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On the Structure of the Discodrilid Nephridium

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On the Structure of the Discodrilid Nephridium Voizzlme XIII. May, 1897. Number 3. JOURNAL OF MORPHOLOGY. ON THE STRUCTURE OF THE DISCODKILID NEPHRIDIUM.’ BY J. PERCY MOORE. I. Nzimber and Position of the Nephridia. ALTHOUGHno less than six2 accounts of the discodrilid neph- ridium are already extant, these fail to furnish us with the information necessary to a careful comparison of the organ with its homologue in other annelids. Five papers dealing with the general anatomy of Branchiobdella parasita and its varieties contain accounts of the excretory organs, but of these only one, that of Lemoine (25), was based upon investigations conducted with a knowledge of modern methods of research. Though in some respects more complete than its predecessors, Lemoine’s description adds little of importance to the earlier accounts, 1 Contributions from the Zoological Laboratory of the University of Pennsyl- vania. No. 5. Voinov has recently contributed, after the present paper was in the hands of the publishers, a more complete and accurate account of the nephridium of Rranchiobdella parasita. I have seen only the preliminary account (Comptes Rendus Acad. Sci., Paris, CXXII (1896), 1069-1071) with which in the main my results agree. This author is to be credited with the first correct description of the relations of the several regions of the nephridium. The “ red gland ” is identi- fied as a folded canal; and excretion by fragmentation of the peritoneal corpuscles described. 328 MOORE. [VOL. SIII. with which it is in substantial agreement. The sixth and latest paper (28) mentions a few novel facts observed in Bdellodrilus illuminatus, but is very incomplete. A new investigation being therefore desirable, I am led to present the results of some recent studies. Odier and Henle, writing respectively in 1823 and 1835, noticed that the nephridia were reduced in number to two pairs, a fact which has since been repeatedly verified for Branchiob- della parasita, and in the American discodrilids Bdellodrilus illuminatus, B. philadelphicus, B. manus, Branchiobdella insta- bilia, B. pulcherrima, Pterodrilus distichus, and P. alcicornus, as well as in several undescribed species. Consequently two pairs of definitive nephridia may safely be stated as characteristic of the family. This is a remarkable peculiarity in an annelid, in which group the metameric repetition of the nephridia is so general as to have gained for them the name of segmental organs, first proposed by Williams (37). Although the neph- ridia are not infrequently absent from a few of the anterior segments, and in Uncinais litoralis (U),and perhaps a few other naids, entirely wanting, no case esactly similar to the discodrilids is known. When, however, we consider the cepha- lization of the anterior somites, with the nearly complete obliteration of their coelom by encroaching muscle fibres and glands, the parallel concentration to form the sucker and its support at the posterior end, the shortening of the discodrilid body until it represents only the anterior (genital and pre- genital) segments, plus the anal and preanal segment of, for example, an enchytraeid; and when we consider the further fact that nephridia are absent from the genital segments of the latter, the important factors in an explanation of the discodrilid condition are in our possession. This does not, however, explain why with four possible neph- ridia-bearing somites anterior to the genital region, only one pair of pre-genital excretory organs should persist in the adult worm. In this connection the following associated facts may be considered as possible, but not necessarily complete expla- nations, viz., the large size of the uninary calculi and granules, necessitatinz a tube of considerable calibre to permit their No. 3.1 THE UISCODRILID LVZPHRIDIULW. 329 ready passage, the large heart with its prominent loop in the first and second somites, the enlargement of the alimentary canal in the fourth somite, the very narrow septa, and the active muscular movements of the worms. These conditions would evidently render a spreading longitudinal arrangement of the tubules an advantageous one, inasmuch as it would per- mit a more ready adjustment of the organs to the animal’s movements. Sections of embryos indicate the presence of provisional nephridia in certain anterior somites, as has been shown to be the case in certain Oligochaeta and Hirudinea. Individual nephridia are highly developed and conspicuous, the principal portion of each one consisting, as was originally described by Henle (21), of four distinct tubules arranged in two long loops, and passing into an opaque granular mass, through which previous writers have failed to trace them. In the more transparent species, and especially in the colorless-blooded Branchiobdella instabilia and pulcherrima, these granular