Earthworms and Their Allies
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The remaining family of terricolous Oligochaeta found in India is the family Moniligastridae which is practically limited to this part of the world and consists of at least twenty species distributed among the genera Moniligaster, Eupolygaster, Desmogaster and Drawida, the majority belonging to the last-named genus.
This quarter of the globe is therefore inhabited by 18 genera which are certainly truly endemic, and which comprise between them about 120 species. But only four or five genera are peculiar.
The remainder of the Asiatic Continent is not very well explored with regard to its earthworm inhabitants. It seems clear however that the southern and coastal region of China and Japan with the Malay peninsula are really continuous with the mass of islands which lie between India and Australia and form together a tract of land which is characterised by an Oligochaetous fauna differing from both that of India on the one hand and Australia on the other. We shall therefore consider this huge portion of the globe as one region comparable to the other divisions that have been hitherto considered. With reference to the Lumbricidae and Geoscolecidae the same remarks may be made as in the case of India. The indigenous forms of the latter family are to be looked upon as outside of the present survey since they are largely or entirely aquatic forms. Pontoscolex corethrurus, and Eudrilus eugeniae, need not detain us for reasons already amply stated. We now come to the great family Megascolecidae. Of this family the genus Pheretima stands at the head; and of the 200 or so species that have been or can be assigned to this genus all, with merely two or three exceptions, are natives of the Eastern Archipelago and adjoining mainlands of Asia. Of other Megascolecinae the region has yielded the following genera. In Java one species of Woodwardia (W. javanica) has lately been described. It is regarded by Michaelsen however as doubtfully indigenous. And the same remark may be made of Perionyx. Plionogaster, however, with four or five species, is limited, as far as our present knowledge goes, to the Philippines and to neighbouring islands. The Acanthodrilinae and Octochaetinae are totally absent from this part of the world, there being no record even of peregrine species of these sub-families. The sub-family Trigastrinae is not however unrepresented; for of Dichogaster several species occur, such as D. malayana and D. saliens. But these are by no means certainly to be looked upon as real natives of the situations within this area where they occur. This completes the scanty list of genera found in the region under consideration; for one Moniligastrid (M. barwelli) is hardly to be looked upon as indigenous. We have therefore to record here but five genera, of which only two are certainly indigenous and probably also confined to the region; they contain between them certainly two hundred species.
We next come to the Continent of Australia. The earthworm fauna is again quite without Geoscolecidae and of course Lumbricidae. As to the former there are not even doubtful cases like Glyphidrilus of the East; for we find only recorded Pontoscolex corethrurus, a species concerning whose extraordinary powers of migration there is no possible doubt. The same may be said of Eudrilus also recorded from Australia. Here the Megascolecidae are as conspicuous as in the old world generally. We find, however, a great many members of the sub-family Acanthodrilinae. Of the antarctic genus Notiodrilus there are some six species. Microscolex occurs; but the real habitat of this genus is very doubtful. Diplotrema, with one species, D. fragilis, is not only indigenous to, but confined to, Australia.
The most prevalent sub-family is that of the Megascolecinae. Of the genus Pheretima there are two species which may or may not be truly indigenous. Of Plutellus there are forty species; there are over thirty of Diporochaeta, while Notoscolex (with which Dr Michaelsen associates Digaster, Didymogaster, Perissogaster) includes more than forty species, Fletcherodrilus has but one species, Megascolex has seventy species, Woodwardia sixteen, Spenceriella five, and Megascolides seventeen. There are four species of Perionyx which were at one time regarded by Michaelsen as necessitating a new but allied genus Perionychella; the two are now merged. Thus there are not far short of 150 species and eleven genera represented, of which only two are limited to Australia.
Having completed the survey of the central and southern land masses of the globe we next direct attention to the northern land masses, viz. North and Central America on the one hand, and Europe and northern Asia on the other. With regard to America we find besides many species of Lumbricidae the genera Notiodrilus, Microscolex among the Acanthodrilinae, Megascolides and Plutellus of the Megascolecinae, no member at all of the Octochaetinae, a considerable number of species of Ocnerodrilus and sub-genera belonging to the Ocnerodrilinae, a good many species of Dichogaster and at least one of Trigaster among the Trigastrinae, while one sub-family, that of the Diplocardiinae, is only found here and contains two genera Diplocardia and Zapotecia with quite ten species between them of which only one belongs to the last-named genus. There are no Geoscolecids (except the chiefly aquatic Sparganophilus); this family stops short in the West Indies where their presence has been briefly referred to in considering the worms of South America. The vast majority of the genera enumerated here are only found in the warmer parts of the North American continent. We have therefore in this division of the world some nine genera of which at least one, viz. Diplocardia (and its close ally, hardly perhaps to be separated, Zapotecia), is confined to it, while Trigaster only extends as far south as to the West Indies.
