Studies in Earthworms, XLII.
The Occurrénce Of The genus Pheretima in New Zealand.
By Sir William Benham, K.B.E., F.R.S.
[Read before the Otago Branch, August 13, 1946; received by the Editor, August 28, 1946.]
The first of this series of my “Studies in Earthworms” was published in 1886 in the “Quarterly Journal of Microscopic Science.” The present is the forty-second.
The fact mentioned in the title does not perhaps mean anything to the general zoologist, but to the student of earthworms it is rather a striking fact, involving a problem of distribution. Some twenty genera of earthworms have been recorded as living in these islands, including the Sub-antarctic Islands, and most of these genera are endemic, but members of the sub-family Megascolecidae are few and related species occur in various widely distant regions.
Now the genus Pheretima—the largest genus of Oligochaeta, with more than 350 species—has its headquarters in the islands of the south-western Pacific and adjacent shores of the Malay Peninsula and extending northward to include Japan. It is true that a number of species are “peregrine” and have been recorded from a variety of separate countries. Some have been evidently carried accidentally by human agency; others have been able to travel owing to former land connections. It is also possible that cocoons containing, as they do, developing young ones may have been carried by currents in the sea or even by birds, when the distance has not been great. We have, however, no definite knowledge as to the affect of sea water on these cocoons: as to whether the chitinous envelope can withstand the salt water or, if so, for what length of time. Michaelsen, whose death has deprived zoologists of the most competent and most experienced worker on the group, with a wider knowledge of the Oligochaeta than any of us, remarks (1903) that the sporadic occurrence of a single species, far removed from its headquarters, must be attributed to human agency.
The subject of the present communication was found in large numbers at Te Araroa, on the East Coast of the North Island. What is the nearest area in which the genus, and especially the species, has been found elsewhere? Where does the most closely related species occur? Has there been commercial or other traffic between the two areas by which this species may have been introduced into the Dominion? A glance at the useful maps showing the distribution of the genus in Michaelsen's work on the geographic distribution (1903) show that two species occur in Australia, one in New South
Wales, the other in Queensland. A discussion, however, on these matters must be postponed until an account of this new species has been given.
In the meantime attention must be directed to the use of the title “Pheretima,” for any zoologist consulting text books or the literature published previously to 1899 would not find this word, for all the species were described as “Perichaeta.” And thereby hangs a tale, such as often is met with in systematic zoology. This title had been bestowed by Schmarda (1861) on certain earthworms which presented an arrangement of chaetae utterly different from that which had been regarded as “typical” and which is familiar in the European worms and those from elsewhere, namely, instead of the eight chaetae, usually in four couples in each segment, this “perichaeta” has numerous chaetae forming a nearly complete circle surrounding each segment. At that date this was a novelty, and Schmarda naturally seized on it as a generic name. But since that time, this condition has been found in various other genera, such as Megascolex (Templeton, 1844), Plagiochaeta (Benham), Diporochaeta (Beddard), Perieodrilus (Michaelsen). But this name “Perichaeta” was retained for the genus until 1899, when Michaelsen examined the types of certain worms described by Kinberg in 1866 under various names, “Pheretima,” “Amyntas,” “Nitocris.” Kinberg had given only very brief accounts, derived from “external features.” Michaelsen examined the internal anatomy and found that under these cognomens were worms previously entitled “Perichaeta.” But the matter is rather more complex owing to the fact that both Beddard and Michaelsen used Kinberg's title “Amyntas,” the former in 1899, the latter in 1900, his article being dated “May.” But in October of that year Michaelsen's valuable “Monograph” was published, and having discovered the annoying fact that Amyntas had been used for a beetle a year before Kinberg had used it for an earthworm, he adopted Kinberg's other title “Pheretima.” And every lumbricologist since that date has adopted the name. But what will happen should some investigator discover that this, too, had been used for some animal in another group? Such are the snares and pitfalls for the systematist in every group of animals. Yet it is virtually impossible for a student of one group or class to be acquainted with every generic name in all the other classes; for example, all the thousand of names in the Class Insecta. Yet we have to do our best to avoid duplication and to abide by the Rules of Nomenclature.
So Kinberg's generic name was resurrected. But one may well ask “Why?” since Schmarda had published his account in 1861. The fact it that it was found that the name “perichaeta” had already been used for a Dipterous Insect in 1853. According to the Rules of Nomenclature, this word cannot be employed for any other animal, however remote in the zoological world.
So we have the following heading:
Perichaeta, Schmarda et auctores numerosi.
Pheretima clerica n.sp.
Species are often named after the country or locality in which they occur: thus “Queenslandica” (Fletcher), “Novae Britannicae” (Benham), “Neoguinensis” (Michaelsen); but in the present case I refrain, for the genus is so foreign to New Zealand that it would be misleading. So I will name it after the discoverer, Mr. J. Clarke, disguised in its Latin form of Clerica.
