Peculiarities in the Segmentation of certain Polychætes

Amongst the Polychætes collected by Professor Haddon in the Torres Straits, which have been for some time, and still are, in my hands for the purpose of identification, there are sixteen specimens of the genus Amphinome sufficiently well preserved and complete for examination with regard to the point at issue. Of these fourteen belong to the sub-genus Eurythoe, and six of these have each a half-segment completely or incompletely intercalated. The position of the intercalated segments will be seen from the following table:

The other two members of the genus in the collection belong to the sub-genus Linopherus, and of these one is regularly segmented, while the other has two and a half of its segments arranged as a spiral, beginning on the ventral and ending on the dorsal surface.

The spiral is therefore a right-handed one as viewed from the dorsal surface. This will be understood by reference to fig. 1 (Pl. XLII), although in the specimen there represented the spiral is half a turn longer.

In another collection of Polychætes from shallow water off Ramesvaram Island in the Gulf of Mauaar, sent to me for identification by Mr. Thurston, of the Madras Central Museum, there are seventeen specimens ofAmphinome (all belonging to the sub-genus Eurythoe), of which two have intercalated halfsegments, and six (one of them the same as one of the two with intercalated half-segments) have spirals of varying lengths. The position of both these variations with regard to the whole animal in the different specimens will be seen from the following table:

In the spirit collections of the British Museum there are fifty specimens belonging to the genus Amphinome well enough preserved for one to be certain whether or not the segmentation is regular and bilaterally symmetrical. Of these, twentyseven present variations in symmetry of one kind or another. It is the presence of another species of Amphinome (A. [s. str.] rostrata, Pall.), of which there are no specimens in the other two collections, which raises the proportion so greatly. Of this species there are seventeen specimens, of which only one is regularly segmented throughout. The other sixteen all have their segmentation irregular towards the posterior extremity of the body, and the irregularity may take the form either of a spiral or of intercalation of half-segments, but more frequently a condition obtains which would seem to be intermediate between regular and spiral segmentation,—that is to say, the furrows which separate the segments form short spirals instead of circles; but, as they are not continuous with one another, the segments themselves do not take the form of a spiral. The arrangement will be best understood by a glance at fig. 4, where I have represented five successive segments whose intervening furrows do not meet on the dorsal surface. Sometimes it is on the ventral surface that they do not meet, and sometimes they do not meet on either in which case the furrows form a number of half-rounds of spirals. Very frequently there are one or more complete segments in the midst of a succession of incomplete ones. These and other details are shown in the following table:

No. 16 of the above list deserves especial mention. Unfortunately it is broken posteriorly, and so near to the critical point that it is difficult to be quite certain of what one sees. I have, however, represented what I saw, to the best of my ability, in fig. 5. It will be seen, by reference thereto, that the forty-ninth and fiftieth segments on the left side have no dorsal setae nor branchiae, and that on the right side between those two segments there are three other segments (a, b, and c) intercalated. I think there is. a fourth segment intercalated on the right side, but of this I cannot be quite certain, as I could only see it on the dorsal surface. At any rate, the result is to make the posterior extremity of the body, as much as there is of it, coil bodily to the left.

Of the other thirty-three Amphinomes in the British Museum collection, twenty-six belong to the sub-genus Eury-thoë, seven to the sub-genus Her mod ice. Of these ten (? eleven), nine Eurythoes and one (? two) Hermodices present irregularities in segmentation, the nature of which will be seen from the following table:

Of these, No. 24 is the most remarkable. The irregularly segmented part is represented in fig. 3. From this figure and the key attached (3 B) it will be seen that there is one spiral of two coils beginning in the middle of and intertwined with another of seven.

With regard to the internal organs: wishing to preserve one intact specimen of each kind of spiral, 1 have only been able to cut sections of two of the specimens at my disposal (namely, Nos. 5 and 6 from the Madras collection). The condition in which the specimens were was not such as to guarantee any results from dissection, and horizontal sections seemed, therefore, the best method of examination. The sections of one of these specimens, No. 6, were worth very little, as the animal was rather broken in the twenty-first segment, and consequently only the anterior part of the spiral could be cut. From the other specimen, of which I cut sections, No. 5, all I have been able to make out is that the nephridia, or at least renal organs of some sort (for, in the state of preservation the specimen was in, all one could see of a renal organ was a solid mass of tissue containing yellowish concretions, and a short duct leading to the exterior), occur regularly one in each halfsegment; that the blood-vessels spring very irregularly from the dorsal vessel and form plexuses in the segments on each side of the alimentary canal, and a sinus round the alimentary canal itself. They seem to be equally irregular in segments which are regular externally, but in both cases injury to the tissues generally, owing to the state of preservation,¹ may have caused the blood-vessels to break, and thus assume an irregularity when put together from sections. The nerves go out in this specimen quite regularly in pairs; one pair to both the thirty-second parapodia, one to both the thirty-third, and one to both the thirty-fourth. The septa are throughout the body incomplete, but as far as they do project into the body-cavity they seem to follow the external boundaries of the segments. I hope, in the course of time, to obtain specimens which will enable me to come to more definite conclusions with regard to the internal organs in the different varieties of spirals. It would also be very interesting to examine specimens of A. rostrata internally, and so get different intermediate conditions.