Europe and northern Asia, of which Europe alone and certain limited tracts of Asia are at all known, contain all the genera of the family Lumbricidae which are (according to Michaelsen) Eiseniella, Eisenia, Helodrilus, Octolasium, Lumbricus with various sub-genera of Helodrilus, such as Dendrobaena and Allolobophora. Besides these forms, which amount to at least 130 species, we find the genus Hormogaster, with two species, the only genus of the Geoscolecid sub-family Hormogastrinae; there is also a species apiece of the genera Criodrilus and Sparganophilus (sub-family Criodrilinae). But as these are at least largely aquatic they come under another set of rules and are not important in the present survey of the earthworms of the world. It will be thus seen that with these two last exceptions the area in question contains but six genera of which all are peculiar.
We have now completed the survey of the principal land-masses of the globe. We shall conclude with a reference to one of the largest islands of the world, viz. New Zealand, with which will be included a few outlying islands such as the Aucklands, Snares Island, etc. The reason for not ignoring these islands as we have ignored certain other islands of large size, such as Japan, will be apparent from the peculiarity of the earthworm fauna which they show. From New Zealand the Moniligastridae, Geoscolecidae, Lumbricidae, and, of course, the Eudrilidae, are absent, save the ubiquitous Eudrilus. The only family of earthworms which is here represented is that of the Megascolecidae. Of this family the Acanthodrilinae are very well represented. We have at any rate seven species of the genus Notiodrilus of which the bulk are from the small adjacent islands and not from the mainland. The genus Rhododrilus with nine species is actually limited to the New Zealand group and so is an allied genus consisting of two species only, viz. Leptodrilus. This latter genus is confined, so far as present information goes, to the Auckland and Campbell islands. Dinodriloides with two species is also limited to New Zealand and to the North Island. Maoridrilus with some ten species is another native and restricted genus. So too is the allied Plagiochaeta with numerous setae on each segment but with the alternating and single nephridial pores of Maoridrilus. There are several species of Plagiochaeta of which one has been lately regarded by Dr Michaelsen as really falling within the otherwise Indian genus Hoplochaetella, while for another he has formed the genus Pereiodrilus. Neodrilus with but one species N. monocystis is another peculiar New Zealand genus. The family Octochaetinae contains only four genera, of which one, viz. Dinodrilus (with four species), is limited to New Zealand, while Octochaetus has about five representatives. Hoplochaetella (if Dr Michaelsen's surmise referred to above be correct) has one species in New Zealand. The Megascolecinae are less numerous than the Acanthodrilinae, but there are ten species of the genus Megascolides (which includes Benham's genus Tokea), perhaps seven species of Diporochaeta, and two other species which Michaelsen has removed from the genus Diporochaeta and placed in Spenceriella. One Plutellus (which however may have been introduced) completes the New Zealand Megascolecines. We have therefore in this part of the world fifteen genera including between them some 58 species; eight of the genera are peculiar to the islands.
From this brief statement of facts some inferences of interest can be drawn. It is in the first place plain that every part of the world except the extreme north and south has a considerable fauna of earthworms. The one exception would appear to be the northern part of the North American continent. Here we meet with members of the family Lumbricidae which are however species that are met with in the Euro-Asiatic province and are thus to be regarded as possibly later immigrants introduced probably by man. Thus temperature short of a constantly frozen condition of the ground is not a bar to the existence of earthworms. Even a freezing of the ground for lengthy periods is not a complete obstacle to the existence of those Annelids; for I have myself received examples of Lumbricidae from the arctic island of Kolguev. Moreover the temperate regions would seem to be as fully populated in the way of individuals, and even of species, as are the tropical regions. Indeed as to individuals it seems that the temperate regions are more fully supplied than much of the tropics. This however is not quite the object of the present section to discuss. We are here concerned with the relative frequency of genera and species. There are according to a recent estimate of the Rev. H. Friend some forty species recognisable in Great Britain. And as already has been stated the earthworms of Europe amount to perhaps 130, – at any rate well over one hundred. In tropical America there are hardly more. But in the latter case the number of genera is very greatly in excess of that of Europe. We cannot however say that an abundance of generic types is quite characteristic of the tropics. For the Eastern Archipelago, though rich in species, is but poor in genera, not possessing more than half a dozen or so. And on the other hand the temperate climate of New Zealand has produced a very considerable series of genera, much more than those of the islands of the East and nearly as many as those of, for instance, Central America and the West Indies.
