SENSE ORGANS AND SENSES OF EARTHWORMS
As this is not an anatomical treatise we shall not attempt any
detailed anatomical and histological account of the sense organs in
this group of worms. But a few facts must be given in illustration
and explanation of the senses of touch and sight that the Oligochaeta
undoubtedly possess. These Annelids, unlike their allies the marine
Polychaeta, and even their allies on two other sides, the leeches and
flatworms, have no complexly organised eyes or other sense organs.
They have in fact no organs to which a definite sense can be attached
on histological grounds. There is nothing comparable to an eye or to
the auditory sacs of other low worms. There are only particular cells
of the epidermis, often associated into small groups, and those again
into larger associations of rows of such groups of cells. It is to be
presumed that these modified groups of cells have a sense function;
but no more can be said than that they are doubtless tactile and
also to some extent receptive of the influence of light. True visual
cells have been asserted to exist in earthworms, consisting of cells
of which a part is clear and transparent and has been supposed to
serve as a lens for the rest of the cell which represents a retina.
But belief in the function of these cells is by no means unanimous.
On the other hand many investigations have proved the existence
of groups of epidermic cells of an oval form which are at present
arranged in definite rows upon the segments of both terrestrial and
aquatic forms, which are moreover connected with nerve terminations,
or are at least--according to more modern views--in close contact
with the terminations of nerve fibres. These are furnished often at
their free tips with minute sensitive processes. There is nothing in
the structure of these to associate them definitely with any sense in
particular. They suggest of course tactile organs more than organs
of any other sense. In addition to these are certain problematical
organs which are found in the Eudrilidae and are present in
the members of one section of that group, the section which is
represented by the universally found _Eudrilus_.
These bodies have been compared to a Pacinian body (a sense-organ
found in Vertebrates) and they bear no little resemblance to it.
For each consists of a darkly staining core surrounded by a layer
of cells arranged like the coats of an onion. In any case it would
appear that these bodies must be looked upon as of a sensory nature,
though they do not reach the surface of the body but underlie the
epidermis. Their function must remain purely a matter of guesswork
at present, for nothing to the point is known of the habits of the
Eudrilidae. It has been suggested by Dr Gustav Eisen that these cells
are auditory and serve to warn the worm of the footsteps of birds and
other enemies. That too is his view of certain peculiar but different
cells found in the epidermis of _Pontoscolex_. In the latter
something like an otosome has been found which is certainly lacking
in the Eudrilidae, whatever may be the function of the cellular
epidermic bodies here briefly referred to.
While there is thus nothing very positive to be gleaned from an
examination of the structure of the Oligochaeta as to the senses
which they may possess, experiment has done something towards an
elucidation of their behaviour under stimuli and their reaction
to light and to other forces which come into play during their
lives. There is some evidence that earthworms can see, using that
expression of course in a very broad way. At any rate they react to
changes in the intensity of light. The gross experiment of flashing
a lantern upon earthworms which are reaching out from their burrows
with the tail end inserted in those burrows shows that they have an
appreciation of light. More refined experiments have been conducted
upon the sense of light. Dr Graber used a box with two compartments,
the one of which was dark and the other illuminated with diffused
daylight, _i.e._ not direct sunlight. The worms were allowed equally
free access to both and were examined at the close of every hour, and
their positions noted. The investigator found that on the average the
dark half of the box contained 5·2 times as many worms as the light
chamber, thus indicating a very marked preference for absence of
light.
Not only is this the case, but the same observer proved that
earthworms can distinguish between degrees of intensity of light.
This obviously indicates a more complete 'visual' sense. He
illuminated the light-box of the former experiment with light
admitted through a ground glass screen, thus diminishing its
intensity. The other chamber was left as before but the screen was
removed, thus admitting full daylight. In this experiment the number
of worms in each compartment proved to be different. The results were
not so striking as before, since only rather more than one-half were
found in the more dimly illuminated chamber. It is a well-known fact
that if earthworms are abroad at all from their burrows, it is during
the night that this movement takes place, the numbers decreasing
towards morning though worms are often seen crawling about well after
sunrise. Some experiments have been made in attempt of explanation of
this apparent anomaly. It would appear from these experiments that
while worms are negatively phototropic to strong and moderate light
as has already been pointed out, they are positively phototropic to
very dim light; hence the advent of evening calls them forth from
their burrows. It will be noted that this perception is of very great
advantage to the worm since its more active enemies above ground are
diurnal. It was held originally that the head end of the worm only
was thus sensitive to light; but more recent experiments have shown
that this is not the case, and that all of the body is sensitive.
This disposes of course of the existence of special light-receiving
organs in the anterior part of the worm's body. Not only this, but an
interesting extension of the view has been promulgated. It has been
shown by Prof. G. Parker and a colleague that in the common Brandling
worm, _Allolobophora foetida_, the response to light stimuli was
related to the amount of the body exposed to its influence. This
is very important as showing that the light perception in these
creatures is probably not due to special organs having a limited
position on the body, but is due to collective sense impressions of
many cells scattered over the whole body, the impression being the
greater when the whole body is exposed and less when only parts of
it are exposed. Furthermore, and this has quite another importance,
these observers noted that the reaction effects differed when only a
part of the body was exposed; that they were greater in the front of
the body, less in the middle, and less still at the tail end. Indeed
they found that the reactions in the case of the front end of the
body alone being exposed were rather more than one-third as compared
with those which were shown when the whole body was subjected to the
light stimulus. The fact that the least sensitive region of the body
is the posterior end has, it is true, only been definitely proved in
the case of the worm whose specific name has been mentioned. But
it is possible that others are similarly affected. And it is highly
important to note the prevalent habit among the Tubificidae of lying
with the head end imbedded in the mud of the pool which they inhabit,
while the tail end emerges and waves freely in the flood. The
additional fact that this tail end occasionally bears gills (as in
_Branchiura sowerbii_ and _Phreodrilus branchiatus_) has a collateral
importance not to be mistaken.