There are but few other genera in the family Amphinomidæ, and there are none of them very common. There is, however, one other genus at least in which irregularity in segmentation occurs, though I have only noticed it at present in one specimen. This is the genus Chloeia, of which I suppose I have seen about twelve specimens altogether, and the one specimen I refer to is in the possession of the Royal College of Surgeons’ Museum. It has thirty-five segments, and of these the halves of each of the last eight are not opposite one another on the ventral surface, so that a condition somewhat similar to that in most specimens of Amphinome rostrata obtains.

I had not noticed spiral segmentation in any other family of Chætopods before the publication of Cori’s paper (except for a doubtful case of it in a Sabellid, which I shall have to refer to later on). Seeing that he found it in such a different group as the earthworms (and apparently it was only in earthworms that he found the spirals), I thought it would be worth while to look through the specimens I had. of other families, and those in the Zoological Museum of this College, paying especial attention to the segmentation. Although I did not look through by any means all the specimens in my charge, I looked through a fairly large number, and I looked through almost all the specimens in the Zoological Museum here. The result was curious: only in two species out of the many examined did I find spiral segmentation again, but in these two species it was more common even than in the genus Amphinome (taking it as a whole); and not only was the proportion of individuals that had this kind of segmentation very great, but also the number of spirals in each individual was most striking.

There are six specimens of Lumbriconereis impatiens, sent from Naples, in the Zoological Museum here. Of these, two (young ones) seem to be regularly segmented throughout; the other four (one young one) have spirals of different lengths, as seen in the following table:

The other species in which the spiral arrangement is so common is one of which Cori also examined a specimen, and he remarks on the large number of so-called abnormalities in it, though he only describes two cases of intercalation. This speciesis Halla Parthenopeia. Of the two specimens of it in the Zoological Museum here (both of them from Naples), one has eleven and the other twelve distinct asymmetrical parts, the nature of which and their relation to the whole length of the animal will be seen in the following table:

The occurrence of right-handed and left-handed spirals not only in the same individual, but very close together in the same individual, is well shown. The three spirals bracketed in the second specimen deserve special mention, as we have here a second case of spiral within spiral. The arrangement is shown in fig. 6, and can easily be understood by reference to its key (fig. 6 B). It will be seen that there are two short spirals of one coil each, both inside a larger one of two coils. I hope in a short time to have specimens of Halla at my disposal for dissection, and feel sure they will yield interesting results.

With regard to other Eunicidae, I have looked through a fairly large number of specimens belonging to the genus Eunice and to the genus Marphysa, a few Diopatras, one very long Arabella (with over nine hundred segments), and two or three other species of the genus Lumbriconereis, but without finding any irregularities in the segmentation.

The only other Polychætes in which I have, so far, noticed what appears to be spiral segmentation is a Sabella. There are in the Madras collection four specimens of what I believe to be a new species of Sabella. The change of setae in three of them takes place between the eighth and ninth segments. In one specimen, however, there are nine pairs instead of eight thoracic parapodia, and the change on the left side takes place distinctly in front of the ninth ventral shield, whereas that on the right takes place distinctly behind it (fig. 8). This is accounted for by the fact that the second ventral shield is divided on the left side, and there is a separate parapodium to each half. Thus in taking a ventral view of the animal we see what appears to be a spiral beginning in the third halfsegment on the ventral surface, left side, and going on into the abdomen to end (?) in the hundred and second segment on the ventral surface, right side, about twelve segments before the end of the body. The reason why one cannot say with absolute certainty that this is a case of spiral segmentation is that the segments are not marked off from one another on the dorsal surface, and one cannot therefore trace them right round the body. But the ventral view of the animal shows exactly the condition which obtains throughout the length of any spiral beginning and ending on the same surface in the other Chætopods. If it is a spiral it is a right-handed one, and remarkable for its length. I cannot be quite certain of its ending in the hundred and second segment, as the segments at the posterior end of the body are extremely narrow and very difficult to follow; but it certainly does not end before, at least not on the ventral surface.

Before leaving the subject I should like to draw attention to the existence of spirals similar to those we have been considering in another group of animals altogether. In a recent number of the ¹ Quarterly Journal of Microscopical Science’ (vol. xxxiv, 1892) Baldwin Spencer, in describing the female of Pentastomum teretiusculum, Baird, says that there are occasionally incomplete segments wedged in on one side or the other between the complete ones. He figures one specimen with two of these so-called wedges, but to me it looks very much as though there were a spiral going four times round the body (i. e. between his two “wedges”). I was confirmed in this opinion by the examination of a specimen of Pentastomum which I happened to have by me (from Professor Haddon’s collection), and which is apparently a female of the same species. In this one specimen there are three spirals to be traced:

Baldwin Spencer’s casual mention of variation in symmetry, and the fact of my finding three spirals at once in the only specimen of Pentastomum available, lead me to infer that spirals and wedges are probably as common, at any rate in this one species of Pentastomum, as in the special Polychætes we have been considering. And it seems to me interesting for the whole theory of segmentation to find such asymmetrical variations occurring almost normally in two very different groups of segmented animals. Cori has already drawn attention to the intercalation of half-segments in the Cestodes (and it would seem that it is especially in Bothriocephalus latus that such variations occur). So that we have here a third group of segmented animals in which variations in symmetry may be common.

