ANNELIDA - OLIGOCHAETA Next to the Polychaeta, the largest division of the Annelida, are the Oligochaeta. More than four-fifths of the species are earthworms; also comprised within the group are a number of small or minute worms, some of fresh-water habit, others (belong ing to the family Enchytraeidae) plentifully present, though incon spicuous, in cultivated ground and under stones on the shore between tidemarks. Wholly marine Oligochaeta are rare.
Oligochaeta have the wormlike segmented body with spacious body-cavity characteristic of most members of the Annelid phylum. In common with the Polychaeta they possess organs of locomotion in the form of setae (chaetae) ; the bundles of setae are not, as in the Polychaeta, set in parapodia, they merely pro trude from the general level of the body surface. The most important distinction of the Oligochaeta from the Polychaeta, however, is that they are hermaphrodite, and have a much more complicated system of reproductive organs.
The Oligochaeta are in all probability descended from marine ancestors belonging to the group Polychaeta; the Polychaete stock has remained marine, the Oligochaete branch has wandered inland and taken possession of the earth and fresh waters; those that are now found on the shore and below low-water mark are descendants of terrestrial or fresh-water worms which are, as it were, returning towards their ancestral home. On the other hand, the Oligochaeta have in turn given rise to the leeches (Hirudinea).
The terms Terricolae and Limicolae, i.e., earth-dwellers and mud-dwellers, are sometimes used, rather loosely perhaps, and correspond generally to Megadrili and Microdrili (exclusive, per haps, of the Enchytraeidae) respectively. There are however not a few Megadrili, "earthworms" by outward appearance and affini ties, which live in marshy or even aquatic habitats.
Comparative Anatomy.—In fig. 13 a marked external ringing indicates the division of the body into a series of more or less simi lar segments. In size, selected in dividuals of some of the largest species measure between three and four feet, while Megascolides australis is recorded as having at tained a length of six or even seven feet. But taking earthworms in general, the British Luri bricidae are perhaps rather above the average in size. The Micro drili are usually much smaller; some species of Aeolosomatidae and Naididae are only about one millimetre in length, while the majority of Enchytraeidae are less than an inch. In fully mature worms the clitellum appears as a thickening of the body, girdle like and extending over several segments; the surface cells of this region secrete, at the time of egg laying, a cocoon in which the eggs are received, and in which the early stages of the development of the young are passed through.
The setae (fig. 14) are rodlike or sometimes hairlike structures implanted in the body-wall ; the rodlike setae are relatively short and project only slightly, while the hairs are longer and project to a greater extent ; the hairs are found only in a few families of Microdrili. The setae of earthworms are in general like that repre sented for Lumbricus (fig. 14a) ; a greater variety occurs among the Naididae and Tubificidae. They are organs of locomotion ; an earthworm uses them for obtaining a hold on the substratum, and then draws up the hinder part of the body ; it then extends the anterior end, takes a fresh purchase with the setae of its front region, pulls up the hinder part again, and so on. The Limicolae use their rodlike setae on the substratum in the same way; but the bundles of hair-setae are swimming organs. In most earthworms there are eight setae per segment, arranged as two couples on each side, one couple (ventral) low down, the other (dorsal or lateral) higher up on the side of the worm. In some earthworms the num ber of setae is much larger ; there may be any number from eight to ioo or more, and they are then arranged in a ring round each segment. In the commoner forms of Microdrili the setae, while arranged, as in most earthworms, in dorsal and ventral bundles, are not limited to two per bundle ; the number per bundle varies, but may be as many as 16 or occasionally even more.
The body-wall, body-cavity, vascular and nervous systems are of the general Annelid type. The excretory organs are nephridia.
In connection with the alimentary canal may be mentioned (a) the gizzard, a strongly muscular section of the gut, present in most earthworms, the function of which is to grind up the ingested earth and so facilitate digestion of the contained organic matter; it is absent in the Limicolae. (b) The calciferous glands appear in many forms; they are essentially a part of the oesophagus, which is either merely dilated or pouched outwards so as to form saccular appendages; the lining of the oesophagus, or of the sacs, is thrown into an often very complex system of folds, within which there is a copious circulation of blood. The glands are present in many but by no means all earthworms ; their function is hardly yet fully known. In the Lumbricidae, and in a few worms belonging to other families, small crystals of calcium carbonate are produced between the folds and are discharged into the main cavity of the alimentary tube; this may serve to neutralize the humus acids in the earth ingested by the worms; it may also be a means of getting rid of the car bon dioxide produced in the or ganism, and the glands may in this sense be respiratory organs.
But it is not certain that such a secretion is produced by all the structures grouped together un der the name calciferous glands.
