Orginal Articles.
VI.—On Sphærularia Bombi.—By John Lubbock, F.R.S., F.L.S., F.G.S. (With Plate I.)
This very curious creature was first discovered by Leon Dufour, and described by him in the "Annales des Sciences Naturelles" for 1836. He at first supposed that it was a dipterous larva, but soon saw that it belonged to the Entozoa; and as it certainly could not be referred to any other genus, he gave it the appropriate name of Sphærularia.
Von Siebold is, I believe, the only other naturalist who has recorded any personal observations on the subject; and as the remarks of both these excellent observers are very much to the purpose, and at the same time very short, I may, perhaps, be permitted to quote them in full.
M. Leon Dufour' s description is as follows:—
"SPHÆRULARIA BOMBI.[1]
"Teres, albido-pellucida, mollis, filiformis, haud annulata, undique sphærulis vesiculæ formibus granulata, antero posticeque obtusa subrotundata.
"Hab. in abdomine Bombi terrestris et B. hortorum. Long., 6-8 lin. J'ai vainement cherché a rapporter ce singulier Entozoaire a quelqu'un des genres consignés dans l'ouvrage de Rudolphi; j'ai cru pouvoir en constituer un nouveau sous le nom de Sphæ- rulaire qui exprime sa structure extérieure. Je l'avais d'abord pris pour une larve de Diptere, mais l'absence de toute segmentation et sa forme cylindrique, me ramenerent a un Entozoaire. Il n'est pas tres greles, puisque sur six a huit lignes de longueur il en a pres d'une de largeur. Il n'offore aucune distinction ni de tête ni de queue, et il est obtus ou même arrondi par un bout ou par l'autre. Toute la surface de son corps est couverte, soit au dessus, soit au dessons, de granulations spheroidales semblables a des vesicules subdiaphanes.
"Je l'ai rencontré plusieurs fois dans la cavité abdominale des especès preciteés de Bombus, en dehors du tube digestif et toujours libre. En Juin, 1833, j'en trouvai deux ensemble dans le meme individu du B. hortorum, et cette circonstance me fortifie encore dans l'idée que c'est un Entozoaire."
V. Siebold says ("Müller's Archiv.," 1838):—
"Who would not be surprised at the appearance of the Sp. bombi, figured by Léon Dufour, and ask himself in which of the five orders of Helminths this bee-worm should be placed. I have been fortunate enough to find this worm in the cavity of the body of B. terrestris, muscorum, and sylvarum, together with completely formed young ones, and have made out from the form and manner of development of the latter that this parasite can be nowhere better placed than among the Nematoids. Besides which, the formation of the female generative organs corresponds exactly with those of Filaria; but, on the other hand, the rest of the worm presents some peculiarities: its digestive apparatus differs remarkably from that of the Nematoids, and I could remark no trace of movement in any individual,—all of which were females,—however fresh I examined them: the young ones, on the contrary, moved about in a lively manner. In this animal the interesting occurrence takes place, that the young are entirely unlike the full-grown animal, their skin being quite smooth, while that of the mother animal is studded with vesicular projections, giving to it a very pretty appearance."
The only other original notice of this extraordinary creature which I have met with is a note to Siebold and Stannius' "Anatomie comparée." They say:—
"On ne trouve ni bouche ni anus chez la Sphærularia Bombi, et le canal intestinal est remplacé par une série d'utricules allongées, adhereutes ensemble, et autour desquelles s'enroulent les organes genitaux."
Except that the series of large cells is double, instead of single, and that one end of the worm is easily distinguishable by the presence of the vulva, these statements are all, I believe, perfectly correct; they still, however, leave a great many points to be ascertained, and it was with a hope of supplying the deficiency that I undertook the subject. My good fortune has been smaller than my hopes; but, though the present memoir is lamentably incomplete, it may not, I hope, be found entirely without interest.
