Chapter XX: Introduction: Anatomy—reproduction—bionomics—distribution—classification (2)

A mapping of the world in regions indicative of the distribution of earthworms produces a result which is slightly different from the accepted division. North America, Europe, and Northern Asia so far as is known agree in having as their distinctive earthworms the family Lumbricidae, which is very nearly the only one represented in these parts of the world. The majority of the species are common to the two continents; there cannot, in fact, be a separation of Nearctic and Palaearctic; we must accept the Holarctic region of Professor Newton. The Ethiopian region, on the other hand, is quite as it is in other groups, being bounded to the north by the desert of Sahara. The Neotropical region is quite distinct, and includes Central as well as South America, and the West Indian islands, even the Bermudas. It is, however, a question whether the more southern portions of the continent should not be cut off from the rest and joined with New Zealand, to form an Antarctic region. In these two countries, and also in Kerguelen and Marion Islands, the prevailing genera are _Acanthodrilus_ and _Microscolex_. In America _Acanthodrilus_ is found nowhere but in the more southern regions of the southern continent, as well as in the Falklands and South Georgia. New Zealand is characterised by other genera of Acanthodrilids besides _Acanthodrilus_ itself; but the bulk of the species belong to the latter genus. _Acanthodrilus_ also occurs (three species only) in Queensland and at the Cape of Good Hope. _Microscolex_ is rather more widely dispersed, being found in other parts of America and in Europe, the island of Madeira (? accidentally imported); but it is undoubtedly chiefly concentrated in South America and in New Zealand. Apart from New Zealand, which, as already said, can only be doubtfully referred to the Australian region, the latter appears to form one with the Oriental region (to which, on account of its Perichaetidae, Japan should be added) of other writers. There is, so far as earthworms are concerned, no "Wallace's line" at all. The characteristic genera _Perichaeta_ and _Megascolex_ range from one extremity of the Indo-Australian region to the other. It is true that _Cryptodrilus_ and _Megascolides_ are limited to Australia itself (with the apparent exception of a species or two in America, for I can hardly separate _Argilophilus_ of Eisen from _Megascolides_); {373}but they are not at all well-defined genera, and indeed the generic distinctions of the whole family Cryptodrilidae are not in a satisfactory condition.

CLASSIFICATION.—The Oligochaeta do not shade into the Polychaeta so imperceptibly as might be inferred from the current schemes of classification. Apart from minor points, which are not universally characteristic of the two groups, though never found except in one or the other, the Oligochaeta are to be defined by the complicated reproductive system; although in a few undoubted Polychaets there is a faint approach to this in the specialisation of some of the nephridia as sperm-receptacles and even as sperm-ducts. But nowhere among the Polychaeta are there the diversified sperm-ducts and oviducts, spermathecae and sperm-sacs, that are universal among the Oligochaeta. Moreover, no Polychaet has a clitellum, which is so distinctive of the Oligochaeta, and of their near allies the Leeches. Dr. Eisig has compared the glandular modification of the integument at the mouths of the sperm-ducts in the Capitellidae to the beginnings of a clitellum. This may be the case, but it is, in my opinion, more comparable to the similar glandular spots at the male pores in earthworms. The reproductive glands in the Oligochaeta (save for a few apparently abnormal cases) are restricted to at most two pairs of each, which occur in the same individual; the Polychaeta being dioecious. There is, in short, no form known which cannot be definitely referred to either the Polychaeta or the Oligochaeta, excepting perhaps _Ctenodrilus_, the anatomy of whose reproductive organs is at present unknown.

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 {374}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]

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 {375}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 {376}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.

_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 {377}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 {378}_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 {379}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 {380}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.

FAM. 9. _Perichaetidae._[440]—The Perichaetidae comprise a larger {381}number of species than any other family of earthworms; but it is a matter of considerable difficulty to divide the family satisfactorily into genera. The family as a whole may be defined as having numerous chaetae in most of the segments of the body.

