Stercutus is a singular genus which was originally found in manure, and has the peculiarity that the alimentary canal is often aborted; this degeneration seems to bear some relation to the food and conditions of life.
Fam. 3. Discodrilidae.[[432]]—This family consists of small parasitic forms which were at one time assigned to the Hirudinea; there seems, however, to be no doubt that they are rightly included in the present Order. Branchiobdella is found upon the gills of the Crayfish, Astacus fluviatilis; the American Bdellodrilus upon Cambarus. The chief reason for the former inclusion of these worms among the leeches was due to the absence of chaetae and to the presence of chitinous jaws and of suckers; apart from these structures there is nothing whatever leech-like about the worms. Bdellodrilus has two pairs of testes in segments 5 and 6; there are two pairs of sperm-ducts, all opening, however, by a common "atrium" on the sixth segment; on the fifth open a pair of spermathecae, likewise by a common pore. The ovaries are in segment 7, and the ova escape by a pair of pores apparently like the single pore of the Enchytraeidae. The entire worm consists of only eleven segments.
Fam. 4. Phreoryctidae.[[433]]—This family contains only two genera, Phreoryctes and Pelodrilus. The former is widely spread, occurring in Europe, North America, and New Zealand. Pelodrilus is limited to New Zealand. Most species of Phreoryctes are distinguished by their extraordinary length and thinness, and there is frequently a tendency to the disappearance of the chaetae. The most important anatomical fact about Phreoryctes (at any rate P. smithii) is that there are two pairs of ovaries as well as two pairs of testes, and that the ducts of all are simple and very much alike. This seems to argue the low position of the family in the series.
Fam. 5. Naidomorpha.[[434]]—This family contains eight or nine genera, perhaps more; they are all of them aquatic and of small size, and they multiply by fission as well as sexually. The most noticeable peculiarity of the family is the "cephalisation" which occurs in the head segments. In some genera, in Pristina for example, there is no such cephalisation to be observed; but in others the dorsal bundles of chaetae commence a few segments farther back than the ventral, the segment where they commence being different and characteristic in the various genera. Thus in Dero the first four segments are without dorsal chaetae, and in Nais the first five are in this condition. There is thus a kind of "head" formed, whence the expression "cephalisation." Dero, Nais, and Pristina are commonly to be met with in ponds, lakes, etc., in this country. Bohemilla is rarer, and is to be distinguished by the remarkable serrated chaetae of the dorsal bundles. Of Dero and Nais there are a considerable number of different species; indeed it is usual perhaps to regard as distributable among three genera, Nais, Stylaria, and Slavina, the species which I am disposed to place in one genus, Nais. Stylaria is defined on this view by its extremely long prostomium, which has given rise to both its popular and technical names. "Die gezungelte Naide" was the term applied by one of its earliest investigators, and the name Stylaria proboscidea signifies the same peculiarity. But as the same inordinately long "proboscis" occurs in the South American Pristina proboscidea, belonging to a genus of which the other member does not possess so well developed a prostomium, it seems too variable a character upon which to differentiate a genus. Chaetogaster and Amphichaeta have been placed by some systematists in a separate family. The first named contains four species which are fairly common. It is one of those worms in which the chaetae are not exactly related to the segmentation of other organs, which moreover sometimes show an independence in their segmentation; thus there are more nerve ganglia in the anterior segments of the body than there are septa.
Fam. 6. Tubificidae.—The worms belonging to this family are of small size, and are all inhabitants of fresh or salt water, or the margins of pools and the sea. They differ from the last family in that asexual reproduction never occurs, and that the reproductive organs are situated rather farther back in the body. The male pores are upon segment 11, and the oviduct-pores upon the following segment. This family differs from the Lumbriculidae in the fact that there are only a single pair of sperm-ducts.
