MODE OF LIFE
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.
[2] 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.