Darwin has also pointed out the benefits to the agriculturist which accrue from the industry of these Annelids. The soil is thoroughly mixed and submitted to the action of the atmosphere. The secretions of the worms themselves cannot but have a good effect upon its fertility, while the burrows open up the deeper-lying layers to the rain. Mr. Alvan Millson,[[421]] in detailing the labours of the remarkable Yoruba worm (Siphonogaster millsoni Beddard), hints that they may serve as a check upon the fatal malaria of the west coast of Africa. By their incessant burrowings and ejecting of the undigested remains of their food many poisonous germs may be brought up from below, where they flourish in the absence of sunlight and oxygen, and submitted to the purifying influence of sun and air.
Phosphorescence.—Phosphorescence has been observed in several species of Oligochaeta. The most noteworthy instance of recent times is the discovery by Giard of the small worm which he called Photodrilus phosphoreus at Wimereux. During damp weather it was sufficient to disturb the gravel upon the walks of a certain garden to excite the luminosity of these Annelids. In all probability this species is identical with one whose luminosity had been noticed some years before (in 1837) by Dugès, and named by him Lumbricus phosphoreus. According to Giard, the light is produced by a series of glands in the anterior region of the body debouching upon the exterior. This same worm has since been found in other localities, where it has been shown to be phosphorescent, by Moniez[[422]] and by Matzdorf[[423]]. It is remarkable that in some other cases the luminosity, though it exists, is very rarely seen. The exceedingly common Brandling (Allolobophora foetida) of dunghills has been observed on occasions to emit a phosphorescent light. This observation is due to Professor Vejdovsky,[[424]] and was made "upon a warm July night of 1881." He thinks that the seat of the light is in the secretion of the glandular cells of the epidermis, for when this and other worms are handled the phosphorescence clings to the fingers, as of course does the mucous secretion voided by the glands.
Phosphorescence has been observed also in some other families of Oligochaetes. The late Professor Allen Harker noticed a small worm in marshy ground in Northumberland which emitted a distinct light, and which was subsequently identified as a member of the family Enchytraeidae.
Geographical Distribution.[[425]]—In the succeeding pages some of the details of the geographical range of the Oligochaeta will be found. The present section deals with a few generalities, which appear to result from an examination of the facts.
As to the aquatic genera but little is known at present with regard to their range; they have not been widely collected in extra-European countries. What little is known points to the conclusion that while many parts of the world have their peculiar genera (such as Hesperodrilus in South America, Phreodrilus and Pelodrilus in New Zealand), some of the common European species are widely distributed. I have, for example, received Henlea ventriculosa from Kirghiz Tartary, and from New Zealand; and a New Zealand Tubifex appeared to me to be indistinguishable from the common T. rivulorum of our rivers and ponds. It is possible that these and similar instances may, at least in some cases, be due to accidental importation at the hands of man, a matter into which we shall enter later. But the aquatic genera have, many of them, facilities for extending their range in a natural fashion, which are greater than those possessed by earthworms. It has been pointed out that the chaetae of the aquatic Oligochaeta are generally hooked at the extremity and bifid, which would give them a greater chance of holding on to the feet or feathers of aquatic birds; I am not myself disposed to lay much stress on the possibilities of migration by these means, since the tender bodies of the small worms would be liable to be soon dried up by wind while in the act of migration. More likely in every way is a migration when enclosed in the cocoon. The cocoons being small, and often deposited at the edges of ponds frequented by aquatic birds, there would be many chances of their being carried away with tolerable frequency; moreover, as Dr. Michaelsen has pointed out, the cocoons of some species, particularly among the Enchytraeidae, contain a large number of embryos; so that when such a cocoon reached a foreign shore there would be a better chance of the species establishing itself there. I have referred elsewhere[[426]] to the singular habit of forming a temporary cyst which characterises one species of the genus Aeolosoma; this would perhaps tend to facilitate its transference in the way indicated from one spot to another.
Earthworms, on the other hand, have not such easy means of travelling from country to country; the assistance which the cocoons in all probability give to the smaller aquatic Oligochaeta cannot be held to be of much importance in facilitating the migrations of the earthworms. In the first place, the animals themselves are of greater bulk, and their cocoons are naturally larger, and thus less easy of transportation. Secondly, they are deposited as a rule upon dry land, where the chances of their sticking to the feet of birds would be less; and thirdly, they are often deposited deep in the ground, which is a further bar to their being taken up. Another possible method by which earthworms could cross the sea is by the help of floating tree-trunks; it is, however, the case with many species that they are fatally injured by the contact of salt water. There are, it is true, a few species, such as Pontodrilus of the Mediterranean coast, which habitually live within reach of the waves; but with the majority any such passage across the sea seems to be impossible.[[427]] On the other hand, rivers and lakes are not a barrier to the dispersal of the group. There are a few species, such as Allurus tetragonurus, which live indifferently on land and in fresh water; and even some habitually terrestrial species can be kept in water for many weeks with impunity. A desert, on the other hand, is a complete barrier; the animals are absolutely dependent upon moisture, and though in dry weather the worms of tropical countries bury themselves deep in the soil, and even make temporary cysts by the aid of their mucous secretions, this would be of no avail except in countries where there were at least occasional spells of wet weather.
