THE PHYSICAL CONDITIONS OF THE ABYSS
It is not surprising that the naturalists of the early part of the
present century could not believe in the existence of a fauna at the
bottom of the deep seas.
The extraordinary conditions of such a region—the enormous pressure, the
absolute darkness, the probable absence of any vegetable life from want
of direct sunlight—might very well have been considered sufficient to
form an impassable barrier to the animals migrating from the shallow
waters and to prevent the development of a fauna peculiarly its own.
The fragmentary accounts of animals brought up by sounding lines from
great depths might, it is true, have thrown doubts on the current views;
but they were not of sufficient importance in themselves, nor were the
observations made with such regard to the possibility of error, as to
withstand the critical remarks that were made to explain them away.
The absence of any evidence obtained by accurate systematic research,
together with the consideration of the physical character of the ocean
bed, were quite sufficient to lead scientific men of that period to
doubt the existence of any animal life in water deeper than a few
hundred fathoms.
We now know, however, that there is a very considerable fauna at
enormous depths in all the great oceans, and we have acquired, moreover,
considerable information concerning some of those peculiar physical
conditions of the abyss that fifty years ago were merely matters of
speculation among scientific men.
The relation between animals and their environment is now a question of
such great interest and importance that it is necessary in any
description of the fauna of a particular region to consider its physical
conditions and the influence that it may be supposed to have had in
producing the characteristics of the fauna.
The peculiar physical conditions of the deep seas may be briefly stated
to be these: It is absolutely dark so far as actual sunlight is
concerned, the temperature is only a few degrees above freezing point,
the pressure is enormous, there is little or no movement of the water,
the bottom is composed of a uniform fine soft mud, and there is no plant
life.
All of these physical conditions we can appreciate except the enormous
pressure. Absolute darkness we know, the temperature of the deep seas is
not an extraordinary one, the absence of movement in the water and the
fine soft mud are conditions that we can readily appreciate; but the
pressure is far greater than anything we can realise.
At a depth of 2,500 fathoms the pressure is, roughly speaking, two and a
half tons per square inch—that is to say, several times greater than the
pressure exerted by the steam upon the piston of our most powerful
engines. Or, to put the matter in other words, the pressure per square
inch upon the body of every animal that lives at the bottom of the
Atlantic Ocean is about twenty-five times greater than the pressure that
will drive a railway train.
A most beautiful experiment to illustrate the enormous force of this
pressure was made during the voyage of H.M.S. ‘Challenger.’ I give the
description of it in the words of the late Professor Moseley.
‘Mr. Buchanan hermetically sealed up at both ends a thick glass tube
full of air, several inches in length. He wrapped this sealed tube in
flannel, and placed it, so wrapped up, in a wide copper tube, which was
one of those used to protect the deep-sea thermometers when sent down
with the sounding apparatus.
‘This copper tube was closed by a lid fitting loosely, and with holes in
it, and the copper bottom of the tube similarly had holes bored through
it. The water thus had free access to the interior of the tube when it
was lowered into the sea, and the tube was necessarily constructed with
that object in view, in order that in its ordinary use the water should
freely reach the contained thermometer.
‘The copper case containing the sealed glass tube was sent down to a
depth of 2,000 fathoms and drawn up again. It was then found that the
copper wall of the case was bulged and bent inwards opposite the place
where the glass tube lay, just as if it had been crumpled inward by
being violently squeezed.
‘The glass tube itself, within its flannel wrapper, was found when
withdrawn, reduced to a fine powder, like snow almost. What had happened
was that the sealed glass tube, when sinking to gradually increasing
depths, had held out long against the pressure, but this at last had
become too great for the glass to sustain, and the tube had suddenly
given way and been crushed by the violence of the action to a fine
powder. So violent and rapid had been the collapse that the water had
not had time to rush in by means of the holes at both ends of the copper
cylinder and thus fill the empty space left behind by the collapse of
the glass tube, but had instead crushed in the copper wall and brought
equilibrium in that manner. The process is exactly the reverse of an
explosion, and is termed by Sir Wyville Thomson an “implosion.”’