There is no other definition which will distinguish this family from the next two families, and even this definition is not absolutely distinctive. There are Acanthodrilids which have a large number of chaetae in each segment. The only difference is that in this case—in the genus _Plagiochaeta_—the chaetae are implanted in twos; this is not the case in the Perichaetidae. In all Perichaetidae that are known the sperm-ducts open in common with the ducts of the spermiducal glands; they generally open into them at some distance from the common external pore. In _Megascolex_, _Perichaeta_, and _Pleionogaster_ the nephridia are of the diffuse type so widely spread among these worms, and the spermiducal glands are lobate. _Megascolex_ differs from the others in the fact that in addition to the small scattered nephridia there are a pair of large nephridia in each segment, and the chaetae do not form absolutely continuous circles, but are interrupted above and below. _Pleionogaster_ has more than one gizzard but otherwise agrees with _Perichaeta_; it is confined to the East. _Perichaeta_ is tropical and occurs—no doubt introduced—in Europe and America. _Megascolex_ is Old World only, and, like _Perichaeta_, Australian as well as Oriental. But whereas _Perichaeta_ is rare in the Australian region, _Megascolex_ is common there. _Perionyx_ and _Diporochaeta_ are the other genera which it is possible to recognise. Both of them have paired nephridia, and neither of them have intestinal caeca, a peculiarity which they both share with _Megascolex_ and _Pleionogaster_. _Perionyx_ principally differs from _Diporochaeta_ in that the spermiducal glands are lobate, whereas in the latter they are as in the Acanthodrilidae. _Perionyx_ is Oriental; _Diporochaeta_ occurs in Australia and New Zealand.

A very distinctive feature of _Perichaeta_—perhaps only of the {382}genus _sensu stricto_—is its exceeding activity. The first specimens ever noticed in this country, or at least of whose existence printed notice was taken, were exhibited by the late Dr. Baird of the British Museum, at a meeting of the Zoological Society. He remarked in that communication upon the agile fashion in which these tropical Annelids will spring off a table when touched or in any way interfered with. Numerous other observers have seen the same manifestations, and the name of "eel-worm" has been given to these _Perichaeta_ by gardeners. It is worth putting on record here that in a species of _Acanthodrilus_ (_A. capensis_) the same irritable behaviour is visible. When a _Perichaeta_ moves it helps itself greatly by extending, or rather protruding, the buccal cavity, which serves as a sucker, and grips the ground in front until the rest of the body is brought forward. It is possibly on account of this extra facility for movement that the genus can climb trees with such ease. A species of _Perichaeta_ has been recorded by Mr. Willey upon an epiphyte of a palm, and Dr. Benham has found that it is a new species, to which the name of _Perichaeta willeyi_ has been given. The Lumbricid genus (if it be admitted as a genus), _Dendrobaena_, was so named on account of a similar habit of climbing trees. Very singular in its habit is the not inaptly-named _Perichaeta musica_ of Java. It is a monster of its kind, several feet in length, and during the night makes "a sharp interrupted sound," apparently by the friction of the chaetae against stones. The species figured (p. 381) is, as are a few others, remarkable for the presence of twelve or seventeen spermathecae in segments 6 and 7.

FAM. 10. _Cryptodrilidae._[441]—This family is one of the largest of the Oligochaeta; there are rather over 120 different species, which can be arranged in at least sixteen genera. They are found in most parts of the world, but abound principally in the tropics. Australia may be considered to be the headquarters of the family, which form its principal earthworm-inhabitants. Peculiar to this continent, or at least mainly confined to it, are the genera _Megascolides_, _Cryptodrilus_, _Fletcherodrilus_, _Trinephrus_, and _Digaster_. _Microscolex_, though occurring in many parts of the world, is characteristic of the more southern regions of South America and of New Zealand. Tropical Africa has the genera _Nannodrilus_ and _Millsonia_ limited to itself, and has besides nearly {383}all the species of the genus _Gordiodrilus_. This family is one which it is exceedingly difficult to define and to split up into different genera. It shades almost imperceptibly into the Perichaetidae on the one hand, and is very hard to differentiate from the Acanthodrilidae on the other. A Cryptodrilid, like any member of the genus _Cryptodrilus_, with complete circles of chaetae would be a Perichaetid; and as there are species of _Perichaeta_ in which the anterior segments have only a few chaetae in each segment, it is perhaps wrong to separate the two families at all. Apart from the chaetae, there is no peculiarity in the organisation of the family Perichaetidae that is not also met with in the Cryptodrilidae. Even the highly characteristic intestinal caeca so distinctive of the genus _Perichaeta_ itself, as contrasted with _Megascolex_ and the other genera, occur, though more numerously, in the African _Millsonia_, where there are forty or fifty pairs of them. A fairly common feature in the family is the presence of two, or even three, pairs of gizzards, a character which is also met with in the genus _Benhamia_ among the Acanthodrilidae, and occurs also in some other families. The names _Digaster_, _Didymogaster_, _Perissogaster_, and _Dichogaster_ have been founded upon this character. The excretory organs may be paired (in _Trinephrus_ there are three pairs to each segment) or of the diffuse kind. The male pores are usually upon the eighteenth segment, but not unfrequently upon the seventeenth, and are often armed with long and ornamented chaetae. Spermiducal glands are invariably present, and may be lobate or tubular. There are two groups of small-sized genera, which in their simplicity of organisation stand at the base of the series; but it is very possible that the simplification is rather due to degeneration than to primitive position. One of these groups includes the semi-marine genus _Pontodrilus_ (with which I include the phosphorescent _Photodrilus_) and _Microscolex_. In these forms the gizzard has disappeared, or is represented by a rudimentary structure, and the male pores are upon the seventeenth segment. In the other group are the genera _Ocnerodrilus_,[442] _Gordiodrilus_,[443] and _Nannodrilus_, which are of even smaller size, and have in the same way the male pores upon the seventeenth segment. The species of this group are often aquatic, and there is not only no gizzard {384}(in most of the species), but the calciferous glands have been reduced to a single pair, which lie in the ninth segment. The latter character is also found in the Acanthodrilid _Kerria_, which has been associated with the above named. _Gordiodrilus_ has the peculiarity that there are, as in Acanthodrilids, two pairs of tubular spermiducal glands.