A few aquatic Oligochaeta pos sess branchiae (gills), processes of the body-wall with a copious blood supply, as organs of respi ration ; but in general respiration is carried out through the un modified body-wall. Sense organs are not well developed in the Oligochaeta. Earthworms avoid light, except of a very low intens ity; the perceptive organs are cer tain specially constituted cells of the surface layer ; the segments at the anterior and posterior ends are the most sensitive to light, and in these regions the sensory cells are most numerous. Some of the small freshwater Naididae have a pair of black eye-spots on the head. Other specially modified cells of the surface layer, in both Mega- and Microdrili, and the free endings of fine nerve twigs between the surface cells, are also doubtless sensory, and may have to do with the perception of mechanical and chemical stimuli. Earthworms can distinguish between the taste of differ ent vegetable foods ; but no special organs are known. All Oligo chaeta are hermaphrodite. The testes are either one or two pairs, the ovaries usually a single pair; they are situated in the anterior part of the body—in the Lumbricidae in segments x. and xi. (testes) and segment xiii. (ovaries). Before being shed, the male cells usually develop and ripen in sacs known as seminal vesicles, the female cells in the cavity of the ovarian segment or in special ovisacs; the genital products reach the exterior by special ducts, the endings of the male ducts being seen in she common Lum bricidae on segment xv. as slitlike apertures on prominent papillae. In many worms (but not in the Lumbricidae) glandular organs (so-called prostate glands) open near or in common with the end ings of the male ducts. Spermathecae are generally present.
In the families Aeolosomatidae and Naididae the above (or sexual) mode of reproduction is largely replaced by fission, an asexual process, in which a worm divides into two, with the forma tion of a new tail in the case of the anterior, a new head in the case of the posterior of the two animals so produced.
Zoogeographical Distribution.—Aeolosoma and several gen era of Naididae (small fresh-water forms) are cosmopolitan ; even the same species is sometimes found in widely separated local ities, even in different continents. This is due to the ease with which they or their cocoons can be transported; they can without difficulty colonize the whole of a river system once they gain an entry; they, or more often their cocoons, may be carried in the mud on the feet of water-birds from one pond or lake to another, or from one river-system to another. The same considerations apply to the Tubificidae and other Limicolae.
The species of earthworms are on the whole much more local ized; instead of being passively carried by the stream, or casually transported (themselves or their cocoons) in mud, their migra tions are in the main due to themselves alone, and largely to their slow tunnelling through the soil; streams, deserts, rocky or snow clad mountain ranges form insuperable obstacles. Yet even among earthworms there are many species which have a wide distribu tion; some of the common Lumbricids are found in all parts of the world where European intercourse has penetrated (e.g., Allo lobophora caliginosa, perhaps the commonest of all earthworms) ; some species of the Far Eastern genus Pheretima are now circum mundane in the warmer regions of the globe. It is evident that this can only have come about by their (or their cocoons) having been passively transported. Certain species, in fact, are now recognized as being very liable to accidental transportation, and as being able, when thus transported, to establish themselves with ease in new surroundings ; insomuch that near South American and Australian towns with considerable European intercourse the indigenous earthworms have been entirely displaced by Lumbricid intruders, and have to be sought in the remoter parts of the country. Species which spread in this way are known as peregrine.
It is obvious that no general conclusions can be drawn from the distribution of the peregrine earthworms and the Limicolae. But the case is different with those genera of earthworms which do not bear artificial transportation readily, which spread slowly and which are limited by the physical features of the land; if, for example, the same genus is found in two regions now entirely cut off from each other by sea, this is presumably because these regions were at one time united by a land bridge. The Oligochaeta are thus of much value to the science of Palaeogeography.
The number of worms in the soil varies widely according to the nature of the soil, and may be very large ; the numbers per square metre given by different observers range from about 3o to 2,000; in meadows and garden earth 300-1,000 seem to be usual. But the small Enchytraeidae are much more numerous; in Switzerland, Bretscher found 1,650-8,00o per square metre.
Earthworms are nocturnal animals, retreating into their bur rows in the daytime, and emerging at night. As a rule, while pro truding and gathering the vegetable debris within their reach, they maintain their hold on their burrows by their tail. In forming their castings, however, the hinder end is protruded; the ejected earth is deposited with some care, first on one then on the other side around the mouth of the burrow. The castings thus surround and cover the aperture ; the mouth of the same burrow is used for this purpose for some time, so that each mass is the product of several operations or of several worms. Castings may be of considerable size ; the largest Darwin could find at Down weighed nearly four ounces; in warmer countries they may be larger.
The burrows of earthworms are excavated in two ways, by pushing the earth away on all sides, and by swallowing it. The burrows run more or less perpendicularly, and seldom branch; they are lined with a firm smooth layer of fine earth voided by the worms, and so may be compared to tunnels lined with cement.
Earthworms drag leaves, seeds and other portions of plants into their holes, and parts of these, at least, they eat ; they also ingest earth, utilizing the organic matter contained in it as food. They will devour raw fat, raw and roasted meat and dead worms if placed within their reach.
A number of earthworms are luminescent, among them the common Eisenia foetida (the brandling). The light is given off by a fluid which escapes from mouth and anus, or in some cases from the dorsal pores (apertures in the body-wall along the middle line of the back) ; the ultimate source of the fluid is in either case the body-cavity.
The chief economic use of Oligochaeta at the present day is as bait in fishing. Worms from Thames mud, doubtless Tubificidae, used to be sold to feed fish in aquaria ; and Tubificid worms are sold as food for goldfish in Japan. Certain earthworms were until recent times used as food by the Maoris. Earthworms have often been employed in medicine ; passing over their use in antiquity and the middle ages, they are given at the present day in China and Japan in fever, in Burma in a variety of affections and in Gippsland in Australia for rheumatism. See EARTHWORM.