It is but reasonable to suppose that the ability to sustain this
enormous pressure can only be acquired by animals after generations of
gradual migrations from shallow waters. Those forms that are brought up
by the dredge from the depths of the ocean are usually killed and
distorted by the enormous and rapid diminution of pressure in their
journey to the surface, and it is extremely probable that shallow-water
forms would be similarly killed and crushed out of shape were they
suddenly plunged into very deep water. The fish that live at these
enormous depths are in consequence of the enormous pressure liable to a
curious form of accident. If, in chasing their prey or for any other
reason, they rise to a considerable distance above the floor of the
ocean, the gases of their swimming bladder become considerably expanded
and their specific gravity very greatly reduced. Up to a certain limit
the muscles of their bodies can counteract the tendency to float upwards
and enable the fish to regain its proper sphere of life at the bottom;
but beyond that limit the muscles are not strong enough to drive the
body downwards, and the fish, becoming more and more distended as it
goes, is gradually killed on its long and involuntary journey to the
surface of the sea. The deep-sea fish, then, are exposed to a danger
that no other animals in this world are subject to, namely that of
tumbling upwards.
That such accidents do occasionally occur is evidenced by the fact that
some fish, which are now known to be true deep-sea forms, were
discovered dead and floating on the surface of the ocean long before our
modern investigations were commenced.
Until quite recently, every one agreed that no rays of sunlight could
possibly penetrate the sea to a greater depth than a few hundred
fathoms.
Moseley says that ‘probably all is dark below 200 fathoms excepting in
so far as light is given out by phosphorescent animals,’ and Wyville
Thomson speaks of the ‘utter darkness of the deep-sea bottom.’
Within the last few years a few authors have maintained that it is quite
possible that a few rays of sunlight do penetrate even to the greatest
depths of the ocean—a view mainly based on the fact that so many
deep-sea animals possess extremely perfect and complicated eyes and very
brilliant colours. Verrill says: ‘It seems to me probable that more or
less sunlight does actually penetrate to the greatest depths of the
ocean, in the form of a soft sea-green light, perhaps at 2,000 or 3,000
fathoms equal in intensity to our partially moonlight nights and
possibly at the greatest depths equal only to starlight. It must be
remembered that in the deep sea far away from land the water is far more
transparent than near the coast.’ Packard is of a similar opinion.
There seem to me to be very slight grounds for this view. The fact that,
comparatively speaking, shallow-water fish avoid nets that are rendered
phosphorescent by entangled jelly-fish does not justify us in assuming
that deep-sea fish avoid regions where there are phosphorescent
Gorgonians or Pennatulids. It is not by any means certain that fish
avoid sunken nets on account of their phosphorescence. Most fish
possess, as is well known, a very acute sense of smell, and it is very
probable that they avoid such nets on account of the putrid odours of
the dead animals that remain attached to them.
Nor is there much strength in the further argument that it can hardly be
possible that there can be an amount of phosphorescent light regularly
evolved by the few deep-sea animals, having this power, sufficient to
cause any general illumination, or powerful enough to have influenced,
over the whole ocean, the evolution of complex eyes, brilliant and
complex protective colours, and complex commensal adaptations.
We have no sound information to go upon to be able to judge of the
amount of light given off by phosphorescent animals at the bottom of the
deep sea. The faint light they show on deck after their long journey
from the depths in which they live to the surface may be extremely small
compared with the light they give in their natural home under a pressure
of 2½ tons to the square inch. The complex eyes that many deep-sea
animals exhibit were almost certainly not evolved as such, but are
simple modifications of eyes possessed by a shallow-water ancestry.
The more recent experiments that have been made, tend to show that no
sunlight whatever penetrates to a greater depth, to take an extreme
limit, than 500 fathoms.
Fol and Sarasin, experimenting with very sensitive bromo-gelatine
plates, found that there was no reaction after ten minutes’ exposure at
a depth of 400 metres on a sunny day in March.
But although it is highly probable that not a glimmer of sunlight ever
penetrates to the depths of the ocean, there is in some places,
undoubtedly, a very considerable illumination due to the phosphorescence
of the inhabitants of the deep waters.
All the Alcyonarians are, according to Moseley, brilliantly
phosphorescent when brought to the surface. Many deep-sea fish possess
phosphorescent organs, and it is quite possible that many of the
deep-sea Protozoa, Tunicates, Jelly-fish, and Crustacea are in their
native haunts capable of giving out a very considerable amount of
phosphorescent light.
If we may be allowed to compare the light of abysmal animals with that
of surface forms, we can readily imagine that some regions of the sea
may be as brightly illuminated as a European street is at night—an
illumination with many very bright centres and many dark shadows, but
quite sufficient for a vertebrate eye to distinguish readily and at a
considerable distance both form and colour.