M. Léon Dufour and Von Siebold met with Sphærularia in the four species of humble bees—namely, Bombus terrestris, hortorum, sylvarum, and muscorum. I have found it in the females of B. terrestris, lucorum, pratorum, lapidarius, subterraneus, hortorum, and mus- corum, which increases to eight the number of species in which Sphærularia is known more or less frequently to reside. The proportion of specimens attacked is, however, very different in the different species, and the parasite appears to be most common in B. terrestris, lapidarius, and lucorum. Out of thirty-three specimens of B. terrestris examined by me in the months of May and June, no less than nineteen—that is to say, more than one-half—contained these parasites. The following table shows the number of bees examined, and the proportion which were affected:—
| No. of large Females examined in May and June. |
No. which contained Sphærulari. |
|
33 | 19 | |||
|
21 | 7 | |||
|
16 | 1 | |||
|
13 | 1 | |||
|
12 | 6 | |||
|
6 | 2 | |||
|
4 | 2 | |||
|
7 | 0 |
I have not had any opportunity of examining B. sylvarum; and it will be observed that B. muscorum and hortorum, in which the parasite was found by V. Siebold, have only supplied me with a single infected specimen each, out of twenty-nine which I examined.
Neither Léon Dufour nor Siebold say anything about the sex of the infected specimens. All, however, that have come under my notice were large females, and I have never seen a single Spæerularia in a worker or a male.
The worms lie free in the cavity of the body, and are somewhat curled up. The largest number of full-grown females which I ever found in a single bee was eleven, but the usual numbers were from five to eight. The two infected specimens of B. pratorum, however, contained only one specimen of the parasite apiece.
Anatomy of Sph. bombi.
FEMALE.
Von Siebold was quite correct in asserting that all the specimens observed by him were females; the males being, as mentioned below, very much smaller in size, and quite different inform and appearance. The full-grown females, as they are met with in May, June, and July, are nearly an inch long, more or less curled up, white in colour, sometimes opaque, sometimes more or less transparent, and of equal thickness from one end to the other, being everywhere about 115th of an inch in diameter. The whole surface is covered with button-like projections, Pl. 1, Fig. 1, from which the very appropriate generic name is derived. These buttons are situated at equal distances from one another, and are of more or less equal size; each one is from 41000ths to 71000ths of an inch in diameter, and the intermediate spaces are a little smaller. There are, therefore, 10 longitudinal, and about 80 transverse rows, making, in all, about 800 of these projections; and each of them projects from 41000ths to 61000ths of an inch above the general surface of the body.
Generally these spherules are nearly as transparent as the rest of the skin; here and there, however, some of them are rendered quite opaque by the presence of innumerable, minute, greenish, elliptic bodies, each about 15200th of an inch in length, by 110000th of an inch in breadth. These darkened spherules are comparatively few in number, only one here and there being affected in this manner, except round the vulva, where from eleven to fourteen were generally in this condition. No other Nematoid worms have wart-like projections so much developed; many species, however, have, on particular parts, and especially in the male sex, buttons, much less than, but doubtless homologous with, those which are so much developed in Sphærularia, and have suggested for it a name so characteristic. Léon Dufour and Siebold considered the Sphærulari from the different sorts of humble bees, as belonging to one species; and all the specimens which have come under my notice have been very similar to one another, and have presented no differences of specific value. One specimen, however, was a little narrower than the rest, and more transparent; the buttons, also, were smaller than usual, and the body tapered a little towards the end which contains the vulva.
In turning to the internal anatomy, one can, with reference to some highly important organs, and systems of organs, only parody Van Troil's celebrated chapter on the snakes in Iceland, and say simply that there are in Sphærularia, no muscles, no nervous or circulatory systems, and no intestinal canal.
A priori it would seem almost impossible that an animal could exist without these organs. Muscles, however, would be useless, or even destructive. So long as the Sphærularia remains quiet, the Bee does not seem incommoded by its presence, which perhaps produces scarcely any abnormal sensations; but if the parasite, being so large in proportion to its victim, were to move about, it would probably so affect and disarrange the viscera of the Bee, that the poor insect would be quite unable to pursue its usual avocations, and would quickly perish. The female Sphærularia being thus, when full-grown, reduced to a merely vegetative existence, the nerves of motion and of sensation must, of course, be useless, and would soon become atrophied. Under these circumstances, however, it might have been expected that the digestive organs and their nerves would have been highly developed. That, on the contrary, these organs are also absent, is probably to be explained by the fact that the animal is bathed on all sides by the blood of the bee, and thus lives in a medium which is highly organized, and requires, probably, scarcely any further elaboration.
