The members of this order, comprising the Earth-worms (Lumbricidae) and the Water-worms (Naididae), are distinguished by the fact that their locomotive appendages are in the form of chitinous setae or bristles attached in rows to the sides and ventral surface of the body. No branchiae are present. They are all hermaphrodite; and the young pass through no metamorphosis. The Oligochaeta are divided into the two groups of the Terricolae or Earth-worms, and the Limicolae or Mud-worms and Water-worms (Saenuridae and Naididae).
Fig. 129. - Hirudinea. a The Medicinal Leech (Sanguisuga officinalis), natural size; b Anterior extremity of the same magnified, showing the sucker and triradiate jaws ; c One of the jaws detached, showing the semicircular toothed margin.
In the common Earth-worm (Lumbricus) the body is" cylindrical, attenuated at both extremities, and carrying in the adult a thickened zone, which occupies from six to nine rings in the anterior part of the body, is connected with reproduction, and is termed the "clitellum," or "saddle." Locomotion is effected by eight rows of short bristles or setae, four of which are placed laterally and four on the ventral surface of the body; these representing the foot-tubercles of the higher Annelides. The mouth is edentulous, and opens by a muscular pharynx into a short oesophagus, which leads to a muscular crop, or "pro-ventriculus," succeeded by a second muscular dilatation, or gizzard. Salivary glands open into the pharynx, and other glands, probably digestive, open into the gullet. The intestine is continued straight to the anus, and is constricted in its course by numerous transverse septa, springing from the walls of the perivisceral cavity. The perivisceral cavity (as in all the Oligochaeta) is lined by a cellular membrane, which is continuous with a yellow cellular layer covering the intestine and large vessels, and which casts off its cells into the perivisceral fluid. The pseudohaemal system consists of three principal longitudinal trunks and their branches, filled with a red non-corpusculated fluid; and there exists, in even greater numbers, the same series of lateral sacculi or " segmental organs" which we have seen in the Leeches, and which have either a respiratory or a renal function. In all the Oligochaeta the segmental organs communicate internally with the perivisceral cavity as well as externally with the outer medium. A portion of the segmental organs is ciliated, and in all cases the segmental organs of certain of the segments have the special function of acting as efferent ducts for the generative organs. The body-cavity is filled with a colourless corpusculated "blood." The reproductive organs consist of two pairs of testes, opening on the fifteenth segment, and one pair of ovaries, of which the oviducts open on the fourteenth segment. In addition, the animal also possesses a pair of seminal reservoirs in the tenth and eleventh rings, and five pairs of glands for secreting the capsules of the eggs. The ova are deposited in chitinous capsules, and the young pass through no metamorphosis.
Of the little Naididae, the most familiar is the Tubifex rivu-lorum, which is of common occurrence in the mud of ponds and streams. It is from half an inch to one inch and a half in length, and of a bright-red colour. The pseudohaemal system is provided with two contractile cavities or hearts; and there is present the same system of lateral tubes, opening externally by pores, as occurs in the Earth-worms.
The Naididae are chiefly noticeable on account of the singular process of non-sexual reproduction which they present before they attain sexual maturity. In this process the Nats throws out a bud between two rings, at a point generally near the middle of the body. Not only is this bud developed into a fresh individual, but the two portions of the parent marked out by the budding point likewise become developed into separate individuals. The portion of the parent in front of the bud develops a tail, whilst the portion behind the bud develops a head. Prior to the detachment of the bud, other secondary buds are formed from the same segment, each in front of the one already produced; and in this way, before separation takes place, a chain of organically connected individuals is produced, all of which are nourished by the anterior portion of the primitive worm. Besides their non-sexual reproduction, the Naididae possess generative organs when adult, and exhibit true sexual reproduction. With the development of the generative organs, a new segment is added to the body, and certain other modifications take place ; so that the process of attaining sexual maturity is actually attended with a species of metamorphosis.
As regards their distribution in space, the Oligochaeta have a cosmopolitan range. The most are either terrestrial, or inhabit fresh waters, burrowing in mud or sand; but a few are marine.