To give an example of the extent to which the illumination due to
phosphorescent organisms may reach, I may quote a passage from the
writings of the late Sir Wyville Thomson.
‘After leaving the Cape Verde Islands the sea was a perfect blaze of
phosphorescence. There was no moon, and although the night was perfectly
clear and the stars shone brightly, the lustre of the heavens was fairly
eclipsed by that of the sea. It was easy to read the smallest print,
sitting at the after-port in my cabin, and the bows shed on either side
rapidly widening wedges of radiance so vivid as to throw the sails and
rigging into distinct lights and shadows.’
A very similar sight may frequently be seen in the Banda seas, where on
calm nights the whole surface of the ocean seems to be a sheet of milky
fire. The light is not only to be seen where the crests of waves are
breaking, or the surface disturbed by the bows of the boat, but the
phosphorescence extends as far as the eye can reach in all directions.
It is impossible, of course, to say with any degree of certainty whether
phosphorescence such as this exists at the bottom of the deep sea, but
it is quite probable that it does in some places, and hence the
well-developed eyes and brilliant colours of some of the deep-sea
animals.
On the other hand the entire absence or rudimentary condition of the
eyes of a very considerable proportion of deep-sea animals seems to
prove that the phosphorescent illumination is not universally
distributed, and that there must be some regions in which the darkness
is so absolute that it can only be compared with the darkness of the
great caves.
It is difficult to believe that the eyes of such animals as crabs and
prawns for example would undergo degeneration if there were a glimmer of
light in their habitat, a light even so faint as that of a starlight
night in shallow water. With the faintest light the eyes would be of use
to them in seeking their prey, avoiding their enemies, and finding their
mates, and any diminution in the keenness of this sense would probably
be of considerable disadvantage to them and tend to their ultimate
extinction.
It might be argued that the animals of the abysses of the ocean probably
feed chiefly upon the carcases of pelagic animals that have fallen from
the upper regions of the sea, and that the sense of smell is probably
the most important for them in searching for their food. That is quite
probable; but many shallow-water animals invariably seek their food by
their sense of smell without showing any traces of a weakness in their
sense of sight. It may be taken as an axiom of biology that unless a
particular sense is absolutely useless to an animal or a positive
disadvantage to it, that sense will be retained.
It may be stated then with some confidence that in the abysmal depths of
the ocean there is no trace of sunlight. It is highly improbable, on the
face of it, that any ray of light could penetrate through a stratum of
water four miles in thickness, even if the water were perfectly pure and
clear, but when we remember that the upper regions, at least, are
crowded with pelagic organisms provided with skeletons of lime and
silica, we may justly consider that it is impossible.
The temperature of the water in the abyss is by no means constant for a
constant depth nor does it vary with the latitude. It is true that, as a
rule, the water is colder at greater depths than in shallower ones, and
that the deeper the thermometer is lowered into the sea, the lower the
mercury sinks. This is consistent with physical laws. If there is any
difference at all in the temperature of a column of water that has had
time to settle, the thermometer will always reach its highest point at
the top of the column and its lowest at the bottom, for the colder
particles being of greater specific gravity than the warmer ones will
sink, and the warmer ones will rise.
The truth of this will be clear if we imagine a locality at the bottom
of a deep ocean with a source of great heat such as an active volcano.
Such a source of heat would, it is true, raise the temperature of the
water in its immediate vicinity, but the particles of water thus heated
would immediately commence to rise through the superjacent layers of
colder water, and colder particles would fall to take their places. Thus
the effect of an active volcano at the bottom of the deep sea would not
be apparent at any very great distance in the same plane. In fact,
unless the bottom of the ocean was closely studded with volcanoes we
should expect to find, as indeed we do find, that the temperature of the
sea rises as the water shallows.
If then we were to consider a great ocean as simply a huge basin of
water, we should expect to find the water at the surface warmer than the
water at the bottom. The temperature of the surface would vary
constantly with the temperature of the air above it. That is to say, it
would be warmer at the equator than in the temperate regions. The
temperature at the bottom would be the same as the lowest temperature of
the basin, that is, of the earth that supports it.
The great oceans however cannot be regarded as simple basins of water
such as this. The temperature of the surface water varies only
approximately with the latitude. It is generally speaking hottest at the
equator and coldest at the poles, but surface currents in the
intermediate regions produce many irregularities in the surface
temperature.
