It is a difficult task to classify the different families of the Oligochaeta; and to enter into the historical aspect of the matter would take too much space. I am myself disposed to divide them first of all into two main groups, for which I use Dr. Benham's[[428]] names of Microdrili and Megadrili.
The Microdrili are, as a rule, small and aquatic in habit; they have short sperm-ducts which open on to the exterior in the segment which immediately follows that which contains the internal aperture; the clitellum is only one cell thick; the egg-sacs are large; the epoch of sexual maturity is at a fixed period. This group, to my thinking, includes the Moniligastridae; although Professor Bourne has denied my statement with regard to the clitellum, and in this case it is not so easy to decide their systematic position.
The Megadrili are characterised by the precisely opposite characters. The sperm-ducts are longer; the clitellum is composed of many layers of cells; the egg-sacs are rudimentary; sexual maturity appears to be more or less continuous.
There is, however, a substantial agreement about the families which I here adopt, which may be fairly taken to express our present knowledge of the Order. For fuller details the reader is referred to my Monograph of the Order Oligochaeta.[[429]]
Fig. 195.—Aeolosoma hemprichii dividing transversely, × 30. (After Lankester.)
I. Microdrili.
Fam. 1. Aphaneura.[[430]]—This name was originally given to the present family by Vejdovsky; the family contains a single genus, Aeolosoma, of which there are some seven species. The name is taken from, perhaps, the most important though not the most salient characteristic of the worms. The central nervous system appears in all of them to be reduced to the cerebral ganglia, which, moreover, retain the embryonic connexion with the epidermis. The worms of the genus are fairly common in fresh waters of this country, and they have been also met with in North and South America, and in Egypt, India, America, and tropical Africa. They are all small, generally minute (1 to 2 mm. long), and have a transparent body variously ornamented by brightly-coloured oil globules secreted by the epidermis. These are reddish brown in A. quaternarium, bright green in A. variegatum and A. headleyi, in the latter even with a tinge of blue. In the largest species of the genus, A. tenebrarum they are olive green. In A. niveum the spots are colourless, and A. variegatum has colourless droplets mixed with the bright green ones. Fig. 195 shows very well the general appearance of the species of this genus. The body has less fixed outlines than in most worms, and the movement of the creatures is not unsuggestive of a Planarian. As the under side of the prostomium is ciliated, and as the movements of these cilia conduce towards the general movement of the body, the resemblance is intelligible. One species of Aeolosoma, at any rate, has a curious habit which is unique in the Order. At certain times, for some reason at present unknown, the worm secretes a chitinous capsule, inside which it moves about with considerable freedom; these capsules when first observed were mistaken for the cocoons of the worms; they are really homologous with the viscid secretion which the common earthworm throws off when in too dry soil, and with which it lines the chamber excavated in the earth in which it is lying. The worms of this genus multiply by fission; sexual reproduction has been but rarely observed.
Fam. 2. Enchytraeidae.[[431]]—This family consists at present of rather over fifty well-characterised species, which are distributed into eleven genera. It is common in this country and in Europe generally; it has been met with in Spitzbergen and the extreme north; it occurs in the American continent from the north to the extreme south; it is also an inhabitant of New Zealand. The worms of this family are nearly always of small size, sometimes minute; they never exceed an inch or so in length, and that is a rare occurrence. They are equally at home in water and in soil, some species being common to the two media; a few are marine or littoral in habit, while others are parasitic in vegetable tissues. Like most earthworms, and unlike the majority of aquatic worms, the chaetae are without a bifid termination; the body-wall, too, is comparatively thick. The perivisceral fluid is often (as in certain Naids) loaded with elliptical or rounded corpuscles. Resemblances to earthworms rather than to the aquatic families of Oligochaeta are suggested by the long distance which separates the spermathecae from the male pores (segments 5 and 12), and by the paired or unpaired glands that have been already compared to the calciferous glands so universally present among earthworms. On the other hand, the male ducts are confined, as in the lower Oligochaeta, to two segments, upon one of which the internal, upon the other the external orifice is situated, and the oviduct is reduced to a simple pore, as in Naids; but this may be merely a matter of convergence by degeneration. Perhaps the most remarkable genus in the family is Anachaeta, which has no chaetae, but in their place a large cell projecting into the body-cavity, which appears to represent the formative cell of the chaeta. The integument of this genus contains true chlorophyll, according to Vejdovsky.
A singular character, found, however, also in Rhynchelmis and Sutroa among the Lumbriculidae, is the opening of the spermathecae into the alimentary canal. This was originally discovered by Dr. Michaelsen, but has been abundantly confirmed.