We have now completed a brief survey of the general characters of the group of the Oligochaeta which will at least serve to impress upon the reader the fact that these animals are somewhat diverse in structure, and that even as regards outward appearance it is not difficult to distinguish a large number of different types. These facts become all the more remarkable when we reflect upon the very similar conditions which surround all the species of earthworms. It is not clear how far the influence of the soil differs in a tropical forest in South America and in Central Africa. With divergent external conditions anatomical differentiation becomes more accountable. But in this case we have a lavish anatomical variation which would appear to have no connection with any kind of need that we can as yet appreciate. Comparing the Terricolous Oligochaeta with other large groups of the animal kingdom, all or nearly all the members of which lead a closely similar life, such as the frogs and toads, or the land mollusca, or snakes, we get a much wider range of structural change in the Oligochaeta than in any of these.
We shall now consider their mode of life and their relations to the environment.
The mode of life of earthworms seems at first sight to need no special chapter or section. They simply live in and beneath the soil, leaving it at times to wander over the surface especially at night and during wet weather. But there are a number of species which habitually lead an aquatic life and whose characteristics therefore demand consideration.
Aquatic Earthworms.
Although it is perhaps somewhat of a contradiction to speak of aquatic earthworms the collocation of words will serve to emphasise the fact that there are species of Oligochaeta belonging to the tribe Megadrili or terrestrial group, which are as purely aquatic in their habits as is a Tubifex or Limnodrilus. In such cases we may fairly assume rather a return to an aquatic life than the persistence of such a habit. For we do not find among these genera and species much evidence of particular resemblance with the purely aquatic familes of Oligochaeta. It is therefore particularly interesting to examine into the characteristics of these water-living genera; for we may expect to be able to deduce from them some hints as to what characters are really to be associated with the purely aquatic life. We can in fact hope to differentiate between adaptive and fundamental characters in these animals.
These secondarily aquatic species can be referred to two categories. There are examples of particular species which differ from their congeners in being aquatic; and there are whole genera, even sub-families, which are exclusively, or very nearly so, aquatic in habit. The former division need not detain us; for the actual occurrence of the worms in fresh water instead of upon dry land may be a temporary affair and not a mark of habitual sojourn. Thus I myself found the British and European earthworm Eiseniella (Allurus) tetraedrus in the River Plym in Devonshire, while it has been generally met with upon land. The Patagonian and Falkland Island species Notiodrilus aquarum dulcium was so called on account of its having been collected in fresh water. But its near ally N. georgianus (which is perhaps even identical with it) was found on the sea shore in the same region of the world. While the differences which the small species of Notiodrilus shows from other purely terrestrial members of the same genus are trifling, further information may prove that this case is on all fours with that of Eiseniella referred to above. There are plenty of similar instances which I shall not enumerate.
We may now therefore pass on to the second category. These examples are obviously much more important because they are of worms which appear to be wholly aquatic, or very nearly so, and which belong to definite genera easily distinguishable as such from their allies. The examples are not however very numerous. And they belong practically exclusively to the family Geoscolecidae, a family which, it will be seen later, is confined to South America, South Africa, Madagascar, certain parts of India and Burmah and of Europe. It is not a family which has reached the greater part of the East or which has been carried to the Antarctic parts of the globe. It is furthermore very important to bear in mind that there are reasons for regarding this family Geoscolecidae as one of the more modern branches of the Oligochaeta; this latter statement tends to prove that the aquatic life is, as already suggested, a secondary matter in these worms, and is not due to their belonging to an ancient race which has never left the waters of the land.
A very interesting fact offers itself first of all in considering this family of earthworms. The Geoscolecidae are one of the only division[2] of the Oligochaeta terricolae which are generally found to be without those characteristic series of pores in the middle line of the back known as the dorsal pores. They are thus eminently suited for an aquatic life; for it is to be supposed from the fact that the purely aquatic 'Limicolae' are also without these pores that their existence is prejudicial to a water-living worm. Indeed the entrance of water into the body-cavity would presumably be dangerous to the worm. The Geoscolecidae are thus already marked out, as it were, for an aquatic life. No modification is here necessary for them. It is also to be noted in this connection that various species of the genus Notiodrilus to which reference has been made as a partly aquatic genus have no dorsal pores. They too are thus fitted for at least an amphibious life.