The earliest known Tubificid was the common Tubifex rivulorum, so widely dispersed in this country and elsewhere; but with it was at first confounded the somewhat similar genus Limnodrilus, which only differs in that the chaetae are all of the cleft variety, and never capilliform, as in Tubifex. The genera are mainly distinguished by the characters of the chaetae and of the male ducts. At the base of the series perhaps lies Ilyodrilus, which has many points in common with the Naids. The form of the terminal chamber into which the sperm-duct opens has the same simplicity as in that group, and the intestine is surrounded with a network of blood-vessels as in the Naids, a structure which is otherwise wanting in the Tubificidae. The development of the ova also is upon a plan which is met with in the Naids. The atrium (see p. [361]) becomes more complicated in other Tubificidae. The extremity also is as a rule modified into a retractile penis. The discrete "prostate," of which we have already spoken, marks out a considerable number of genera, such as Tubifex, Limnodrilus, Spirosperma, Hemitubifex. In the marine Clitellio there is no such structure at all, and it is also wanting in the South American Hesperodrilus. In Branchiura there is a complete prostatic investment of the atrium, and in Telmatodrilus a large number of separate aggregations forming as many distinct prostates. Vermiculus, a genus consisting of but one species, found by Mr. Goodrich on the sea-shore in the neighbourhood of Plymouth, is remarkable for the unpaired character of the generative organs, a peculiarity which is shared by Stolc's genus Bothrioneuron. The gills upon the posterior segments of Branchiura sowerbyi and Hesperodrilus branchiatus have been already noticed above (p. [352]). A very aberrant genus, perhaps not rightly referable to this family, is Phreodrilus,[[435]] from New Zealand, first collected in water from a subterranean spring. It differs from all other Tubificids except Hesperodrilus in that the spermathecae lie behind the male pores, a state of affairs which is met with in the Lumbriculidae. Another singularity of structure concerns the sperm-duct, which is wrapped in a thin-walled sac, which has every appearance of being simply the outer muscular wall of the duct. Within this are the complicated coils of the duct, and also a quantity of free spermatozoa, whose mode of ingress is difficult to understand. Many of the Tubificidae live in tubes fabricated by themselves, whence the tail end protrudes. The integument in more than one species is vascular. This integumental blood system, universal among the earthworms, appears to be restricted to the present group among the Limicolae of Claparède.
Fam. 7. Lumbriculidae.[[436]]—This family is not a large one, and is nearly limited in range to Europe and North America; indeed, if we omit the doubtful Alluroides, entirely to the Palaearctic region. There are only fourteen species, which are referred to eight genera. A number of dubious forms, as is the case with other families, may possibly ultimately swell this list. The type genus of the family, viz. Lumbriculus, upon which Bonnet made his experiments in section and subsequent regeneration, has only within the last year been thoroughly explored anatomically. But all the other genera are well known. The Lumbriculidae are of small or moderate size, and all of them aquatic in habitat. There are three characters which are nearly or quite universal in the genera of the family. In all of them the chaetae are only eight to each segment, arranged in couples, and are either cleft at the extremity or simple. As a rule which has but two exceptions, the genera Alluroides and Lumbriculus, there are two pairs of sperm-ducts, which, however, communicate with the exterior through a single terminal chamber on each side of the body.
The dorsal blood-vessel has in the Lumbriculidae a series of contractile and blind appendages, which were at first mistaken for caeca of the intestine itself. There are two genera of this family in North America, which are not very different anatomically from their European representatives. The genera described by Eisen are Sutroa[[437]] and Eclipidrilus.[[438]] The latter lives in cold torrents at a great height in the mountains of the Sierra Nevada of California.
Fam. 8. Moniligastridae.—This family, terrestrial in habit, is probably Oriental in range; but I have described a single species from the Bahamas which may possibly be referable to the category of accidentally introduced specimens. Our knowledge of this family is conveniently summed up in Professor Bourne's paper[[439]] upon the genus Moniligaster. There are some eighteen species, which range in size from an inch or so in length (M. bahamensis) to about two feet; this last measurement is that of the huge M. grandis, of which, together with many others, Professor Bourne gives coloured drawings. There is a second genus, Desmogaster, which is mainly characterised by the doubling of the reproductive organs. This was described by Rosa from Burmah. The family is noteworthy on account of the fact that every species belonging to it has at least four distinct gizzards, sometimes more; but as this multiplication of the gizzards has been also found in Heliodrilus among the Eudrilidae, and indeed elsewhere, it is insufficient to define the family. More characteristic is the fact that the sperm-ducts open on to the next segment to, or even the same segment as, that which contains their funnels; consequently the apertures of the oviducts are behind instead of in front of them. These pores are also situated in a very anterior position, the male pores being upon the tenth segment or between the tenth and eleventh, and the oviducal pores upon the following one. In these features the family presents resemblance to the aquatic Oligochaeta, from which, however, its stoutly-built gizzards, and vascular nephridia differentiate it.
II. Megadrili.