I received from Mr. J. Clarke, of Raukumara Station, Te Araroa, Matakaoa County, near Gisborne, a large number of this species; some were collected on rolling hilly country and others from his garden. Those from the hills were found amongst the roots of grass, so evidently the worm is a superficial burrower.* They reached me alive and were very active, climbing out of a glass dish whose sides are one and a-half inches high.
All the specimens had a well-developed clitellum, though the internal reproductive organs were not fully developed.
Colour purplish brown, with a narrow lighter ring round the body where the chaetae are embedded. This is especially noticeable in the postclitellar region.
The ventral surface is quite pale; the clitellum is also pale brown or buff. In this there are no chaetae and the limits are well defined, as is usual in this genus.
Unfortunately the pigment is entirely dissolved in formol, so that the worm becomes a dirty white in tint. In alcohol it changes colour to a pale brown without the purplish tinge.
The length is on the average about 140 to 150 mm. with a diameter of 5 mm. as measured while the animal was crawling.
The prostomium is epilobic, embedded to about the middle of the first segment.
The chaetae number 36 in the mid-body and 16 in the anterior segments. The ring is practically complete, the ventral and dorsal gaps being only twice the distance of that separating individual chaetae.
As is usual in the genus, the preclitellar segments are much longer than the postclitellar. In all the specimens of this species the tenth segment is somewhat longer than its neighbours. This fact surprised me, as I had not known such a thing to occur in any other species. I had not found it actually recorded, though I confess that I have not read the account of every one of the 350 species, but the only really good figures that represent this feature are those illustrating Beddard's description of P. alexandri (1900, p. 988) and of P. palauensis (1900, p. 945). Indeed, in these articles alone do we find a real picture of the external characters.
In order to convince myself of the difference in length, I cleared and mounted the body wall of this region and made a drawing with camera lucida, on which the lengths of the segments were measured. In Fig. 2 this drawing is reduced.
[Footnote] * Since writing this I have received more specimens from another area in that neighbourhood: it is spreading.
The lengths of the successive segments are on the camera drawing:—viii, 20 mm.; ix, 23 mm.; x, 35 mm.; xi, 30 mm.; and xii and xiii, 23 mm. each.
The clitellum occupies the usual segments 14–16, is without chaetae and with margins well defined. There are no intersegmental grooves here, but it measures three times the length of the preclitellar segments.
Genital Apertures.—The pore of the oviduct occupies the usual position in the genus, being median on the 14th segment. The male pores are, as usual in the genus, on the 18th segment; they are slitlike, carried on oval raised areas which are separated from each other by eight chaetae (Fig. 3).
On the mesial side of each of these areas there is a small papilla, though whether to regard it as a “puberty or copulatory papilla” I am uncertain. At any rate, none of the individuals had any other papillae.
In this genus many species carry a varied number of such sexual papillae differently arranged in different species and subject in a given species to a good range of variation in number and position.
It may be that the individuals here described are immature.
The spermathecae are four pairs, situated in segments vi, vii, viii, and ix, and, as usual, opening at the anterior margin in each case.
By dissection and bisection I was able to make the following observations:—
The oesophagus has, in the segments xi and xii, thicker walls than elsewhere and is laterally distended, forming apparently glands, though I did not note any calcareous matter in them.
The gizzard lies behind the septum vii/viii and the two following septa are absent. The longish gizzard occupies the position between the two posterior pairs of posterior spermathecae, and thus lies in the segments viii and ix.
The transition from the posterior region of the oesophagus into the intestine is not at all distinct.
A pair of small intestinal caeca lie either in xxvii or xxviii. They are quite simple, and spring from the underside and are thus dfficult to detect in the preserved worm.
Enlarged hearts occur in segments x, xi, xii, and xiii.
The worm is micronephric.
Spermathecae.—Each one consists of a conical ampulla with a long, thick duct of about its own length. The diverticulum is ovoid, with a very long and delicate duct opening into the main duct close to the body wall (Fig. 4).
The testes are in the usual position, and I have been unable to distinguish “testicular sacs” owing to the softness of the tissues. The sperm sacs are two pairs, each of those in segment x is trilobed. those in xi are much larger, of irregular shape and fill the segment.
The prostate is somewhat squarish in shape—an irregular lobulated gland which lies wholly in segment xviii, with a simple curved muscular duct (Fig. 5). There is no enlarged muscular sac such as occurs in some species and contains an eversible penis.
Fig. 1.—External view of the ventral surface of the genital region (× 3). Note the enlarged size of segment X. cl., clitellum; mp., male pore; op., oviducal pore; sp.th., spermathecal pores.
Fig. 2.—Camera lucida outline of the anterior segments showing their relative lengths. Only part of body wall is visible under the microscope.
Fig. 3.—The male pores.
Fig. 4.—A spermatheca. amp., ampulla; div., diverticulum.
Fig. 5.—The prostate.