This conclusion is in its turn contradicted by the conditions observable in Chili and the temperate regions of South America, where the number of species is large but the number of genera small. In short no general laws, in the present state of our knowledge, can be laid down as to the connection between species and genera on the one hand and climatic conditions on the other. In this department of our subject we cannot do more than has already been done, i. e. to state the actual facts. One is tempted in comparing the rich fauna of tropical Africa with the very limited fauna of Madagascar to associate a richness of types with extent of land surface. In the two cases cited this conclusion is obvious. It may also be extended – if we confine ourselves to species and not to genera. For the two great islands of New Zealand have not between them more than fifty species of earthworms, while Australia has four or five times that number. It will be noticed however that we cannot associate poverty of generic differentiation with limited land masses; for New Zealand has a large number of generic types, very many more than the huge Euro-Asiatic tract of continent.
The Range of Genera
We have seen, and shall again refer to the fact, that individual species of earthworms have not as a rule a range over a great extent of country, save only in those cases such as Pheretima heterochaeta which belong to that physiological section of these worms called 'peregrine' forms; these appear to possess some means of extending their range by the assistance of man which is denied to other forms. Apart from these instances, which do not come under the present category, it is only Lumbricus and its immediate allies, Helodrilus, etc., of which certain species are found to exist over wide tracts of land. There are however many genera which have a wide range and which may be contrasted with others in which the range is very limited. The two extremes are moreover connected by forms with an intermediate range. There is no doubt whatever that the genus with the widest range is Notiodrilus of which species are found throughout the antarctic region, viz., in Patagonia, the islands of the Antarctic Ocean, the Cape of Good Hope, New Zealand, and also further to the north, sometimes even to and beyond the tropics in America, Australia, and Africa. There is no other genus of which the genuine extension (i. e. not in any way due to man) is so great as this genus Notiodrilus. And this fact gains much significance from the now generally accepted view that in its anatomical structure Notiodrilus comes near to the original type of earthworm.
Perhaps the next most widely distributed genus is Helodrilus of the family Lumbricidae which occupies Europe and Asia to the extreme east, and is thought also to be indigenous to certain parts of North America. But this range, though equally wide perhaps in mileage, is less impressive than that of Notiodrilus, since the land areas inhabited by the genus are continuous – almost so if we accept North America as its real habitat. Here we have a case precisely the opposite of that of Notiodrilus; for while there are reasons for regarding Notiodrilus as an ancient form of Lumbricid, there are equally good reasons for regarding the Lumbricidae as the most modern family of earthworms.
To find other instances of widely spread genera we must recur to the great family Megascolecidae. There are practically no Geoscolecidae which have a really extensive range. The only instances are Criodrilus and its ally Sparganophilus which occur in America, whether North or South, and in Europe; but as these forms are at least largely aquatic the facts are not quite comparable with those now under consideration.
The genus Dichogaster (which includes as synonyms Benhamia, Millsonia, Microdrilus) is unquestionably indigenous to tropical Africa and certain parts of America including the West Indies. It has been also met with in the East; but as the species there occurring, such as for example the species originally described by myself as Microdrilus saliens, are of small size, an accidental introduction is quite possible, and it is by no means certain that it has not occurred. In any case the genus is known to possess species which are undoubtedly to be reckoned among peregrine forms – such as D. bolavi, which has turned up in Europe. Gordiodrilus and also Ocnerodrilus with its sub-genera have very much the same range as has Dichogaster. It is to be noted however that these forms are circumtropical, and that their distribution is thus less continuous than that of Notiodrilus; they do not however show the markedly discontinuous range of certain other genera of Megascolecidae. For instance Octochaetus is well known from New Zealand, and, not occurring in the intermediate tracts, is again met with in India. Hoplochaetella is believed by Michaelsen to present us with another precisely similar instance. Then also the genera Woodwardia and Notoscolex are to be found in Australia and again (absent from the immense tract lying in between) in Ceylon. Megascolex has much the same range, showing also this marked and remarkable discontinuity. Stranger still, perhaps, is the range of Plutellus and Megascolides, of which the former, chiefly found in Australia and Tasmania, not only extends its habitat to Ceylon but also to North America; it is there represented by Eisen's species Argilophilus marmoratus, referred by him, and not unnaturally, to a distinct genus, but placed by Michaelsen in Plutellus. Megascolides is Australian and from the North Island of New Zealand, where its species were regarded by Benham as of a distinct genus, Tokea. There is also one form, Megascolides americanus, in the western region of North America.
The two genera Yagansia and Chilota, closely related to Notiodrilus, have a range which is short of that of Notiodrilus, and we shall see later that there are reasons for regarding these genera as derived from Notiodrilus. They are met with only in the south of South America, and in the Cape of Good Hope region.
The range of Microscolex seems to be much the same as that of Notiodrilus; but it is a little uncertain how far the genus is really autochthonous in the countries where it occurs; and in any case it differs from Notiodrilus in occurring in Europe, where the species has been named for a long time M. phosphoreus. We do not positively know whether this is 'peregrine' in Europe or not.