Moreover, although this absence of certain important parts is carried to an extreme in the present animal, we find in other Nematoids a considerable approach to the same condition. Indeed, until within the last few years, we had scarcely any reliable knowledge of the nervous system in any of the Nematoids; lately, however, it has been figured and described at length in several genera, as, for instance, in Strongylus, Ascaris, Oxyuris, Gordius, and Mermis; but even in Van Beneden's Prize Memoir, "Sur les Vers Intestinaux," the nervous system is scarcely so much as mentioned; and it seems very doubtful whether the filaments referred to by Meissner in Mermis as nerves, do not rather belong to the muscular system; while the so-called supra-œsophageal ganglion is asserted by Schneider to be really the œsophagus.
In the Nematoids generally the intestinal canal is a straight tube, reaching from one end of the body to the other. In Mermis and Gordius,[2] however, we meet with a totally different and very abnormal type, which it is unnecessary for me here to describe. It is sufficient to say that, whereas in these two genera there is no stomach, and that, while in Mermis the œsophagus is small, and in Gordius quite rudimentary, I have in the mature female Sphærularia been unable to detect any trace of them at all.
The same is the case with the muscular system. I have often opened the body along one side, and then stretched out the skin. In this manner it may be examined with a high power; but I have never been able to see any structure in the least like muscular filaments. The entire absence of motion confirms this view.
In fact, the interior of the body is wholly occupied by two relatively enormous organs—the double series of secretory cells, and the ovary.
The former of these extends in a straight line from one end of the body to the other, being attached at the extremities, but otherwise lying loose in the interior. The cells lie side by side, and thus form a double series. Some of them are very large indeed, being even as much as 111th of an inch in length by 133rd in breadth. Others, however, are not above half as long, though they do not differ much in width. They are not arranged with any regularity as to size, so that often a long one lies by a short one, in which case, however, there is no gap; but the series becomes more or less alternate, until, perhaps, another difference brings each two cells again nearly opposite to one another.
Each of the large cells contains a thick fluid, and about seven or eight transparent nuclei, which are of tolerably even size, and about 81000th of an inch in diameter.
A very similar organ to this has been described by Meissner in Mermis albicans, where also it consists of a double series of large cells, with nuclei. The large cells, however, are full of oil globules, and the nuclei contain crystals. In Mermis nigrescens and in Gordius the fat-body consists of a large number of much smaller cells. In the last-named genera this fat body is continuous, with a very short œsophagus; and I have therefore examined the two ends of it, to see whether the same was true for Sphærularia. I never, however, found anything in the least like the narrow œsophagus and peculiar stomachal sacs of Mermis, nor the small mouth and short œsophagus of Gordius. It seems, however, that this corpus adiposum must be considered as homologous with the intestine of the Nematodes, although no central cavity has been formed in it.
The ovary is about four inches and a half in length ; it commences near one end of the body, as a fine tube about 11000th of an inch in diameter, and gradually increases to about 1120th, after which it slightly diminishes, then again expands into an uterus 140th in diameter, and then finally contracts to about 190th, and opens externally at the extremity of the other end of the body. It lies perfectly free in the general cavity, but near the vulva is connected with the large fat-cells. The female generative organs of Sphærularia differ therefore considerably from those of Mermis and Gordius, both of which have a double ovary connected with the vulva by a short oviduct.
According to Claparède,[3] in all Nematoidea, the Purkinjean vesicle is the first-formed part of the egg:—
"Il parait certain," he says, "que chez tous les Nematoides la vésicule germinative est l'élément primaire de l'œuf. Le blastogene n'ayant chez l'Ascaris mucronata qu'une largeur d'environ m, 013, ne peut comprendre plus que deux vésicules germinatives dans sa largeur. Ces vesicules s'entourent d'une mince couche d'une substance glutineuse et incolore. C'est là le premier rudiment du vitellus. Nous n'avons pas rencontré d'individus chez lesquels les œufs eussent atteint un developpement plus considérable."
Leuokart, also, expresses himself in a very similar manner. Meissner, however, as is well known, has given a different, and very remarkable acconnt of the development of the eggs in Mermis. According to him, the eggs commence as a cell with a nucleus; the nucleus divides, and the new nuclei become the germinal vesicles, while the old cell-wall is gradually produced into follicles, into each one of which a germinal vesicle enters. Finally, the follicles are, by gradual constriction, separated from one another; and in this manner a whole festoon of eggs, besides several abortive follicles, originate directly from the modification of a single cell.
