It is a somewhat remarkable fact that the domain of human existence is
singularly liable to be intruded upon by lower forms of both animal
and plant life, which may in some cases inflict injury of great extent
upon man’s possessions or even upon his bodily frame. Not so long ago
a foreign member of the beetle-fraternity threatened the interests of
agriculturists in this country, and caused consternation to prevail
throughout the length and breadth of the land. And although the
alarm with which the advent of the insect-intruder was hailed has
now disappeared, agriculturists would inform us that their especial
territory is beset with other insect-enemies which invariably damage
their crops, and which in certain seasons cause the disastrous failure
of many a thriving field. Witness in proof of this the ravages of
the ‘turnip-fly’ and its neighbours, which blight the crops in some
districts to an extent which must be seen to be realised. Or take the
case of the hop-grower, whose favourable prospects largely depend on
the absence of a small species of plant-lice which specially affects
these plants, and which in certain seasons may cause, by their enormous
increase, the total failure of this important crop. Nor do our
insect-foes confine their ravages to growing-crops. When the fruits of
the harvest have been duly gathered in and stored within the granary,
even there they are attacked by minute pests. Numberless insects—flies,
beetles, and other forms—select the granary as a nursery or suitable
place for the upbringing of their young; the larvæ or young insects
feeding on the grain and destroying large quantities by their increase
as well as by their destructive habits. Apart from the domain of
agriculture, however, lower forms of animal and plant life powerfully
affect man’s estate. The growth and increase of lower plants produce
many skin-diseases; and if it be true—already rendered probable—that
epidemics are propagated through the agency of living ‘germs’ which
increase after the fashion of lower forms of life, then it may be
held that we are liable to be attacked on every side by enemies,
insignificant as to size, but of incalculable power when their numbers
are taken into consideration. Parasites of various kinds ravage man’s
flocks and even affect his own health, so that it is perfectly clear
that we do not by any means enjoy any immunity whatever from the
enemies which living nature in its prolific abundance produces, and
which select man and man’s belongings as their lawful spoil.
The animal enemies of man, concerning which we purpose to say a few
words in the present paper, belong to a different sphere from that at
which we have just glanced. Some of the most powerful marauders upon
human territory belong to the Mollusca or group of the true shell-fish,
and present themselves as near relations of the oysters, mussels,
and their allies. The molluscs which become of interest to man in
other than a gastronomic sense, possess, like the famous oyster, a
bivalve shell, or one consisting of two halves. In the first of man’s
molluscan enemies to which we may direct attention, the shell is of
small size, and so far from inclosing the body of the animal, appears
to exist merely as an appendage to one extremity, which for want of
a better term, we may name the head—although, as every one knows, no
distinct head exists in the oyster and its kind. Suppose that from
this head-extremity, bearing its two small shells, a long worm-like or
tubular body is continued, and we may then form a rough and ready, but
correct idea of the appearance of the famous ‘ship-worm’—the _Teredo_
of the naturalist. This animal was first styled the ‘ship-worm’ by
Linnæus and his contemporaries; and in truth it resembles a worm much
more closely than its shell-fish neighbours. As a worm, indeed, it
was at first classified by naturalists. But appearances in zoological
science are as deceptive as they are known proverbially to be in
common life, and the progress of research afterwards duly discovered
beneath the worm-like guise of the teredo, all the characters of a
true mollusc. The long body of the mollusc simply consists of the
breathing-tubes, by which water is admitted to the gills, being
extremely developed, the body proper being represented by the small
portion to which the two small shells are attached.
The importance of the ship-worm arises from the use it makes of these
apparently insignificant shells as a boring-apparatus; and any sea-side
visitor, residing on a coast where an ocean-swell or severe storms
strew the shore with drift-wood, has but to use his eyes to assure
himself of the extent and perfection of the ship-worm’s labours. Pieces
of drift-wood may be seen to be literally riddled by these molluscs,
which live in the burrows they thus excavate. Each habitation is
further seen to be coated with a limy layer formed by the tubular body,
and the boring for the most part is noted to proceed in the direction
of the grain of the wood. The little excavator turns aside in its
course, however, when it meets with a knot in the wood, and an iron
nail appears of all things to be the ship-worm’s greatest obstacle—a
fact which has been taken advantage of, as we shall presently see, by
way of arresting its work of destruction.
