The Zoologist/4th series, vol 1 (1897)/Issue 670/The Dorsal Pores of Earthworms
THE ZOOLOGIST
No. 670.— April, 1897.
THE DORSAL PORES OF EARTHWORMS.
By the Rev. Hilderic Friend,
Author of 'Flowers and Flower-Lore.'
If a specimen of the Common Earthworm is examined, especially after having been preserved for a time in spirits, it will be found that a number of pores exist on the back. They are most readily seen on the girdle as a rule, and look exactly like the holes which result from the puncture of a pin or needle. These openings have been known for a considerable time as the dorsal pores, a name which serves not only to define their position, but also to differentiate them from the other openings which exist on various portions of the worm's body, such as the male or spermiducal pores, the nephridiopores, and the puberty pores (tubercula pubertatis).
It is now many years since these apertures were originally detected. Who first observed them it is impossible to say. Equally difficult would it be to decide who was the first to notice their presence in describing the animals. In 1727 Dr. Derham, Canon of Windsor, wrote a very interesting work entitled 'Physico-Theology,' in which he endeavours to demonstrate "the being and attributes of God from his works of Creation."
He says that under the skin of worms "there lies a slimy juice, that they emit, as occasion is, at certain perforations between the annuli, to lubricate the body, and facilitate their passage into the earth." A little later, however, he shows that a certain Dr. Willis had previously written an account of these "foramina on the top of the back, adjoining to each ring, supplying the place of lungs." Now Willis published his work, 'De Anima Brutorum,' in 1672, so that for upwards of two hundred years the pores have been known to science, to go no further back. It is only in recent years, however, that they have been carefully noted, and the position of the first pore recorded for the different species of worm. It has been thought by some that the first dorsal pore was so uniformly placed in the various species of Earthworms that a specific character might be based thereon. This I am disposed to think is not borne out by facts.
Dr. Benham, one of our few English authorities on the subject, says: "In many Earthworms the cœlom is put into communication with the exterior by means of a series of dorsal pores, placed on the intersegmental grooves. In Lumbricus these pores occur in every somite after about segment eight; in Digaster and Perionyx they commence just behind somite four; in Plutellus behind somite six; in Pleurochæta and Typhæus the pores are present only behind the clitellum. They are present in Acanthodrilus, and in many Perichæetæ.'[1] In Allurus they begin behind segment three or four.
As will be inferred from the foregoing, a variety of ideas have prevailed respecting the use to which these apertures were devoted in worm economy. Willis says they supply the place of lungs, and if Derham's remarks apply to the dorsal pores, he regards them simply as the openings through which lubricants were poured. Lloyd Morgan is as cautious on the subject as he is inaccurate. He says: "Every segment of the body except the first has a dorsal pore opening into the anterior part of the ring in the mid-dorsal line, and two very minute pores, one on each side of the ventral line, which are the external orifices of the nephridia or segmental organs, whose function is excretory."
The dorsal pores are not found in the typical Earthworm on every segment save the first, and if they were, we are not favoured by the Professor with a vestige of an idea as to their use. He says: "There are no specially differentiated respiratory organs, respiration being apparently effected by the surface of the body," so that he does not regard the dorsal pores as lungs.
The most important contribution to the subject is undoubtedly that which was made a few years ago by Hermann Ude, in a paper which deals chiefly with the structure of the bodywall in Earthworms.[2] He points out that "the dorsal pore lies on the anterior edge of the somites in which it occurs, and appears on the intersegmental groove. It is absent in the most anterior somites, but the position of the first pore is constant for a given species." In the Common Earthworm it occurs between eight and nine, and in the Turgid Worm between ten and eleven. We should say between eight and nine and nine and ten respectively. Claparède formerly described the epidermis as being folded inwards at the dorsal pore, just as it is where the setæ are situated, but Ude shows that such is not the case. By stripping off the epidermis I have been able to detect the infolding of the cuticle around the setæ, but not around the dorsal pore, which, as Ude affirms, is a perforation through the epidermis and the muscular layers. The pore is wanting in most Freshwater Worms or Limicolæ. Beddard has dealt with the exceptions. In some worms, when the girdle is fully developed, the pores become closed through the growing up of the cuticle around the edge. This is not always the case, however, for the Mucous Worm has been noted by some to be an exception, while I have found that the dorsal pore on the clitellum or girdle of some species is quite as discernible after the organ has attained full development as before.
If a worm is opened laterally, and the internal organs removed so as to leave only the body-wall, it will be possible so to display this portion of the animal as to see the whole series of pores in regular succession. It will be easy then to observe that they are connected with each other by a kind of tube which runs right along the back of the worm. I am a little doubtful whether or not this is what Ude refers to when he says that the epithelium of the body cavity passes across the muscular layers, and meets the cuticle around the edge of the pore. The pore has a special set of muscle-bundles which form its sphincter muscle.