The range of the antarctic Acanthodrilinae is in a sense continuous; for they argue the former northward extension of the antarctic continent and in any case they occupy neighbouring land masses. In Octochaetus and Plutellus the case is different and one of real discontinuity. There are however cases of wide range which is also actually continuous and such is afforded by the genus Pheretima. This genus appears to be possibly indigenous to Australia; in any case it reaches from the Solomon Islands on the east to India towards the west, being found in all intermediate continents, while it reaches Japan on the north side of this large area.
There are other genera which extend their range over a considerable area, but which are not so widely distributed as these which we have just been considering. Thus Diporochaeta is chiefly Australian but also reaches even the South Island of New Zealand and the southward lying antarctic islands. Desmogaster and Eupolygaster among the Moniligastridae range from Burmah in the east to Sumatra and Borneo further east, though they are not recorded from intermediate islands. Perionyx is found in Burmah, India, Zanzibar, Sumatra, and Java. There are other examples of genera which have much the same range as those enumerated. Finally there are those which are confined to one land mass and very often to a restricted region of that. Thus Kynotus is confined to Madagascar, all the genera of Eudrilidae to tropical Africa, some of them, e. g. Beddardiella and Euscolex, to very limited tracts, others to wider or less wide areas in that continent. Maoridrilus is only found in New Zealand, to the South Island of which also is confined the genus Neodrilus. To the Cape region of Africa is limited Microchaetus; and to a belt running across the northern part of the tropical region and extending down the Nile, the remarkable, partly aquatic, Alma.
As a kind of appendix to these facts and conclusions we shall next deal with certain widely spread forms that have been already referred to, with the range of different genera over great land masses of the world, and with the earthworms of oceanic islands.
CHAPTER VI
PEREGRINE FORMS
Dr Michaelsen has used this term to describe those species which possess some powers of migration over the sea, denied to the majority of worms, and probably due to the direct interference of man. Thus we find in collections of earthworms from various parts of the world not only examples of forms which do not come from other parts of the world, but also a few which occur in many or even most of such collections. It is for example to be actually expected that a collection of earthworms made in South America, the Philippine Islands, or Australia will contain examples of the apparently ubiquitous Pontoscolex corethrurus. This is what has actually happened in cases of which I have personal knowledge, as well as in many others recorded in the literature of the subject. I have myself received this worm from the three parts of the world mentioned, and also from Hawaii. Others have increased its known range to other parts of the South American continent, to Central America, the West Indies, the islands of Sumatra, Java, Borneo, Celebes, Mauritius, and Madagascar, etc. It is in fact found everywhere in the tropics. With this range may be contrasted that of another genus of the same family (Geoscolecidae), viz. Kynotus, which, though consisting of many species, is not found outside of the Madagascar district. It should be added that Pontoscolex does not appear to contain more than two species, the one not mentioned in the above survey of its distribution being P. insignis of Kinberg, which is apparently the same as P. liljeborgi of Eisen, and is limited to certain parts of America.
Before attempting to grapple with the remarkable facts implied by the distribution of this genus, it will be well to survey the whole group of Oligochaeta and to reduce to as short a space as possible the total series of facts which are of the same nature.
A case, even more striking than that of Pontoscolex, is afforded by the Eudrilid genus Eudrilus. As with Pontoscolex there are two species of this genus, one, E. pallidus, being confined to West Africa, the remaining one, E. eugeniae, being world-wide in range. This latter species has received the following names, viz. E. decipiens, E. lacazii, E. peregrinus, E. sylvicola, E. boyeri, E. jullieni, E. erudiens, and E. roseus; they appear to be all synonyms of the name originally given by Kinberg who however did not recognise the distinctness of the form as a genus. It is now known as Eudrilus eugeniae. The variety of names given to supposed different forms (for two of which I am myself responsible) is due to the fact that in earlier days when nothing was known about the geographical distribution of this group of animals it was thought by no means unreasonable that a given genus represented by several species should range over the globe. This fact coupled with imperfect description of structural details led to the multiplication of supposed species, a position which is no longer tenable. This worm is quite as abundant in gatherings from all parts of the world as is Pontoscolex corethrurus; and in addition to the countries inhabited by the latter, Eudrilus eugeniae has been met with in New Caledonia: tropical Africa is probably its original home.
The two families that have been hitherto considered offer no further instances, among their many species, of worms with so wide a range as those just dealt with. There are indeed one or two forms, e. g. Criodrilus and Glyphidrilus, which have a considerable range though not nearly equalling that of Eudrilus and Pontoscolex. These are, however, aquatic forms and the range of aquatic forms is determined as far as we can see by a different series of causes to that of terrestrial forms, which are referred to later.