Fam. 9. Perichaetidae.[[440]]—The Perichaetidae comprise a larger 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.
Fig. 196.—Perichaeta everetti F. E. B. × 1. sp, Spermathecal pores; cl, clitellum; ♀, female pore; ♂, male pore.
A very distinctive feature of Perichaeta—perhaps only of the 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 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 (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 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).