Ude does not think there is the slightest connection between the pores and the nephridia, which are excretory in their function. Yet in a sense the dorsal pores play their part in the excretory process, since the fluid contained in the cœlom or body-cavity, as well as certain other substances which in some species of Earthworm are coloured, can be caused to exude through them. Sometimes the exudation is in drops, but some foreign species are able to squirt it to a distance of a foot, much as Peripatus does. In these cases the process is perhaps protective.
In a memorable article on the Earthworm, published some years ago, Prof. Ray Lankester[3] says: "In the cuticle of the Earthworm a system of very minute canals exists, ... which might either be described in connection with the respiratory mechanism, or here, if we regard these ducts as excretory pores ... It is undoubtedly through these minute canals, which exist throughout the integument of the Earthworm, that water passes to the perivisceral cavity, and a dense fluid passes out." Ude tried a series of experiments to ascertain whether or not water was admitted through these pores, but he failed to satisfy himself that such was the case, though I have many times observed the denser fluid of which Prof. Lankester speaks issuing from them.
It is to Prof. Busk that we are indebted, through Prof. Lankester, for one of the best accounts of these apertures in English. In a remarkable paper on the Earthworm, published by the latter in 1865, we have an illustration of the integument of a Worm with all the various pores found on the dorsal surface carefully represented. "One of these orifices, situated in the median dorsal line of the segment, appears always to be larger than the others, and penetrates directly to the perivisceral cavity. That these openings form a very ready and frequent means of escape to the colourless fluid may be ascertained by handling a large Earthworm, when some considerable quantity is nearly invariably found to escape from its dorsal surface."[4] Nor is this all. Prof. Busk says that the fluid expressed from these pores was of a dirty greyish colour, thin and opaque. Examined under the microscope, it contained numerous spherical particles and pyriform granular bodies, besides irregular organic particles. This coloured fluid differs with the species of Worm examined. In some, as the Brandling and Turgid Worm, it is yellow; in others, as the Mucous Worm, it is white; while the Red Worm yields two-thirds of colouring matter.
Mr. Beddard, in his invaluable 'Monograph of Oligochæta,' unfortunately leaves the subject almost untouched. He says (p. 13): "The cuticle seems undoubtedly to be a formation of the packing cells of the epidermis; the pores upon its surface are the outlets of the gland-cells, and their existence appears to be simply due to the fact that the gland-cells do not secrete a cuticle like the other cells, their secretory activity being taken up in the formation of the granules with which they are laden; hence at the points where they abut upon the cuticle there are gaps—the pores in question." In discussing the question of the cœlom—a subject which has been somewhat fully treated by Mr. Lim Boon Keng, Straits Settlements Scholar, since the 'Monograph' was published—Mr. Beddard again (p. 30) introduces the dorsal pores, and as the paragraph represents the latest results, it will be well to give it almost in extenso. "The cœlom," we are told, "is placed in communication with the external medium in a large number of the Oligochæta by a series of pores, one to each segment; in addition to these structures, which are called the dorsal pores, there is, in a certain number—most of the aquatic Oligochæta—a single pore on the prostomium, which is generally spoken of as the head pore" (and is found in the embryo Lumbricus (p. 32), though not in the adult). The dorsal pores are never developed upon the first one or two segments of the body, and the point where they commence is characteristic for the species. The dorsal pores were considered at one time to lead into sacs, the function of which was believed to be respiratory; it is now known that the pores are simply perforations of the integumental layers just at the anterior boundary of the segment to which they belong; there is no lining of epithelium, as has been erroneously stated to be the case; there is simply a discontinuity of the muscular and epidermic layers where the pores exist. The structure of these pores has been more particularly studied by Ude. Their structure in Fridericia has been studied by Vejdovsky and Michaelsen: "in these Worms the pore is bordered by large round glandular cells on each side; no such cells are visible in the case of the dorsal pores of Earthworms. We are at present completely in the dark as to the morphological meaning of these pores." No relations are apparent between the dorsal pores and the nephridia. Michaelsen thinks the dorsal pores "have the function of nourishing the body and prevent its becoming unduly dry; it is certain that the cœlomic fluid is pressed out through the pores, and their occlusion is regulated by longitudinal muscles.... Perhaps in the Oligochæta the dorsal pores pass out the waste fluids, while the remaining excretory products are elaborated and passed out by the nephridia."
Thus we see that even now, notwithstanding the large amount of attention which has been paid to Earthworms during the past decade, we are very badly informed on many points connected with their economy; and there is great need that some one, with the necessary leisure, means, and scientific training, should investigate some of the details more fully. I have been able to make great progress with my work on the distribution and revision of the British Lumbricidæ, till recently almost totally neglected; and hope by the due publication of the new and interesting results to stimulate further research on the part of others.
Meanwhile, so far as the dorsal pores are concerned, they appear to be for the emission rather than the introduction of fluids; and are apparently lubricative, excretory, and protective. Their homology with certain organs found in other annelids does not seem to have been carefully ascertained; at any rate I know of nothing on the subject in English.
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