With regard to Cori’s suggestions as to the origin of these variations, we have as yet too few data to say anything either for or against them. His statement that the irregularities occur more frequently in the middle of the body than at either end is not confirmed by the specimens I have examined, as reference to the above tables will show. As far as Ehlers’ suggestion is concerned, that such irregularities are the result of regeneration of lost parts, I will mention that 1 have frequently seen Polychætes regenerating heads or tails (and amongst others four or five Amphinomes, one Halla, and one Lumbriconereis impatiens), but in all cases except one the regenerated part was attached quite regularly to the rest of the animal. This of course shows very little, as it may have been that in all these cases the body of the animal had been broken off symmetrically previous to regeneration, and one could only expect to get asymmetry where the body had been broken in a plane not at right angles to the long axis of the body. The one specimen that I have seen asymmetrical at the point of juncture of the regenerated and main portion of the body is the No. 5 of the Australian Amphinomes, where the anterior eleven and a half segments have been regenerated, and the half-segment on the right side is apparently the first of the new segments. It would be interesting to know whether the species or genera in which asymmetrical variation particularly occurs are especially remarkable for their power of regeneration. Even if the intercalation of half-segments were shown to be due to regeneration, it would still remain to be shown that the spiral arrangement is likewise due to it. The fact of spirals occurring in Pentastomum, where, as far as we know, regeneration does not take place, seems to me against this hypothesis. It would be very interesting to make experiments on the living animals of those species in which irregular segmentation is common with regard to regeneration of lost parts, and also to watch their development.

In conclusion, I should like to thank Professor Weldon, in whose laboratory I am working, for his kindness in giving advice with regard to this and other work. My thanks are also due to the authorities of the British Museum for kindly giving me access to their collections during the last few years; and to Mr. Stewart, of the Royal College of Surgeons’ Museum, for letting me examine the Polychsetes which are in the possession of the Museum.¹

Illustrating Miss Florence Buchanan’s paper on “Peculiarities in the Segmentation of certain Polychsetes.”

FIG. 1.—Six segments of an Amphinome (Madras Collection, No. 5), of which three form a simple right-handed spiral, beginning and ending on the dorsal surface. Dorsal view. Enlarged.

FIG. 1A.—Ventral view of the same segments.

FIG. 1B.—Diagrammatic representation of the spiral.

FIG. 2.—Six and a half segments of an Amphinome (Madras Collection, No. 6), of which three and a half form a left-handed spiral, beginning on the dorsal and ending on the ventral surface. Dorsal view. Enlarged.

FIG. 2 A.—Ventral view of the same segments.

FIG. 2 B.—Diagrammatic representation of the spiral.

FIG. 3.—Ten segments of an Amphinome (British Museum Collection, No. 24) in which there are two right-handed spirals, one inside the other. Dorsal view. Enlarged.

FIG. 3 A.—Ventral view of the same segments.

FIG. 3 B.—Diagrammatic representation of the spirals. The longer one is represented as though lined with black, the smaller one as though with a shaded lining.

FIG. 4.—Five segments of an Amphinome rostrata (British Museum Collection, No. 3), showing a condition intermediate between normal and spiral segmentation. Dorsal view. Enlarged.

FIG. 4 B.—Diagrammatic representation of the same.

FIG. 5.—Incomplete posterior end of an Amphinome rostrata (British Museum Collection, No. 16), showing segments intercalated on the right side causing the body to coil to the left. Dorsal view. Enlarged.

FIG. 5 A.—Ventral view of the same.

FIG. 6.—Six segments of a Halla Parthenopeia (in U. C. L. Museum, No. 2), showing two small spirals inside one larger one. Enlarged.

FIG. 6 A.—Ventral surface of same.

FIG. 6 B.—Diagrammatic representation of the spirals.

FIG. 7.—Two and a half segments of a Lumbriconereis impatiens (in U. C. L. Museum, No. 1), showing a spiral of half a round. Dorsal and ventral view, a—a_t marks the course of the groove. Enlarged.

FIG. 8.—Ventral view of the anterior region (without the branchire) of a Sabella (probably a new species) from Ramesvaram Island, showing a spiral beginning on the left side in the third half-segment and continued into the abdominal region. Slightly enlarged.

¹In all cases where spiral segmentation is involved the total number of segments means the total number of pairs of parapodia + the segments (if any) at the anterior end without parapodia. In the same way when a spiral is said to begin (or end) on a segment x, it means that it begins (or ends) opposite

the xth parapodium on the side of the body mentioned (or the æth parap. —2 in the case of the Eunicidæ).