Art. XXIII.—A Remarkable Case of Bifurcation in Lumbricus rubellus.
[Read before the Otago Institute, 1st December, 1914.]
I Early in the present year I received from Mr. James Jefferys a specimen of Lumbricus rubellus (an introduced European earthworm) which bears on its right side a short narrow branch or outgrowth.
The worm is adult, in that the clitellum is well developed, and the peculiarity about this particular example of what is not a very rare occurrence is that the branch is developed in this clitellar region. This anterior situation of the bifurcation is, so far as I can gather, unique, as it is usually much farther back.
The clitellum commences on the 27th segment, and occupies seven segments. The first of these segments is distinct from the 2nd over the
ventral surface and on the right side, but on the dorsal surface and on the left side it is merged into the 2nd. The 3rd segment as seen from above is unsymmetrical, but owing to a wound in the skin on the dorsal surface the exact course of the intersegmental furrow cannot be traced. It is from the hinder face of this segment that the branch arises, on the right side and nearer to the dorsal than the ventral surface. As seen from below, this segment has the form usually seen in bifurcated worms—that is, its posterior boundary on the right side is oblique, forming an angle of nearly 45° with the long axis of the body. Consequently a backwardly directed angular projection exists between the branch and the body. This angle, or “axilla,” is clearly seen ventrally, but is merged into the next segment on the dorsal surface.
Fig. 1.—Outline of Lumbricus rubellus, with a branch on its right side springing from the clitellum. Natural size.
Fig. 2.—Dorsal view of the clitellar region: the dotted area represents a damaged portion of the skin. X 4.
Fig. 3.—Ventral view of the same. X 4.
The 4th segment of the clitellum, again, is unsymmetrical, as seen from above, being wider on the left and narrow on the right side, just below the axilla.
The remaining segments, 5, 6, and 7, are normal. It is usually the case in these bifurcated worms that some segments are abnormally and irregularly developed.
Traces of the tubercula pubertatis are present in the form of a slight ridge on each side of the 3rd, 4th, 5th, and 6th segments, at the line where the glandular epidermis ceases latero-ventrally.
The branch thus springs from the 3rd clitellar segment; and the following three segments that form its base are also glandular—that is to say, the clitellum is continued on to the branch, and the ridge (or tubercula pubertatis) is continued along the outer side of these three segments.
The body is 73 mm. in length, with a diameter of about 4 mm. just behind the clitellum; the branch measures 14 mm. by 1.75 mm., and arises, as the figure shows, at a point about 18 mm. from the anterior end. It contains 15 segments, mostly biannulate, as on the body of the worm.
The chaetae on the body are, of course, in four couples, the individuals of a couple being close together; but the only chaetae that I can detect
on the branch occur on the inner side of the 3rd segment (corresponding to the 6th clitellar segment). Possibly they are also present on the preceding segments, embedded in the skin; but there are none on the rest of the branch.
At the end of the branch is a slight pit, which suggests that an anus is present here; it is, however, imperforate; and on opening the branch at two places I find that the intestine does not enter it, but the nerve-cord and the nephridia are continued to the end of it.’
It is, of course, difficult, if not impossible, to say how this bifurcation or branching has come about in the free state. It may be that the worm was injured at the commencement of the clitellar region, and that a lateral outgrowth developed instead of a mere healing of that wound.
The specimen is remarkable not only on account of the forward situation of the bifurcation, but also on account of the great inequality in size between the two forks, and finally on account of the absence of the intestine in the smaller branch.
II. In 1886 Mr. T. W. Kirk described and figured a “Curious Double Worm.”* This specimen remained in the Dominion Museum till about two years ago, when the late Director, Mr. A. Hamilton, was good enough to send it to me for examination. When it reached me it was glued to a piece of card. It was evident that at some time it had become dry, owing to the evaporation of the alcohol, for the tails are shrunken and the skin a good deal shrivelled. However, it is in sufficiently fair preservation for me to add something to Mr. Kirk's account.