FAM. 11. _Acanthodrilidae._[444]—This family is only with difficulty to be distinguished from the last. The following definition applies to all the members of the family with one exception, and does not apply to any Cryptodrilid with, so far as is known, one exception only. There are two pairs of spermiducal glands, opening upon the segments in front of and behind that which bears the apertures of the sperm-ducts.

The one exception to this definition is the species _Acanthodrilus monocystis_, which I formerly placed in a distinct genus, _Neodrilus_. _Microscolex modestus_ is the exception among the Cryptodrilidae; in that worm the male pores are upon the segment which follows that upon which the spermiducal glands open. The Acanthodrilidae show a considerable range of structural variation. This enables them to be separated into several well-marked genera. The type genus _Acanthodrilus_ has a pair of nephridia in each segment. It contains thirty-five species, which are all from the southern hemisphere. These species show but little variation among themselves. _Benhamia_ is a genus that differs from _Acanthodrilus_ in the fact that the nephridia are of the complex type, so often met with in earthworms with many external pores. The segment that bears the male pores is entirely without any traces of the ventral chaetae. Here again there are a large number of species which are nearly confined to the continent of Africa. Dr. Michaelsen is indeed of opinion that the few species found in the East Indies and America are accidental importations. I have proposed to separate some of the New Zealand Acanthodrilids into a distinct genus, _Octochaetus_, which is somewhat intermediate between _Acanthodrilus_ and _Benhamia_. They have multiple nephridia, but only a single gizzard. _Plagiochaeta_ of Benham, from New Zealand, is in any case clearly a distinct form. It is mainly to be distinguished by the numerous chaetae in each segment. _Trigaster_ Benham, is West Indian. _Deinodrilus_ (New {385}Zealand) has twelve chaetae in each segment. _Diplocardia_, from North America, has the male pores on segments 18, 19, 20.

FAM. 12. _Eudrilidae._[445]—This is perhaps the most remarkable family of terrestrial Oligochaeta. Its distribution is no less curious than its structure. Up to the present it is not known outside tropical Africa, with the exception of the genus _Eudrilus_ itself, which is almost world-wide in range. As, however, but one species of _Eudrilus_ is found out of Africa, and as that species is so common in gatherings from various tropical countries, it seems to be an instance of a species with large capacities for accidental transference from country to country. The type genus, _Eudrilus_, has been known since 1871, when it was originally described by M. Perrier.[446] Since that date nineteen other genera have been described from Africa by Dr. Michaelsen, Dr. Rosa, and myself. The most salient external character of the group, not universal but general, is the unpaired male and female orifices. The orifices are commonly very conspicuous (see Fig. 197).

The peculiarities of internal structure mainly concern the reproductive organs, the differences in which from genus to {386}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 {387}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.

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 {388}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. {389}_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_.

This is the only family of earthworms which, so far as is known, can brave the ice and snow, and what is still more {390}difficult to understand, the perpetually frozen undersoil of the Arctic regions. Eisen has described a number of species from Spitzbergen, and Colonel Feilden recently sent me an example of _Allolobophora octoedra_ from Kolguiev, where Mr. Trevor-Battye also saw another specimen. The family is characteristic of the Nearctic and Palaearctic regions, and though found beyond them, is probably elsewhere an accidental importation (see p. 371). There are at least fifteen species of this family found in England and Ireland, and probably more will be identified.