In the Eudrilidae also these pores are very frequently absent.
The rule however regarding the absence of dorsal pores in the Geoscolecidae is not absolute. A few species and among them two species at any rate of the aquatic genus Sparganophilus have a few pores between some of the anterior segments which have been spoken of as 'neck pores.' They are not, it is to be believed, of a different nature from the generally distributed dorsal pores of other worms but are in fact limited to the 'neck' region.
There are no other obvious characters of the family Geoscolecidae as a whole which might be regarded as fitting them for a purely aquatic life.
Of this family one entire sub-family, the Criodrilinae, is very nearly purely aquatic in habit. Two genera, viz. Callidrilus and Glyphidrilus, out of another sub-family, Microchaetinae, which contains perhaps five other genera, are also aquatic in their mode of life. In examining into the characters of the various aquatic species with a view to searching for common characters which might be put down to modifications induced by the aquatic life, there are two or three which arrest attention. In the first place the Criodrilinae never possess a well-developed gizzard, having at most a rudimentary gizzard, or even two. However this character is not of universal applicability, for both Callidrilus and Glyphidrilus have got a gizzard and a strong one. These later genera however have no calciferous glands or oesophageal pouches of any description, structures which are also absent among the Criodrilinae. It will be remembered that the purely aquatic families, Tubificidae, Lumbriculidae, etc., rarely show signs of a gizzard and rarely possess oesophageal pouches of any kind. In view of the fact that in the case of a life in fresh water no calcareous salts are necessary to resist the acids of the soil, and that the mud passed through the alimentary canal is already finely divided, it is not surprising to find gizzard and calciferous glands absent or rudimentary.
Another not unusual feature among these aquatic Geoscolecidae is the quadrangular form of the posterior end of the body. This is shown—as its specific name denotes—by Glyphidrilus quadrangulus, by species of Alma and in all the species of the genus Criodrilus. It is to be noted in this connection that a species of the partly aquatic Eiseniella has been named tetraedrus on account of precisely the same phenomenon. In these cases it is the posterior part of the body which is thus quadrangular; the anterior segments down to the ninth in Criodrilus being rounded in the usual Oligochaetous fashion. As the paired setae are apt to lie in the four projections of the quadrangular body, one is tempted to see in this arrangement of structures a faint approach to the dorsal and ventral parapodia of the marine worms, and in any case it seems possible that by this means the worms can cling more effectively and continuously to the stems and leaves of aquatic plants among which they so largely live.
It is a very remarkable fact that in the genera Criodrilus and Alma the vent is dorsal in position instead of being surrounded as in earthworms generally by the last segment of the body. This fact might be put down to the near affinity between these two genera, were it not for the fact that Glyphidrilus shows precisely the same state of affairs. These facts gain additional significance in my opinion from the fact that among the leeches which are now universally admitted to be allies of the earthworms the same position of the vent is met with. This abnormal position of the end of the alimentary canal may thus be fairly quoted in connection with structures modified by the aquatic life.
Finally, and this seems to be very important, the only genus among the Megadrili which possesses gills is the Nile worm Alma nilotica.
Marine Species.
