Fig. 197.—Libyodrilus violaceus F. E. B. × 2. sp, Spermathecal pore; cl, clitellum; ♂, male pore.
The peculiarities of internal structure mainly concern the reproductive organs, the differences in which from genus to genus are often very great. We have already referred to the remarkable branching of the nephridial duct in the body-wall, and to the much modified calciferous glands of Stuhlmannia and some other genera. These structural variations perhaps permit the family to be divided into two sub-families. In one there are calciferous glands of the normal type, though peculiar in that one or more are median and ventral in position, and are unpaired; there is no branching of the nephridium in the body-wall; there are always, so far as is known, the Pacinian-corpuscle-like bodies in the integument. In the other sub-family the calciferous glands, if present (they are absent, for instance, in Libyodrilus), have undergone much modification in structure; the nephridia, where they have been investigated, have been found to branch copiously in the body-wall; the peculiar integumental bodies hardly ever occur.
Fam. 13. Geoscolicidae.[[447]]—This family is essentially tropical, being found in South America and the West Indies, in tropical Africa, in India, and in some of the islands of the Malay Archipelago. But it also occurs (Sparganophilus and Criodrilus) in Europe and in America. A good many of the genera are aquatic. This is the case with the two already mentioned; the genera Glyphidrilus and Annadrilus of the Malay Archipelago can live in water. The family is easily definable if we take the more typical forms; but at one end of the series it fades into the next family, that of the Lumbricidae. Criodrilus is one of the genera which is difficult to place. As is the case with many Geoscolicidae, Criodrilus has ornamented chaetae not only upon the clitellum, but upon the other segments of the body. This character was until recently unknown among the Lumbricidae; it has been lately found in Allolobophora moebii and A. lonnbergi. The absence of spermathecae characterises Criodrilus as well as other Geoscolicidae; but here again the character is not by any means distinctive, for in Allolobophora constricta there is the same absence of these organs. In Criodrilus the male pores are upon segment 15, as in the Lumbricidae, but a species of Kynotus, which is certainly a Geoscolecid, has these pores upon precisely the same segment. The only point in which Criodrilus is definitely a Geoscolecid, or rather not a Lumbricid, is in the forward position of the clitellum, which begins upon the fifteenth segment, far earlier than it does in any undoubted Lumbricid. The peculiar elongated cocoon, which much resembles that of Sparganophilus, is another character which favours its Geoscolecine affinities. Dr. Michaelsen has proposed to unite Criodrilus and Alma into a family intermediate between the Geoscolicidae and the Lumbricidae.
Fig. 198.—Alma millsoni F. E. B. × 1.
Perhaps the most remarkable genus in the whole family is Alma. One species lives in the Nile mud; another is the "Yoruba worm" of West Africa, whose habits have been described by Mr. Millson. The most marked character of this genus, apart from the branchiae (see p. [352]) which apparently may be present or absent according to the species, is in the two enormous processes of the body-wall, which are illustrated in Fig. 198. These contain the sperm-ducts, which, however, open some way in front of the free end; they are provided on the ventral surface with a series of sucker-like structures and with peculiar chaetae. Another interesting genus is Pontoscolex, which was originally described from the sea-shore of Jamaica by Schmarda; there are only two species which are certainly characterised, though a variety from the Hawaian Islands may be a "good" species. It possesses the remarkable peculiarity that the chaetae at the end of the body are disposed in a perfectly irregular fashion, which earned for it the name of brush-tail at the hands of its discoverer, Fritz Müller. This worm, which is universal, or nearly so, in its range, doubtless having been transferred accidentally from country to country, invariably shows a light spot not far from the tail; when this is examined with the microscope it is seen that the chaetae are here absent or very small, and that the muscular structure of the body-wall is slightly different; it was thought that this spot was a zone of growth where fresh segments could be added after the fashion of some of the aquatic Oligochaeta, to which, it may be remarked, the present genus shows a curious point of likeness in the bifid character of the chaetae. It seems, however, that there are really no grounds for the supposition, and it is possible that we have here a "weak" spot, such as that in the foot of certain land snails, which readily gives way when the worm is picked up by a bird, and allows the "better half" of the creature to escape. The Bermudian genus Onychochaeta offers a very strange peculiarity in that the chaetae on the hinder segments of the body are enormously larger than those in front, and end in strong hooks; it seems likely that their function is to maintain a tight hold of the ground while the worm is leaning out of its burrow, as every one has seen the common earthworms of this country do. Onychochaeta has the same irregular arrangement of the chaetae upon the greater part of the body, as has Pontoscolex. This family, like so many others, has its giants and its dwarfs. At one extreme is the great Antaeus of South America, several feet in length; at the other the inch-long Ilyogenia of Africa. The American Urobenus has a pair of intestinal caeca like those of Perichaeta, and placed in the same segment.
Fam. 14.9 Lumbricidae.[[448]]—This family is to be distinguished by the following assemblage of characters.
The male pores are usually upon segment 15, and never behind that segment; the clitellum commences some way behind the male pores. The gizzard, which is invariably single, is equally invariably at the end of the oesophagus. There are three pairs of calciferous glands. The nephridia are always paired. The spermathecae never have a diverticulum.
This family only contains three well-known genera, viz. Lumbricus, Allolobophora, and Allurus. The American Bimastos may be distinct. Tetragonurus, not allowed by some, is at present unknown except as regards external characters; it differs from the other Lumbricidae in the fact that the male pores are upon the twelfth segment. In Allurus they are upon segment 13, and in the remaining genera upon the fifteenth. Lumbricus is to be distinguished from Allolobophora by its prostomium, which is continued by grooves on to the buccal segment, so as to cut the latter in half. It has also median sperm reservoirs, as well as the paired sperm sacs which are alone present in Allolobophora.