There does not exist at present any comprehensive account of the British species of earthworms, though all of them are included in Dr. Rosa's recent revision of the family. Most of the British forms belong to the genus _Allolobophora_, which may be divided into two series according to whether the chaetae are quite close together or further apart. The extent of the clitellum and the position of those swollen eminences which appear earlier than the clitellum, and are known as tubercula pubertatis, offer further characters. In the following tables, extracted from those of Rosa, the known British species of this genus are grouped according to these three characters. With the help of these tables and Figs. 199 and 200, any of the species ought to be easily identified.

WITH CHAETAE DISTANT.[449]

---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
|25|26|27|28|29|30|31|32|33|34|35|36|
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | |
_A. putris_ |..|——|——|——|——|——|——|..| | | | |
| | | |..|——|——| | | | | | |
| | | | | | | | | | | | |
_A. constricta_ | |——|——|——|——|——|——| | | | | |
| | | | | | | | | | | | |
_A. veneta_ | |..|——|——|——|——|——|——|——| | | |
| | | | | |——|——| | | | | |
| | | | | | | | | | | | |
_A. octoedra_ | | | | |——|——| |——|——|..| | |
| | | | | | |——|——|——| | | |
| | | | | | | | | | | | |
_A. cyanea_ (subsp. | | | | | |——|——|——|——|——|——| |
_profuga_) | | | | | | |——|——|——|——| | |
| | | | | | | | | | | | |
_A. rubida_ | | | | | |——|——|——|——|——|——| |
| | | | | |——|——|——|——|——|——| |
| | | | | | | | | | | | |
_A. mammalis_ | | | | | | |——|——|——|——|——|——|
| | | | | | | | |——|——| | |
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+

{391}WITH CHAETAE PAIRED.

-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | |
_A. rosea_ |..|——|——|——|——|——|——|——|——| | | | | | | |
| | | | | |——|——|..| | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. foetida_ |..|..|——|——|——|——|——|——|——| | | | | | | |
| | | | |——|——|——|..| | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. eiseni_ |..|——|——|——|——|——|——|——|——| | | | | | | |
| | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. caliginosa_ | | | |..|——|——|——|——|——|——|——|..| | | | |
| | | | | | | |——|..|——| | | | | | |
| | | | | | | | | | | | | | | | |
_A. terrestris_ | | | |..|——|——|——|——|——|——|——|——| | | | |
| | | | | | | | |——|——|——| | | | | |
| | | | | | | | | | | | | | | | |
_A. chlorotica_ | | | | |..|——|——|——|——|——|——|——|——|——| | |
| | | | | | | |——| |——| |——| | | | |
| | | | | | | | | | | | | | | | |
_A. georgii_ | | | | |..|——|——|——|——|——|——|——| | | | |
| | | | | | | |——| |——| | | | | | |
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

There are of course other points by which the different species can be distinguished. Colour in a few cases enables a species to be named at once without any further aid. One of the most striking of these cases is the Brandling, so common upon dunghills, and so dear to some anglers. This worm is ringed with brownish purple upon a yellowish ground. The greenish _A. chlorotica_ is often found under stones, and curls itself round into nearly a complete circle when disturbed. _A. cyanea_, of a bluish grey colour, is one of the earthworms very commonly met with in the early morning in London and the neighbourhood. More generally, however, the colour is of a paler or darker red, verging towards and attaining brown, or even blackish brown; and is so variable that nothing in the way of identification can be attempted from the colour alone, even with the most elaborate description.

_Lumbricus_, as already mentioned, is distinguished from all _Allolobophora_ except _A. eiseni_, by the complete dovetailing of the prostomium into the first segment. There are five species in this country which may be thus distinguished by the position of the tubercula pubertatis. The most familiar species is the common _L. rubellus_.

Tubercula pubertatis are on
_L. rubellus_ Hoff. 28, 29, 30, 31
_L. castaneus_ Sav. 29, 30, 31, 32
_L. herculeus_ Sav. 33, 34, 35, 36
_L. papillosus_ Friend 34, 35, 36, 37
_L. festivus_ Sav. 35, 36, 37, 38.

{392}CHAPTER XIV

HIRUDINEA (LEECHES)