There are a few, but relatively speaking very few, worms of the order Oligochaeta which lead a marine life. And of these the majority are shore forms not extending into the waters of the sea. The most salient example, at any rate the best known perhaps, is the genus Pontodrilus, the name of which fixes its habitat, and was naturally given to it on that account. It was originally found on the sea shore of the South of France, and I have myself examined examples from Nice. The worm lives among bunches of sea-weed cast up by the sea, and which are thoroughly salt. Besides the two forms that have been met with in this Mediterranean region but which are united by Dr Michaelsen into but one species, other Pontodrilus have been described from so many and such diverse parts of the world as the following. The West Indies (Bermudas, Jamaica etc.), the coasts of South America, of both West and Eastern Africa, the Red Sea, Christmas Island near Java, Sharks Bay in West Australia, the Hawaii Archipelago, Celebes, South West Australia etc. In fact there is no great tract of the ocean excepting the antarctic region where this genus is not to be found. It is possible however that this latter statement is not correct and that New Zealand ought to be added. But the species described from those islands, viz. Pontodrilus lacustris, is not a marine form at all as its specific name denotes; nor is it quite certainly to be included in the genus. On the other hand a form from the Chatham Islands in the same quarter of the globe, described originally as Pontodrilus chathamensis, is to be referred to the antarctic region. Altogether some dozen species of Pontodrilus have been described by different naturalists; but quite recently Dr Michaelsen has reduced these to three only, which are P. bermudensis (F. E. B.), P. litoralis (Grube) and P. matsushimensis (Izuka), with the doubtful addition of P. lacustris already referred to. Whatever may be the ultimate verdict upon this question of species it is clear that the genus is widely spread upon the sea shores of the world and that forms from different regions show some fixed variations, which others may eventually agree with their original describers in regarding as definite species.
It cannot be said that any salient characters in the organisation of these worms mark them out from either terrestrial or fresh-water Oligochaeta. There are no such important variations of structure as can be seized upon to characterise them as inured to salt water. The genus agrees with many aquatic forms in the fact that the nephridia are not present in the earlier segments of the body, not indeed putting in an appearance until about the thirteenth segment or even later. They are thin delicate worms; but there is nothing distinguishing about this, while the feeble or absent gizzard is a character which is really difficult to correlate with habitat. Still we have here a whole genus which is marine in its habit. Among the Megadrili or earthworms there are not many other examples of these 'euryhaline' forms as they have been named. On the shores of Patagonia however and Kerguelen shore-living species of the mainly antarctic genus of earthworms Notiodrilus have been met with. And there are a few allied cases among the antarctic genera of Acanthodrilinae.
In addition to these terrestrial forms there are a few limicoline genera which are partly marine in their habit. Thus several species of the prevalently arctic and antarctic family Enchytraeidae are shore living. There are also marine Tubificids such as Clitellio arenarius and Tubifex ater (not uncommon on British shores), marine Lumbriculids and a marine Naid from the Italian coast. These forms show no great differences from their fresh-water allies.
Earthworms originally purely aquatic
animals.
The very name Earthworm, so distinctive as it is of the habitat of these animals, seems to have been expressly invented in order to crystallise into one word the remarkable distributions of these creatures. They are with very few exceptions the most purely terrestrial animals that we know. There are a few Mammals like the mole and several underground Amphibians and Snakes in the tropics which share this habitat with the worms, probably because they chiefly prey upon them. But there is no group of animals that is characterised by a subterranean existence in the way that earthworms are. For we cannot put burrowing animals, such as the prairie dog and many rodents, into the same category. These make and seek their burrows for protective purposes and in order to bring forth their young in security. They do not feed beneath the surface of the ground or pass their entire lives in that situation. We have already in a previous chapter dealt with such exceptional forms of earthworms as do not lead an entirely subterranean existence; but as was pointed out in chapter I these exceptions are but few and the immense bulk of earthworms fully justify their name.
Nevertheless there are many arguments which tend to show that these purely land-dwellers have grown out of exclusively water-dwellers and even that the change from the one mode of life to the other has been accomplished comparatively recently. For there are here and there vestiges of structures which seem only fitted for an aquatic life; and in other cases structural changes have commenced which would appear to be in definite relation to the underground mode of life prevalent to-day. Let us consider for a moment the differences which obtain between the conditions of life in water or in soft mud at the bottom of pool or river, and those which are undergone by a dweller in stiff soil or vegetable dÉbris. In the first case the medium is fluid or at most very soft, while the soil is at least stiffer and harder to traverse.
Secondly the transition between the very bottom of a pool and the top layers of the water is more or less gradual, while the stiff soil ends abruptly in the tenuity of the atmosphere.
A third point of difference is doubtless the smaller supply of readily available oxygen in the still pools and even rapid rivers, which in certain stagnant pools and in the bottom waters of deep lakes must produce a very vast difference in physiological conditions.