Sphærularia offers so many points of agreement with Mermis, that the development of the eggs naturally became specially interesting; and although my observations are very incomplete, I can ac least say, that, if the account given by Meissner is correct, there is in this respect, at least, no similarity between the two genera.
At the extreme end of the ovary I found a large cell with a nucleus. Following this cell are a great number of small vesicles, which much resemble true nucleated cells. They occupy the whole cavity of the ovary, and each of them is about 15000 of an inch in diameter. These are at first transparent, but gradually become more and more opaque on their inner side, from the deposition of minute yolk globules. The Purkinjean vesicle is also distinctly visible, but I could see no macula. As the ovary widens, the eggs gradually become wedge-shaped, the outer, larger portion remaining clear, so that in this part of the ovary there is a transparent border, with an opaque central axis. This axis, which is known under the name of "rachis," becomes gradually smaller and smaller, being absorbed into the growing egg, which becomes more and more opaque, and assumes a round shape, the Purkinjean vesicle remaining for some time visible in it, and containing a single macula.
When, however, it has entered the wide part of the tube, which we may probably call the uterus, it has again become elongated, and has lost the Purkinjean vesicle, and the yolk has begun to undergo segmentation. Pl. I., f 11, represents a very common state of the egg at the beginning of this process: the first two yolk-spheres, each with its nucleus, lie at the two extremities of the egg; and the central part is occupied by a mass of yolk, divided into an uncertain number of irregular masses, which however contain no nuclei, and are not regular spheres of segmentation. Farther down the uterus we find eggs in all stages of segmentation (Pl. I., f. 13); and in several instances I could distinctly see the nucleus dividing, as in Pl. I., f. 12, in preparation for the next division of the yolk. The segmentation is already far advanced when the egg is laid, but I never found in the uterus any eggs with a fully developed embryo.
I noticed a few specimens in which all the eggs near the vulva were broken up into irregular masses, and in one specimen this was even carried so far, that it began when the eggs were only about half grown. In normal eggs, the development of the young takes place in the manner usual among Nematoids.
The young animals are born soon after the eggs are laid. They are about 160th of an inch in length, and 12500th in diameter at the broadest part. They are very active; the skin has the appearance of being ringed. The head is pointed; the tail ends more abruptly, and makes a sudden curve. The anterior end of the body is transparent; but the rest is darkened by minute, round, strongly-refracting globules. As soon as the Humble Bees come out in spring, young Sphærulari may be found together with old ones, in some of them. I have met with them from the beginning of May till the middle of July, and the whole abdominal cavity of the humble bee often swarms with these little worms. In order to ascertain roughly what the number might be, I washed out the inside of a bee, and then collected all the young Sphærulari together. I then put them into a measuring bottle, and after shaking up, poured away half of the contents. Repeating this process, until only about a hundred were left, it was easy to calculate what the number must have been, if half had been removed a given number of times, though, of course, no great accuracy was thus obtainable. I repeated this experiment five times, and thence concluded that one specimen contained about fifty thousand young Sphærulari, three about sixty thousand, and one even over a hundred thousand! It seems almost inconceivable that a bee should live with such an immense number of parasites in its body; and still more so, that it should, meanwhile, go about its daily duties as if nothing was the matter.
These experiments, however, give but a faint idea of the number of young to which a single female Sphærularia might give birth. In every case the whole ovary was full of eggs, in various stages of development; and, considering the minuteness of the eggs, and the size of the ovary, the number present must be enormous. If the young worms can in any manner leave the bee without destroying it, there seems no reason why nearly all of these should not succesively come to maturity, and be hatched; but, even supposing that this is not the case, and that in the preceding experiment I have ascertained the greatest, or nearly the greatest number of young Sphærulari which can be produced in a single bee, still the chances against any one of them attaining to maturity must be very great; for it is evident that if the sexes of a given species are equal in number, and if the species is neither increasing nor diminishing, the chances against any given young one attaining to maturity may be obtained by halving the average number of young ones produced by each female.