A re-examination of this specimen seemed desirable, for Mr. Kirk was one of the first to place on record the occurrence of a bifurcated worm, the only previous accounts being those of Asa Fitch in America in 1865 (in a report on insects in the State of New York), C. Robertson in England in 1867, and Bell in 1885. It is evident that Kirk did not know of either of these. A fair number of similar cases have been recorded since that time, but Kirk's paper has been overlooked by all these writers, even by the most recent, Korschelt (1914).
The worm is a species of the genus Octochaetus, though, as it is immature, it is impossible to define its specific position. Kirk recognized that it was not a Lumbricid, but a native, though he figures the prostomium as being like that of Lumbricus—” tanylobic,” whereas it is “epilobic,” without the prolongation into the 1st segment. The chaetae are all on the ventral surface, small in size and close together, so close that each couple looks at first like a single bristle. The lines are approximately equidistant, though the two ventral rows are rather farther apart than the lateral row is from the ventral of its own side.
Kirk's measurements may be given in his own words: “The anterior portion is about 1 in. in length and ¼ in. in diameter … From the posterior end of the thick part, which terminates abruptly, spring two limbs, each 2 ½ in. in length and of an average diameter of ⅙ in.”
At present the “body” measures 14 mm. in length by 6 mm. in diameter; the two “tails” are unequal, one measuring 33 mm., the other 40 mm., each with a diameter of about 2.5 mm. These two tails are curved and undulating, a good deal shrunken, and one came away from the body on the specimen being removed from the card.
[Footnote] * Trans. N.Z. Inst., vol. 19, p. 64.
The “body” contains about 40 segments, biannulated, and much compressed. I did not count the segments in the tails, but they are very much more numerous, and are about the length of one annulus, and are uni-annulated. The two tails are symmetrically placed, as Kirk's figures show, and are separated by a narrow area at the hinder end of the body. The two rows of chaetae of the right and the two rows of the left side are continued on to the two tails respectively; while on the inner side of each tail two additional rows of chaetae make their appearance, though exactly at what point they commence it is impossible now to say, owing to injuries at their bases.
Kirk states that “the anal aperture is situated immediately at the posterior end of the thick portion, and between these two limbs or tails.” I am unable to confirm or to refute this statement, though from analogy with other cases I doubt whether it is the case here. At any rate, I must correct the statement that follows: “There is no aperture in the end of either limb, though there is a spot which at first sight gives the impression that an opening is present.” In this instance “first thoughts” would have been best, for there is an actual perforation at-each end, and an incision into the tail shows that the intestine is continued into it. This is quite what is to be expected, for in all cases which have been dissected, as Robertson was the first to show, the internal organs are, like the body-wall, bifurcated.
In short, this specimen is very similar to most of those that have been described, except that the body is much shorter than the tails, for generally the bifurcation occurs nearer to the hinder end.
Korschelt has suggested that in these cases, with approximately equal tails, the bifurcation is due to an embryonic process—a kind of twinning. He refers to a young individual of Allolobophora subrubicunda, which he removed from the cocoon, being bifurcated, and was therefore not the result of injury, but was probably due to the partial division of the ovum or embryo. It is known that in some species of Lumbricus the ovum does divide into two embryos at an early stage of its history; in these instances of bifurcation, then, the division has been incomplete.
III. Mr. T. W. Kirk was good enough to send me a water-coloured drawing of a living bifid worm with two approximately equal “tails” and very similar to that described by Robertson, Bell, Williamson, and others. Unfortunately, it has not been identified, but it looks to me like L. rubellus. It was found in Rangitikei.
Andrews, E. A. American Naturalist, vol. 26, 1892.
Bell, F. J. Annals and Mag. Nat. Hist., ser. 5, vol. 16, 1886.
Korschelt, E. Zoolog. Anzeig., vol. 43, 1914.
Robertson, C. Quart. Journ. Micro. Sci., vol. 7, 1867.
Williamson, H. C. Annals and Mag. Nat. Hist., ser. 6, vol. 13, 1894.