We have already dealt with the characteristics of the aquatic genera of earthworms, not only in detailing the general characters of the families which are found in this situation but also in studying the features which earthworms show in those cases where they have reverted to an aquatic mode of life. It remains in the present section to attempt to descry in the purely terrestrial forms the remnants of adaptations to an aquatic life which are no longer of service to them.
It is a noteworthy fact, that the continuous circle of setae which is met with in certain earthworms is by no means a character of such classificatory importance as it was at one time, perhaps, thought to be. It is true that we meet with this character in the genera Megascolex and Pheretima which are not very far from each other in the system and are at any rate members of the same sub-family, the Megascolecinae. But we also find the continuous circle of setae well developed in Plagiochaeta which is not so near to Pheretima, and an approach towards it in Dinodrilus and Dinodriloides which are equally remote perhaps from both Pheretima and Megascolex on the one hand and Plagiochaeta on the other. Still it may be urged that all of these genera are at least members of the family Megascolecidae and that the question of a character which thus merely shows affinity is not yet eliminated. It is therefore of particularly great importance that Dr Cognetti de Martiis should have met with the South American genus Periscolex which, undoubtedly a member of a totally distinct family, the Geoscolecidae, yet shows the same complete circle of setae. The reason for dwelling upon this particular anatomical character in the present connection is because it would seem to be a character specially suited to an underground life where there is an equal pressure all round the body and where progression would seem therefore to be best attained by a continual leverage round the circular body. And this view is strengthened by the sporadic occurrence of this modification in different families. We thus come to the conclusion that the opposite state of affairs is a remnant of an aquatic life, a conclusion which it is the object of the present section to discuss. More than this, it would seem that an equal disposition of the two bundles of setae on each side of the body was a less modified state of affairs than the restriction of the two bundles or pairs of setae to the ventral surface, such as occurs for example in the genus Dichogaster and which is very obvious in some of the larger-sized members of this extensive genus. For the restriction of the setae to the ventral surface obviously favours progression upon a surface and not through a medium. And it is only among the terrestrial Oligochaeta that this mode of progression occurs. It might also be urged, and with some reason, that the retention of rather longer setae upon the clitellum in the Lumbricidae and Geoscolecidae, and the possession of equally long or in many cases much longer setae corresponding to one of the two pairs of setae of the generative segment in certain Megascolecidae, is a feature in which an aquatic condition—so to speak—is retained. The setae would represent a vestige of the general presence of long setae over the body generally such as is convenient or at least not inconvenient to an Annelid living in water or soft mud. But probably it will be thought the modified genital setae are a recent development and not a retention.
There is no more thoroughly terrestrial family of earthworms than that of the Moniligastridae and yet this family in its general anatomical characters shows many points of likeness to aquatic forms as has been now pointed out by many observers. It is true that these characters are not those which might be associated at first sight with an aquatic life. But none the less they are characteristic of most of the families which live in the waters of the earth. Thus Moniligaster and its allies (Eupolygaster, Drawida, etc.) have quite short sperm ducts which open on to the exterior at furthest in the segment next to that in which their internal funnel lies. Again the simple structure of the terminal gland into which they open and which in its turn opens on to the exterior is very like that of such a family as that of the Lumbriculidae. Another fact is the simple undivided cavity of the sperm sacs which is unlike that of typical earthworms but again like that of all of the Limicolous families. We may fairly see in these worms evidence of origin from aquatic ancestors. Evidence of the same nature, i.e. not as showing the retention more or less of anatomical characters commonly associated with a life in water, but as affording indirect evidence of an origin from actually aquatic forms, is to be seen in certain members of the families Geoscolecidae and Eudrilidae. In both of these it not infrequently happens that the sperm sacs are but a single pair and that that pair consists of sacs of extraordinary length. Thus in Trichochaeta (or Hesperoscolex) barbadensis Miss Fedarb and I have shown that the long thin sperm sacs extend through no less than 109 segments, which is vastly in excess of the length of those of the majority of earthworms in which they are most commonly limited to a single segment. In the same way the Eudrilid worm Polytoreutus magilensis has a pair of long and thin sperm sacs which extend through some fifty segments. This elongation of the sperm sacs in the ripe worms is a very common feature of the Limicolous genera.