It would seem, at first sight, that the history of the young Sphærularia was very simple. We might suppose that the infected bees would die in their nests; and that the young worms would then leave them, and immediately eat their way into other bees. This view would also be supported by the fact, that, at least as far as my experience goes, each infected bee contains, on an average, five or six Sphærulari. Two reasons, however, inconsistent though they may appear, militate against this supposition. The first is, that too large a proportion of the young Sphærulari would live; and the second is, that the whole race must soon perish. For, if their history were so simple, there seems no reason why a large proportion of young might not survive; and the species would then continually increase in numbers, which is impossible. This argument is, however, far from conclusive, because the increase may be prevented by disease, or by some enemy. On the other hand, there would, under this theory, be no means by which the parasites could pass from bees of one nest to those of another; so that in each species we should have one race infested by Sphærulari, and another free from them; in which case, it can hardly be doubted that the former race would, in the struggle for existence, gradually be supplanted by the latter, and thus, in time, the Sphærulari would all perish.
That the young Sphærulari can live some time after leaving the body of the bee, and without entering any other animal, I ascertained satisfactorily. On the 25th of last May I took some from the body of a B. lucorum and put them in water, where some of them remained alive until the 9th of August, though, during the latter part of the time, they were far from lively. In this case, therefore, they lived in water for more than ten weeks. Whether they would have lived as long in damp earth, I cannot say, but it seems not improbable; and as we know that humble bees often crawl about on the earth under leaves and grass, they may, in this manner, give the young Sphærulari an opportunity of entering them. I tried to solve this question, by wetting humble bees with water containing young Sphærularias; but, partly owing to the difficulty of keeping these insects in confinement alive for more than a few days, and partly, perhaps, from the difficulty of detecting a single young worm in the abdomen of a bee, my experiments were quite unsuccessful.[4]
I had hoped to have thrown some light upon this question, and also upon the metamorphosis, by obtaining some specimens in autumn and winter. Up to the present time, however, I have only found them in May, June, and July. This is partly, perhaps, owing to the fact, that large females are most easily obtainable in these months; and it is unlucky for me that the last two years have been very unfavourable to bees—1860, indeed, so much so, that it is said (Zoologist, September, 1860), to have been the worst year for Hynmoptera since 1828.
I have, however, examined eight large females of B. lucorum in August, and three in October; two of B. terrestris in August, two in September, and two in October; if, therefore, at this season, the Sphærulari were as numerous and as large as in spring and summer, I should almost certainly have found some. If, on the other hand, they were quite small, they may easily have been overlooked.
From all these facts, I am inclined to think that humble bees, when infested with Sphærularia, live for a while as if nothing were the matter; and that only when the young Sphærulari, or the majority of them, are hatched, the parasites appropriate to themselves so much of the nourishment belonging to the bee, that the latter becomes seriously incommoded by their presence. As from the misappropriation of its blood the bee became weaker and weaker, it would, probably, feeling its end approaching, crawl into some long grass, or other place of concealment.
As soon as the bee was dead, the young Sphærulari probably work their way out of it, and immediately begin to look out for a new victim. Those who are so fortunate as to meet with a large female, or queen, may enter it, as young Gordii have been seen to enter other insects, but do not, in all probability, increase much in size at first. This I infer, firstly, because I have not found Sphærulari in autumn, but principally because they would in this case be much less injurious to the bee than if they immediately began to increase in size. When the spring commences, the female Sphærularia probably begins to grow rapidly, and soon lays eggs. I am inclined to think that young Sphærulari also occur in workers, and that I have overlooked them on account of their minuteness; since there seems no reason to suppose that the young Sphærulari have sufficient intelligence to distinguish Queen Bees from workers, or even from other insects.
M. Fabre, who has so graphically described (Ann. des Sc. Nat, 1858) the extraordinary series of adventures through which the young of Meloë attain to maturity, found that, though their only chance of life was to attach themselves to Anthophora, or to its parasites Melectes and Cælioxys, yet they were equally ready to spring on other insects, or even on pieces of straw, if brought within reach, though, curiously enough, they seem, according to M. Fabre, to gain wisdom by experience, and not to be so easily duped a second time:—
"Il est vrai," says M. Fabre, "qu'arrivés sur ces objets inanimés, ils reconnaissent bientot qu'ils ont fait fausse route, ce que l'on voit aisément à leurs marches et contremarches désesperées, et a leur tendance à revenir sur la fleur s'ils en est encore temps. Ceux qui se sont aussi jetés étourdiment sur un bout de paille, et qu'on laisse retourner sur la fleur, se reprennent diflicilement au même piége. Il y a done aussi pour ces points vivants une mémoire, une experience des choses!"