masses may be seen with the naked eye as conspicuous, dark, and opaque spots, lying within the coelom by the side of the alimentary canal, one each in the second and third somites, and a pair symmetrically placed in the eighth somite.1 Connected with the masses are the nephridial funnels and efferent ducts, as well as the tubule loops, so that an under- standing of their structure is necessary to a knowledge of the relations of the latter. As indicated above, and this is true for all known species, the anterior nephridia are not symmetrical, the opaque mass of one lying anterior to that of the other. This has been observed by all writers on B. parasita, but it is noticeable in their descrip- tions and figures that they fail to agree as to whether the right or left one is the more anterior. Lemoine finally states that the species is variable, some individuals having the right, some the left nephridium in advance. This is the case in the three American species (Bdellodrilus illuminatus, B. philadelphicus, The European writers have variously enumerated the somites, some counting the “head” as one, 2nd each two of the body rings as one; others each ring of head and body as a somite. Inasmuch as the constitution of the “head ” is still a matter of disagreement, I enumerate only post-cephalic sonites, each consisting of two annuli. 330 MOORE. [VOL. XIII. and Branchiobdella instabilia) which were examined with refer- ence to this point. It is of interest that certain localities seem to furnish individuals predominantly of one kind, a fact which will be further developed in another connection. In those species which have well-developed inter-segmental septa, as is the case in Bdellodrilus philadelphicus, Branchiob- della instabilia, pulcherrima, and parasita, the opaque masses are strictly confined to their proper somites. On the other hand, those of B. illuminatus, in which the septa of the seg- ments anterior to the genital region are imperfect or altogether aborted, are more variable in position. The anterior one may lie partly or wholly within the third somite, and the posterior partly or wholly within the fourth, or both may lie -but this is a rare condition - symmetrically within the third.l In this species the greater number of individuals examined up to the present time had the anterior nephridium to the left, the posterior to the right of the intestine. The general arrangement of the tubules can best be seen when living worms are examined under a low power. For convenience, let us suppose an example of B. instabilia in which the right nephridium is anterior - but the windings have prac- tically the same form in all species. Three somewhat complexly looped tubules pass around the margin of the compact granular mass from the ventral to the dorsal anterior border, where they turn backward and are joined by a fourth, which appears from within the opaque mass. The four are now arranged in two pairs, of which one, composed of two of the tubules which wise from the ventral margin, forms a longer loop, and is twisted and foldeu about the other comparatively straight one. The four tubules, lying side by side, now leave the opaque mass at its posterior dorsal region, bend to the left, and extend trans- versely over the intestine, in more or less close contact with the septum lK/KI,, to the left body-wall, to which they are anchored by their peritoneal investment. Turning sharply forward they extend in a loose, wavy loop to the anterior left angle of the same somite, and are here once more anchored to One example of the anterior opaque masses, both l+g in the third somite, was observed in I3. instabilia. NO. 3.1 THE DISCODRILID NE'F'.VRIDIUM. 33 = the body-wall. Again turning sharply, the longer loop being anterior, they cross the alimentary canal transversely, still on the dorsal side, to the right anterior angle of the segment, where the longer loop turns backward around the end of the shorter one, and the peritoneal covering extends as a sheet (with the apparent exception of B. illuminatus) to become con- tinuous with the parietal peritoneum. At this point the two tubules of each pair become continuous with one another to form the two complete loops. The figure formed by the tubules of the anterior nephridium in the case described is seen to be a nearly complete quadrangle, extending around the four sides of the second somite (the anterior transverse leg frequently crowds the septum far forward), and terminating nearly at the point of apparent origin from the granular mass, When the anterior nephridium lies on the left side, which is the more usual condition in B. illuminatus, the tubules are generally disposed in a different way. The tubule group first extends backwards from the anterior margin of the opaque mass to the septum 11/111, then forward dorsally on the left side to the anterior septum, and transversely to the right, so that the tubule loops terminate at the same place as before.
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