Linnæus long ago designated the ship-worm as the _calamitas navium_,
and although perhaps the expression as applied to ships is somewhat
far-fetched—save in the case of broken-down hulks—and utterly
inapplicable in this age of iron, there can be little doubt that
regarded relatively to wooden piles, piers, and like erections, the
ship-worm is unquestionably a calamity personified. So, at anyrate,
thought the Dutch in the years 1731-32, when the teredo began to
pay attentions of too exclusive a nature to the wooden piles which
supported the great earth-works or ‘dikes’ that keep the sea from
claiming the United Provinces as its own. A Dutchman has been well said
to pay great attention to two things which are euphoniously and shortly
expressed by the words ‘dams’ and ‘drams.’ The former keep the sea from
invading his territory, and the latter aid in protecting him personally
from the effects of the perennial damp amidst which he exists. The
ship-worm in the years just mentioned caused terror to prevail through
the length and breadth of the Netherlands, through its appearance in
large numbers in the wooden piles of the dams or dikes. On these piles
the fortunes of Holland may be said to depend; and the foundations of
the Dutch empire might therefore be regarded, correctly enough, as
having been sapped and threatened by an envious enemy in the shape of
a mollusc, and one belonging to by no means the highest group of that
division of animals. The alarm spread fast through the Netherlands,
and the government was not slow to appreciate the danger, or to offer
a reward of large amount for the discovery of any plan which would
successfully stay the progress of such dreaded invaders.
Inventors, it might be remarked, are not slow, as a rule, to accept
invitations of such generous nature; and if report speaks truly, the
office of discriminating between the worthless and feasible projects
which were submitted to the Dutch nation on the occasion referred to,
could not have proved either an easy or enviable one. Then came the
chemists with lotions innumerable, and the inventors of varnishes,
paints, and poisons were in a state of hopeful anxiety. But none of
these preparations was found to fulfil the required conditions, and
the only project which appeared to savour of feasibility was one
which was rejected on account of its impracticable nature—namely that
of picking the teredos from their burrows like whelks from their
shells. The kingdom of Holland thus appeared in a fair way of being
undermined by an enemy of infinitely greater power and one less capable
of being successfully resisted than the Grand Turk, who once upon a
time declared his intention of exterminating the nation with an army
whose only weapons were spades and shovels. But after a period of
unrestricted labour, the ship-worm ‘turned tail’ on the Netherlands,
and disappeared as suddenly as it had appeared, leaving only a few
stragglers to mark the vantage-ground.
Though Britain has not suffered from teredo-epidemics in the same
measure as Holland, there can be little doubt that the ravages of this
mollusc on the timber of our piers and dockyards, cost us a large
sum annually. The stoutest oak is riddled through with the same ease
displayed in perforating the softer pine; and in some of our seaport
towns, especially on the southern coasts, the yearly estimates for
repairs of damage done by the ship-worm form no inconsiderable item
in the government or local expenditure as the case may be. The most
effectual plan for the repression of the teredo and for the prevention
of its work of destruction appears to be that of protecting the exposed
timber by driving therein short nails with very broad heads. These
nails form a kind of armour-casing which is rendered more effective
through the chemical action of the water in producing rust.
Some molluscs, near neighbours of the teredo, and which burrow for
the most part into stone, but occasionally perforate wood, are those
belonging to the Piddock-family—the genera _Pholas_ and _Saxicava_ of
the naturalist—celebrated by Pliny of old as phosphorescent animals.
The _Saxicavæ_ have somewhat elongated shells, by means of which they
burrow in rocks and lie ensconced in their dwelling-places, and whose
perforated rock-homes are eagerly sought after by all who delight in
forming rockeries in their gardens. These molluscs have ere now caused
fears for the safety of Plymouth breakwater, through the persistence
with which they excavated their burrows into the substance of the
stones. And as has been well pointed out, the destructive action of
these molluscs may pave the way for an inroad of the sea; a riddled
mass of rock or stone being rendered through their attack liable to
disintegration from the action of the waves.
A final example of an animal enemy of man which as regards size is
to be deemed insignificant when compared with the teredo, but which
nevertheless adds by its destructive work to our annual expenditure, is
the little crustacean known as the _Limnoria terebrans_, or popularly
as the wood-boring shrimp or ‘gribble.’ This animal belongs to the
group including the familiar ‘Slaters’ or ‘Wood-lice,’ found under
stones and in damp situations, and by means of its powerful jaws
burrows deeply into wood of all kinds. Occasionally, the ship-worm and
gribble have been found at work in the same locality and have committed
ravages of great extent; the latter, on account of its small size,
being more difficult of detection and eradication than its molluscan
neighbour.
The consideration of a subject such as the present, it may lastly be
remarked, possesses a phase not without some degree of consolation
to minds which, if incapable of seeing ‘good in everything,’ may
nevertheless believe in the adjustment and counterbalancing of most of
Nature’s operations. The repression of animal life by parasites may in
one sense prove a gain to nature at large, viewed from a Malthusian
stand-point, although humanly considered, there may be differences
of opinion regarding the applicability of the opinion to the case of
man. But if the ravages of the teredo and its neighbours on the works
of man are to be considered as a veritable affliction, we must not
fail to think also of the service these animals render in clearing the
ocean of vast masses of drift-wood, which, liberated from the mouths
of all the great rivers of the world, would speedily accumulate to
check navigation and impede commerce in many quarters of the world.
The genius of Brunel, which discerned in the manner of the ship-worm’s
burrowing the true method of excavating the tunnel associated with his
name, and which thus improved engineering science by a happy thought
and observation, may also be regarded as bearing testimony to the
consoling fact that there exist few evils which are entirely unmixed
with good.