I have not myself bad any opportunity of repeating these experiments; but some months ago, being on a geological excursion in a sandpit, where there were few, if any flowers, I was surprised to see on a herbaceous plant several yellow flowers with which I was quite unacquainted. On gathering one or two, however, my surprise was increased, when the supposed flower broke up, and ran away, turning out to consist entirely of small, yellow larvæ. Unfortunately I had neither bottle nor pillbox with me, and was unable to carry any specimens home; but it occurred to me at the time that they were young Meloës; and that, in the absence of any flowers near them, they had in this manner attempted to supply the deficiency. I was certainly completely taken in; and as I think that my eyes are better than those of most bees, I have little doubt that they also would have fallen into the trap.
MALE.
All the specimens met with by V. Siebold, and all the large ones which I have seen, were females. I observed, however, in the second specimen which came under my notice, that there was a small nematoid worm attached to the large female, Pl. 1, f. 1, A, near to the end in which lies the free extremity of the ovary. This minute worm was apparently overlooked both by Léon Dufour and V. Siebold; or, if they saw it at all, they probably mistook it for one of the ordinary young ones. It is always, however, in very close connexion with the female, the skins of the two being firmly attached to one another; and, if the small worm is torn away, there is a sort of rent at the spot where the attachment takes place. On the other hand, we know that in many nematoid worms the male is much smaller than the female, and the two are, during copulation, closely connected together; in Syngamus trachealis, indeed, this is so much the case, that the pair have been mistaken for a single animal. Moreover, although the small attached worm in Sphærularia is not altogether exactly like the ordinary young ones, still, in size and general appearance, it remarkably resembles them; and, lastly, unless we may regard it as being the male, that sex is, as yet, entirely unknown. Although, therefore, I have not been able to distinguish any generative organs, or trace of spermatozoa, I think that I am justified in considering that in Sphærularia the male is far smaller than the female, and that the two are fastened together in a certain definite manner and position. The shape of tail is also quite different from that of the larva; in Pl. 1, f. 6, I have represented one of the young worms; and in f. 7, one of the attached specimens; and it will be seen that the tail is quite dissimilar, being straighter, and more pointed in the latter. Of what nature, then, is this minute worm, and what are its relations to the large female Sphærularia? Three possibilities only occur to me, viz.: that it might be the larva, a parasite, or the male.
There is, however, no instance in the Nematoidea of any such mode of metamorphosis; and the little creature, though quite motionless, looks too fresh and transparent to be merely the shrivelled-up skin of the young. The difference of shape just alluded to, also, militates against this view, which is, I think, quite untenable.
Nor is the parasitism of the little creature a more probable supposition. In the first place, the almost, if not quite, invariable presence of the little worm speaks against it; and, secondly, the mode of its attachment is almost equally conclusive, as no Nematoid worms are external parasites.[5] Moreover, it is evident that this little worm must perish at the same time as, or soon after, the Sphærularia, and it is equally clear that in the month of July this latter has not long to live; if, therefore, the small worm was a different species, we ought to see in it eggs in course of development, which, however, I have never found to be the case.
The extraordinary disproportion in size between the sexes, though an extreme case, is not entirely without analogy in the animal kingdom. Nordmann first ("Micrographische Beitrage," Pt. 2—see also Huxley's Lectures, "Medical Times and Gazette," August 22nd, 1857, p. 187), discovered that in certain Crustacea the males are much smaller than the females. This is the case, principally, in the genera Actheres, Brachiella, Chondracanthus, and Anchorella, in which the minute male may generally be found attached to the female in the neighbourhood of the vulva. The minute and "complemental" males, discovered by Mr. Darwin in the genera Scalpellum and Ibla, afford cases in point from among the Cirripedia.
In spite, however, of analogies pointing in the same direction, one cannot but be astounded at the existence of a species in which, as in the present, the male is more than twenty-eight thousand times smaller than the female, which, if we may so say, belongs to, it.
I was not able very satisfactorily to ascertain the manner in which the two are fastened together; but it seemed as if the large worm had a small sac-like depression of the skin, Pl. 1, f. 14, into which a corresponding projection of the small one closely fitted. The inner contents of the body passed into the projection, but I could not perceive any penis or spermatozoa, nor was the ovary of the female connected with the place of attachment. The two creatures adhere together more closely than this condition, taken by itself, could account for; and, as in the somewhat similar case of Syngamus the union is effected by the presence of a sort of cement, it was natural to suppose that the same might be the case here. Neither Mr. Busk, however, who was kind enough to look at the junction, nor I, could see any trace of cement; and it is evident, therefore, that if the two skins are not continuous, they are, at least, perhaps by long contact, very closely united to one another.
