Michaelsen. “Oligochaeten von verschiedener Gebieten.” Mitt. a. d. Naturhist. Mus., vol. 27, 1910.