Again, although we have no means of knowing what the temperature of the
earth is at 1,000 fathoms below the surface of the ocean, it is very
probable that in the great oceans the temperature of the deepest stratum
of water is considerably lower than the true earth temperature. This is
due to currents of cold water constantly flowing from the poles towards
the equator. If these polar currents were at any time to cease, the
temperature of the lowest strata of water would rise.
Although the polar currents cannot be actually demonstrated nor their
exact rapidity be accurately determined, the deduction from the known
facts of physical geography that they do actually exist is perfectly
sound and beyond dispute. A few considerations will, I think, make this
clear.
If the ocean were a simple basin somewhat deeper at the equator than at
the poles, the cold water at the poles would gradually sink down the
slopes of the basin towards the latitude of the equator, and the bottom
temperature of the water would be constant all the world over.
A few hills here and there would not affect the general statement that
for a constant depth the temperature of the lowest stratum of water
would be constant.
But in some places ridges occur stretching across the oceans from
continent to continent, and these ridges shut off the cold water at the
bottom of the sea on the polar side from reaching the bottom of the sea
on the equator side.
If A (fig. 1) represents a ridge stretching from continent to continent
across an ocean, and the arrow represents the direction of the current,
then the water that flows across the ridge from the polar side to the
equator side will be drawn from the layers of water lying above the
level of the ridge, and consequently none of the coldest water will ever
get across it, and from the level of the ridge to the bottom of the sea
on the equatorial side the water will have the same temperature as the
water at the level of the ridge on the polar side.
It follows from this that in places where there are deep holes in the
bed of the ocean surrounded on all sides by considerable elevations, the
temperature of the water at the bottom will be the same as the
temperature of the water on the summit of the lowest ridges that
surround them.
[Illustration: FIG. 1.—Diagram illustrating the passage of an ocean
current across a barrier (A).]
This explains why it is that we find that the bottom temperature for a
given depth is frequently less in one place than it is in another, even
in places of the same parallel of latitude. One or two examples may be
taken to illustrate these points. The temperature off Rio Janeiro in
lat. 20° S. was found by the ‘Challenger’ to be 0·6° C. at a depth of
2,150 fathoms. In a similar latitude north of the equator at a depth of
2,900 fathoms the temperature was found to be 2·2° C., and at a point
near Porto Rico there is a deep hole of 4,561 fathoms with a bottom
temperature of 2·2° C.
Again it has been shown by the American expedition that the temperature
of the water at the deepest point in the Gulf of Mexico, 2,119 fathoms,
is the same as that of the bottom of the Straits of Yucatan, 1,127
fathoms, namely 4·1° C. And, passing to another part of the world
altogether, we find in the small but deep sea that lies between the
Philippines and Borneo that, at a depth of 2,550 fathoms, the
temperature is 10·2° C.
These facts then show that, although at the bottom of the deep seas the
water is always very cold, the degree of coldness is by no means
constant in the same latitude for the same depth.
We must now return to the polar currents. We have assumed above that
these currents do exist, and it is probable that by this time the reader
must have seen why they are assumed to exist.
The water at the bottom of the ocean is exceedingly cold. Where does
this coldness come from? It is obvious that in temperate and tropical
climes it does not come from the surface. Nor is it at all probable that
it comes from the earth upon which the water rests; for, if it were so,
the temperature for water of a given depth would always be the same. We
should not find the bottom temperature of 0·4° C. at 2,900 fathoms off
Rio de la Plata and a temperature of 2·2° F. in 4,561 fathoms off Porto
Rico.
In fact the only hypothesis that can with any show of reason be put
forward to account for the temperature of the bottom of the ocean is
that which derives its coldness from the Polar ice.
We have at present very little evidence to enable us to judge of the
force and direction of the polar currents in the two hemispheres, but
the researches of the ‘Challenger’ prove almost conclusively that in the
Atlantic Ocean there is a very strong predominance of the Antarctic
polar current. In fact it seems very probable that the Arctic polar
current, if it exist at all, is very small and confined to the eastern
and western shores of the North Atlantic.