Considering all these facts, there seems every probability that in this little creature we have the male Sphærularia; but until the Spermatozoa and the transformations are known to us, the fact cannot be regarded as being conclusively established.
It only remains for us to consider the natural position and affinities of Sphærularia, though it will not be possible to come to any satisfactory conclusion until we know more of the anatomy and development of the young. It is, of course, evident that Leon Dufour was right in placing it among the Nematodes; but when that order was limited by the separation of the Gordiacei, it is not so clear that it is correct to leave Sphærularia in its former position. The principal differences between the two orders (Siebold, "Anat. comp.," t. i., p. 113), as given by Siebold, are that the true Nematodes possess an anus and an organ for copulation, while in Gordiacei the one is always, and the other sometimes, wanting. According to both these characters, Sphærularia would belong to the latter order, in which, accordingly, it is correctly classed by Diesing and Meissner, although V. Siebold, Rudolphi, Owen, and other helminthologists, class it with the true Nematodes. The absence of an organ is, however, not generally so important a character as its structure: thus, for instance, we see in insects, that the absence of wings is less significant than are the differences in their structure, so that we have wingless representatives of all the large orders. The characters which induce us to separate Sphæralaria, Gordius, and Mermis, from the Nematodes being principally negative, are not to my mind quite satisfactory. Schneider differs also so much from Meissner as to the anatomy of Gordius and Mermis, that it will be necessary to say a few words on this subject, before considering the affinities of Sphærularia. The so-called supra-œsophageal ganglion of Mermis he denies to be a portion of the nervous system at all, and considers it rather to be an œsophageal bulb, homologous with what is found in many Nematodes. If this be granted, the principal argument in favour of the nervous nature of the so-called peripheral nerve-system falls to the ground, and with it one of the principal differences between Sphærularia and Mermis. While, however, we know nothing about the nervous system of Sphærularia, and are in such a state of uncertainty as to that of Gordius and Mermis, it is evident that we cannot avail ourselves of it for the purposes of classification. Meissner's extraordinary account of the digestive organs in Mermis is well known. According to him, the œsophagus is open along one side, thus constituting a trough rather than a tube, which sends out from time to time lateral branches, each of which terminates in a spherical cavity, which he calls a stomach-cell. According to Schneider, however, the œsophagus is a closed tube, and the "stomach-cell" is only a round, firm body, containing a nucleated structure, but without any central cavity, or any communication with the fat-body. This "fat-body" is probably homologous with the intestine of ordinary Nematodes, but no cavity has been developed in it; and while Meissner describes thirty connecting tubes between it and the œsophagus, Schneider denies that any such junction takes place; the two organs lie side by side, but have no communication with one another.
Schneider exemplifies this by the case of Ascaris rigida, R., in which the œsophagus opens, not at the anterior end, but at the side of the intestine. If, he says, this condition were exaggerated, and the lateral connexion removed, we should have exactly the case of Mermis. In Mermis albicans the fat-body consists of two rows of large cells, as in Sphærularia; but in Gordius the cells are much smaller and more numerous, still, however, solidly filling the tube; while in Mermis nigrescens the cells are smaller, and only clothe the outer tube, and leave a large central cavity; thus completing the series, and giving us a most interesting gradation, connecting the corpus adiposum of Sphærularia with the ordinary intestine of any common Nematode. This corpus adiposum, therefore, is homologous, not with the whole intestinal canal of Nematodes, but only with the intestine; and we find, in fact, that in Gordius the œsophagus is very short, and opens at once into the anterior end of the corpus adiposum; so that, to pass from this genus to Sphærularia, it would only be necessary to shorten the œsophagus a little more, and then the wall of the corpus adiposum would be immediately attached to that of the body. So far, therefore, as concerns the corpus adiposum and the œsophagus, Sphærularia agrees neither with Gordius nor Mermis, nor indeed with one more than the other, since, if it agrees with Mermis albicans in the double series of large fat-cells, it has no œsophagus, and in this respect more nearly resembles Gordius.