The range of the existing genera and species is quite in keeping with the suggestions and facts already put forward. But in considering them we must first of all eliminate the direct influence of man. Every one who studies this group of animals knows perfectly well that importations of plants frequently contain accidentally-included earthworms; and there are other ways in which the transference of species from one country to another could be effected by man. There are various considerations which enable us to form a fair opinion as to the probability of a given species being really indigenous or imported. Oceanic islands afford one test. There are species of earthworms known from a good many, but with a few exceptions they are the same species as those which occur on the nearest mainland; in those cases where it is supposed that the animal inhabitants have reached an oceanic island by natural means of transit, it is a rule that the species are different, and even the genera are frequently different. That the bulk of them are the same seems to argue either frequent natural communication with the mainland or a great stability on the part of the species themselves. It is more probable that the identity is in this case to be ascribed to accidental transference.
Another argument comes from the distribution of the family Lumbricidae. This family forms the bulk of the earthworms of the European and North American continents. But they are also found all over the world. With one or two exceptions, such as Allolobophora moebii, from Madeira, the extra-north-temperate species are identical with those found within that region. Now, if the migration had been by natural means there would surely in the lapse of time been some differentiation of species. Furthermore, Dr. Michaelsen has pointed out that in South America the presumably European forms (i.e. Lumbricus and Allolobophora) are found upon the coast and in cultivated ground; it is inland that the presumably indigenous species are met with. This again looks very like accidental transference.
A mapping of the world in regions indicative of the distribution of earthworms produces a result which is slightly different from the accepted division. North America, Europe, and Northern Asia so far as is known agree in having as their distinctive earthworms the family Lumbricidae, which is very nearly the only one represented in these parts of the world. The majority of the species are common to the two continents; there cannot, in fact, be a separation of Nearctic and Palaearctic; we must accept the Holarctic region of Professor Newton. The Ethiopian region, on the other hand, is quite as it is in other groups, being bounded to the north by the desert of Sahara. The Neotropical region is quite distinct, and includes Central as well as South America, and the West Indian islands, even the Bermudas. It is, however, a question whether the more southern portions of the continent should not be cut off from the rest and joined with New Zealand, to form an Antarctic region. In these two countries, and also in Kerguelen and Marion Islands, the prevailing genera are Acanthodrilus and Microscolex. In America Acanthodrilus is found nowhere but in the more southern regions of the southern continent, as well as in the Falklands and South Georgia. New Zealand is characterised by other genera of Acanthodrilids besides Acanthodrilus itself; but the bulk of the species belong to the latter genus. Acanthodrilus also occurs (three species only) in Queensland and at the Cape of Good Hope. Microscolex is rather more widely dispersed, being found in other parts of America and in Europe, the island of Madeira (? accidentally imported); but it is undoubtedly chiefly concentrated in South America and in New Zealand. Apart from New Zealand, which, as already said, can only be doubtfully referred to the Australian region, the latter appears to form one with the Oriental region (to which, on account of its Perichaetidae, Japan should be added) of other writers. There is, so far as earthworms are concerned, no "Wallace's line" at all. The characteristic genera Perichaeta and Megascolex range from one extremity of the Indo-Australian region to the other. It is true that Cryptodrilus and Megascolides are limited to Australia itself (with the apparent exception of a species or two in America, for I can hardly separate Argilophilus of Eisen from Megascolides); but they are not at all well-defined genera, and indeed the generic distinctions of the whole family Cryptodrilidae are not in a satisfactory condition.
Classification.—The Oligochaeta do not shade into the Polychaeta so imperceptibly as might be inferred from the current schemes of classification. Apart from minor points, which are not universally characteristic of the two groups, though never found except in one or the other, the Oligochaeta are to be defined by the complicated reproductive system; although in a few undoubted Polychaets there is a faint approach to this in the specialisation of some of the nephridia as sperm-receptacles and even as sperm-ducts. But nowhere among the Polychaeta are there the diversified sperm-ducts and oviducts, spermathecae and sperm-sacs, that are universal among the Oligochaeta. Moreover, no Polychaet has a clitellum, which is so distinctive of the Oligochaeta, and of their near allies the Leeches. Dr. Eisig has compared the glandular modification of the integument at the mouths of the sperm-ducts in the Capitellidae to the beginnings of a clitellum. This may be the case, but it is, in my opinion, more comparable to the similar glandular spots at the male pores in earthworms. The reproductive glands in the Oligochaeta (save for a few apparently abnormal cases) are restricted to at most two pairs of each, which occur in the same individual; the Polychaeta being dioecious. There is, in short, no form known which cannot be definitely referred to either the Polychaeta or the Oligochaeta, excepting perhaps Ctenodrilus, the anatomy of whose reproductive organs is at present unknown.