It is very probable, however, that these currents at the bottom of the
ocean are extremely slow, and, as the water is never affected by tides
or storms, the general character of the deep sea is probably one of calm
repose. This is a matter of no little importance; for, in the
consideration of the characters presented by the fauna of any particular
region, it is always necessary to take into account the physical
difficulties the animals have to contend against and the modifications
of structure they present to combat these difficulties.
Thus in a region such as that presented by the deep sea, where there are
no rapid tides, we should not expect to find such a powerful set of body
muscles in the free-swimming forms nor such a firm vertebral column as
in the animals that live in more lively water.
Perhaps it is of the nature of an assumption to say that there are no
rapid currents and tides in the abysmal depths of the ocean, for we have
no means of demonstrating or even of calculating the rate of flow of
these waters. But it is a reasonable hypothesis and one that we may well
use until the contrary is proved.
A fact of some importance that supports this hypothesis, as regards some
parts of the ocean at least, is presented by the sea-anemones.
Many of the shallow-water Actinians are known to possess minute slits in
the tentacles and disc, affording a free communication between the
general body cavity or cœlenteron and the exterior.
In many deep-sea forms the tentacles are considerably shorter and the
apertures larger than they are in shallow-water forms. It is difficult
to believe that such forms, perforated by, comparatively speaking, large
holes, could manage to live in rapidly flowing water, for if they did so
they would soon be smothered by the fine mud that composes the floor of
all the deep seas. In fact anemones of the type presented by such forms
as _Sicyonis crassa_ are only fitted for existence in sluggish or still
water.
[Illustration: FIG. 2.—_Sicyonis crassa._ M, mouth; S, ciliated groove;
T, tentacles. Each tentacle is perforated by a single large aperture.
(After Hertwig.)]
Another character that must be taken into consideration is that
presented by the floor of the great oceans. The floor of the ocean, if
it were laid bare, would probably present a vast undulating plain of
fine mud. Not a rock, not even a stone would be visible for miles.
The mud varies in different parts of the globe according to the depth,
the proximity to land, the presence of neighbouring volcanoes or the
mouths of great rivers.
The Globigerina ooze is perhaps the best known of all the different
deep-sea deposits. It was discovered and first described by the officers
of the American Coast Survey in 1853. It is found in great abundance in
the Atlantic Ocean in regions shallower than 2,200 fathoms. Deeper than
this, it gradually merges into the ‘Red mud.’ It is mainly composed of
the shells of Foraminifera, and of these the different species of
Globigerina are the most abundant. It is probably formed partly by the
shells of the dead Foraminifera that actually live on the bottom of the
ocean and partly by the shells of those that live near the surface or in
intermediate depths and fall to the bottom when their lives are done.
So abundant are the shells of these Protozoa that nearly 95 per cent. of
the Globigerina ooze is composed of carbonate of lime. The remaining
five per cent. is composed of sulphate and phosphate of lime, carbonate
of ammonia, the oxides of iron and manganese, and argillaceous matters.
The oxides of iron and manganese are probably of meteoric origin; the
argillaceous matter may be due to the trituration of lumps of pumice
stone and to the deposits caused by dust storms.
[Illustration: FIG. 3.—Globigerina ooze. (After Agassiz.)]
Globigerina ooze may be found on the floor of the ocean at depths
ranging from 500 to 2,800 fathoms of water in equatorial and temperate
latitudes. The reason that it is not found in Arctic seas may be that
the cold surface waters of these regions do not bear such an abundant
fauna of Foraminifera. This is supported by the fact that it extends ten
degrees further north than south in the Atlantic, the warm water of the
Gulf Stream bearing a richer fauna than the waters of a corresponding
degree of latitude in the Southern Sea.
The Pteropod ooze has only twenty-five per cent. of carbonate of lime.
It contains numerous shells of various Pteropods, Heteropods, and
Foraminifera, but nearly fifty per cent. of its substance is composed of
the siliceous skeletons of Radiolaria and the frustules of diatoms.
According to Murray it is found in tropical and subtropical seas at
depths of less than 1,500 fathoms.
The Radiolarian ooze is found only in the deepest waters of the Central
and Western Pacific Ocean. In some of the typical examples, not a trace
of carbonate of lime was to be found, but in somewhat shallower waters a
few small fragments occurred.
A Diatom ooze, mainly composed of the skeletons of diatoms, has also
been found in deep water near the Antarctic Circle, but it has not
apparently a very wide range.