Sphærularia agrees with Gordius in the possession of a terminal vulva, but differs both from that genus and from Mermis in having only a single ovary. As regards the development of the young, Sphærularia resembles Gordius in undergoing a metamorphosis; but with this remarkable difference, that whilst the former begins with the filiform or Nematoid condition, the latter ends with it. Mermis, on the other hand, undergoes no metamorphosis; in all stages of development this worm, like the embryo of Sphærularia and the adult of Gordius, is filiform and Nematoid; so that we may say of the three genera, that the metamorphosis is progressive in Gordius, absent in Mermis, and retrogressive in Sphærularia.
On the whole, it is, I think, evident that Sphærularia constitutes a group equivalent to Gordius or Mermis, and indeed farther removed from them than they are from one another. Omitting, then, those points as to which, from the imperfection of our knowledge, no conclusions are at present attainable, we shall get the following as the principal characteristics of the three genera:—
Mermis albicans.—Skin partially covered with papillæ. Œsophagus long, contained in an outer tube, within the outer membrane of which is a series of nuclei, at sub-equal distances. No intestine or anus. Organs of excretion three in number, and occupying the ventral and lateral lines of the body. Ovary double; vulva opening at the middle of the body. No metamorphosis, the young being filiform. Males of moderate size, free. Spiculæ two in number.
Gordius.—Skin smooth, or in part provided with short spines. Œsophagus very short. Corpus adiposum containing several series of cells. No intestine nor anus. Ovary double; vulva terminal. Metamorphosis progressive, the young not being filiform. Males of moderate size, free. No spiculæ.
Sphærularia.—Skin covered with spherules. Œsophagus wanting. Corpus adiposum consisting of a double series of large cells. No intestine nor anus. Ovary single ; vulva terminal. Metamorphosis retrogressive, the young being filiform. Males very minute, attached to the females. No spiculæ.
Thus, then, we see that Gordius and Mermis differ very materially from one another, while Sphærularia departs even more from the common type, and indeed agrees with the other two in little except the absence of an anus, and the very peculiar corpus adiposum. M. Schneider is therefore, no doubt, right in proposing to divide the Gordiaceæ into two families, to which we must now add a third—Sphærulariaceæ, for the genus Sphærularia. For the present we must leave the characters of these families to be the same as those of the genera. The arrangement of the cells constituting the corpus adiposum, the relative size of the males, and the position of the vulva, will probably, however, be found somewhat variable, and are perhaps characters of not more than generic importance; in which case the arrangement of the nervous system, the presence or absence of an œsophagus, the presence of a single or double ovary, and the development of the young, will, with the absence of the anus, remain as the principal family characters.
I shall endeavour to get some humble bees in the course of this winter, in order if possible to determine some of the many points which yet remain to be ascertained; and I should feel very grateful to any one who would send me even a single specimen of any species of Bombus between the months of December and April. In the meantime, Sphærularia still remains, as it was when Diesing wrote the "Systema Helminthum," a "genus inquirendum."
DESCRIPTION OF PLATE I.
1. Sphærularia bombi x 15. A. Small male.
2. Part of corpus adiposum x 10.
3. Free end of ovary x 250.
4. Two young eggs with rachis x 250.
5. Portion of ovary x 250.
6. Outline of young x 60.
7. Outline of male (?) x 60.
8. Head of male x 60.
9. Tail of do. do.
10. Ovary x
11. Egg, showing the commencement of segmentation, x 250.
12. Do., in a more advanced stage, x 60.
13. Young egg, still more advanced, x 250.
14. Place of union of male and female x 250. a. Part of the body of male. b. Part of skin of female, c. Projection of male fitting into sac-like depression of female.
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- ↑ Léon Dufour, Ann. Sc. Nat. 1836, 2nd Ser., vol. vii.
- ↑ The intestinal canal is quite short also in some other worms, as, for instance, in Nemertes.
- ↑ De la Formation et de la Fecondation des Œufs chez les Vers Nématodes, p. 38.
- ↑ I found the best plan was to put the bees in a glass with moist sugar. They seemed also to live longer if put in the dark, probably from the soothing effect upon their nerves. In this manner I kept one bee alive for more than a month.
- ↑ It might, however, be said, that as this law arises from the necessity that the external surface should be bathed by animal fluids, the present case might be an exception caused by the fact that the little worm, though external to the female Sphærularia, was internal as regards the Bee.