Now let us try to solve the questions put previously. What is the nearest locality in which the genus has been found? Two species have been recorded from Australia by Fletcher (1886), P. queenslandica, in the north, and P. peregrinia, in Victoria; the latter is regarded as possibly introduced from Mauritius.
Some of the characters by which the numerous species of the genus are differentiated are:—
The number of spermathecae, which varies in different species from one pair up to six pairs, but a good number have four pairs, as does the present species.
The second character lies in the number of chaetae per segment in the mid-body.
A third feature is in the form and structure of the spermatheca.
There are, of course, several other differentiating characters, but let us take these first.
We have, then, to look for those species with four pairs of spermathecae in segments vi, vii, viii, and ix and consider these species inhabiting localities near New Zealand. Both the Australian species agree with this.
Our worm has 36 chaetae in mid-body per segment. P. peregrina has 40–46 and P. queenslandica about 60 per segment.
Size perhaps is useful. Whereas our species is 140 mm. in length, P. peregrina is 130 mm. and queenslandica 150 mm., so this feature is not distinctive. As to the shape of the spermatheca, in the present species the “ampulla” has a long, thick duct as long as the “conical” ampulla, while the diverticulum is ovoid with an even longer narrow duct.
Unfortunately, Fletcher does not give figures of this organ, and a verbal description is rather difficult to interpret. Of P. peregrina he writes: “Each spermatheca consists of an elongated depressed stalked pouch, with a long filiform caecum with a dilated tip, about as long as the main pouch.”
Unfortunately, he does not refer to the length of the main duct, which in the present species is as long as the “ampulla” (or “main pouch”), nor does he state where the duct of the diverticulum (his “caecum”) enters.
Moreover, there is little doubt but that the shape and size of this “caecum” depends to some degree on the sexual season, for after copulation it will be empty, while before that operation it will be filled with spermatozoa and consequently will be enlarged.
As to P. queenslandica, he writes: “Each spermatheca is a stalked pouch, the stalk or duct about half as long as the distal dilated portion, and giving off a very short and rudimentary caecum from its upper part.” Our species evidently differs from this, for the duct of the diverticulum opens into that of the ampulla close to the body wall, that is, in the “lower” part.
We may then neglect this species and consider only the other Australian form. Are there any other differences? There are one or two other points of differences. Thus he has not mentioned the enlargement of the tenth segment in his species; the absence in the New Zealand worm of “copulatory papillae” may be due to inimaturity.
There is but a single median ovipore, whereas in peregrina the “two are close together.”
Another difference from P. peregrina is that there are “about 14 setae between the two male pores,” whereas in the present species there are only 8. Such differences seem rather small, but there are sundry little divergences in the internal structures. It is, of course, the accumulation of small differences that serve to distinguish species.
Further afield we meet with four pairs of spermathecae in two species from New Britain described by me in 1897, namely, P. novae britannicae, and Beddard records P. pacifica (1899).
P. novae britannicae has an average length of 110 mm.; the condition of the male pores is very different. There are many more chaetae in the segment xxvi, to wit, 72, while only four chaetae lie between the two pores. These are enough to distinguish the two species.
In P. pacifica there are 40 chaetae on segment xvii, 10 between the male pores, large puberty papillae. The worm measures only 80 mm. in length.
Similar differences, and more extensive ones, serve to distinguish the many other species having four pairs of speramtheca, so that I am justified in making the present worm new, for neither Michaelsen nor Ude (1932) find similar conditions in the many species studied by them. Eight species have been described by Michaelsen (1913) as occurring in New Caledonia and neighbouring islands, but as none of them have four pairs of spermathecae, they have no relation to our species.
Beddard, F. E., 1899. In Willey. Zool. Results, II, p. 190.
—— 1900. “Revision of the Genus Amyntas.” Proc. Zool. Soc., p. 609.
—— 1900. “Earthworms Collected During the Skeat Expedition in the Malay Peninsula.” Loc. cit., p. 891.
—— 1900. A New Species Belonging to the Genus Amyntas from India. Loc. cit., p. 998.
Benham, W. B., 1897. Linn Soc. Journ., xxvi, p. 201.
Fletcher, J. J., 1886. Notes on Australian Earthworms.” Proc. Linn. Soc. N.S.W., Vol. 8.
Kinberg, J. G. H., 1866. “Annulata nova.” Kongl. Vetenskap. Alad. Forhandlingar.
Michaelsen, W., 1900. Oligochaeta (Monograph).
—— 1903. Die Geograph. Verbreitung v. Oligochaeten.
—— 1899. Revision d. Kinbergischen Oligochaet. types, Ofversigt. Kongl. Vetenskap. Akad. forhandlingar.
—— 1899. “Terricolen v. verschiedenem Gebieten.” Mitt. a.d. Naturhist. Mus. Hamburg, Vol. xvi.
Schmarda, L. K., 1861. Neue Wirbellose Thiere.