Of all the deep-sea deposits, however, the so-called ‘Red mud’ has by
far the widest distribution. It is supposed to extend over one-third of
the earth’s surface. It is essentially a deep-sea deposit, and one that
is found in its typical condition at some considerable distance from
continental land. Like the Globigerina ooze it is never found in
enclosed seas. To the touch it is plastic and greasy when fresh, but it
soon hardens into solid masses. When examined with the microscope it is
seen to be composed of extremely minute fragments rarely exceeding 0·05
mm. in diameter. It contains a large amount of free silica that is
probably formed by the destruction of numerous siliceous skeletons, and
a small proportion of silicate of alumina. It usually contains the
remains of diatoms, radiolaria, and sponge spicules, and occasionally
lumps of pumice stone, meteoric nodules, and, in colder regions, stones
and other materials dropped by passing icebergs.
In the great oceans, then, we find in the deepest places red mud, or,
where there is an abundant Radiolarian surface fauna, Radiolarian ooze;
in water that is not deeper than about 2,000 fathoms, we find the
Globigerina ooze; in shallower waters and in some localities only
Pteropod ooze.
It must not be supposed that sharp limits can anywhere be drawn between
these different kinds of deposits, for they pass gradually into one
another and present many intermediate forms.
It is probable that the sea water by virtue of the free carbonic acid it
contains in solution is able to exert a solvent action upon the calcium
carbonate shells of animals as they sink to the bottom, and during the
long and very slow journey from the surface to the bottom of the deepest
seas these shells are completely dissolved.
The first to be dissolved would be the thin delicate shells of the
Pteropods and Heteropods, for besides the fact that they present a wider
surface to the solvent action of the water they are probably influenced
more by tide and currents, sink more slowly and erratically, and thus
have a longer journey to perform.
Then the smaller but more solid and compact shells of the Foraminifera
are dissolved, and lastly, in the deepest water only the siliceous
skeletons of the radiolaria and diatoms are able to reach their last
resting place at the bottom of the ocean.
These four oozes then are characteristic of the floor of the deep
oceans. In the proximity of land and in inland seas where deep water
occurs, other muds are found differing from one another in accordance
with the character of the coasts in their vicinity. It is not necessary
to give a detailed account of them, but a few remarks on some of the
more pronounced forms may not be without interest.
The blue mud contains eighty per cent. of a mixture of quartz, mica,
felspar, hornblende, and other minerals, mixed with a considerable
quantity of decomposing animal and vegetable substance, the calcareous
remains of foraminifera, mollusca, worms, echinoderms, alcyonaria and
corals, and the siliceous skeletons of radiolaria and diatoms.
The green mud is characterised by a large percentage of glauconite.
The red muds characteristic of the Brazilian coast contain a large
amount of ochreous matter brought into the sea by the great rivers.
In the neighbourhood of active volcanoes there is a characteristic
volcanic mud, and in the coral seas the deep-sea deposits contain a
large percentage of the calcareous remains of dead corals.
One more character of the deep-sea region must be referred to before we
pass on, and that is the absence of vegetable life. It has not been
determined yet with any degree of accuracy where we are to place the
limit of vegetable life, but it seems probable that below a hundred
fathoms no organisms, excepting a few parasitic fungi, are to be found
that can be included in the vegetable kingdom. While then the researches
of recent times have proved beyond a doubt that there is no depth of the
ocean that can be called azoic, they have but confirmed the perfectly
just beliefs of the older naturalists that there is a limit where
vegetable life becomes extinct. It is not difficult to see the reason
for this. All plants, except a few parasites and saprophytes, are
dependent upon the influence of direct sunlight, and as it has been
shown above that the sunlight cannot penetrate more than a few hundred
fathoms of sea water, it is impossible for plants to live below that
depth.
The absence of vegetable life is an important point in the consideration
of the abysmal fauna, for it is in consequence necessary to bear in mind
that the food of deep-sea animals must be derived from the surface. It
is possible that deep-sea fish, in some cases, feed upon one another and
upon deep-sea crustacea, that deep-sea crustacea feed upon deep-sea
worms, that deep-sea echinoderms feed upon deep-sea foraminifera, and so
on through all the different combinations; but the fauna would soon
become exhausted if it had no other source of food supply. This other
source of food supply is derived from the bodies of pelagic organisms
that fall from the upper waters of the ocean, and is composed of
protozoa, floating tunicates, crustacea, fish, and other animals,
together with diatoms and fragments of sea-weed.