J J. JOh

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LIFE IN THE SEA

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LIFE IN THE SEA

JAMES JOHNSTONE

Fisheries Laboratory,
University of Liverpool

Cambridge :
at the University Press

Cambridge :

PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS

With the exception of the coat of arms at
the foot, the design on the title page is a
reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521

PREFACE

THE history of every science affords examples of
the enormous stimulus to further research which
arises from the discovery of some new and sound
method of research ; and two such advances in
methods have transformed marine biology during
the last ten years. Late in the eighties of last
century some German physiologists began to estimate
the quantity of microscopic life contained in the sea
under each square metre of surface, and nets and
other apparatus were invented for this purpose.
About the beginning of the nineties some Scandi-
navian geographers and zoologists also began to
investigate the microscopic life of the sea with
respect to the physical conditions of the water. The
object of the Kiel quantitative plankton investigations
was to estimate the production of a sea area in much
the same way as a scientific agriculturalist attempts
to estimate the productivity of a cultivated land
area ; and that of the Scandinavian hydrographers
was to attempt to forecast seasonal changes in the
climatic conditions of their countries by studying the
currents of the sea.

Now apart altogether from the primary objects
of these researches results were obtained which have
proved to be of great theoretical interest, so that the
original methods employed have been improved, and

249491

vi PREFACE

their use has been extended. There is of course a
very great amount of such work still to be done but
we know enough even now to make it well worth
while to attempt to make a broad survey of life in
the ocean ; to consider the way in which the different
kinds of organisms affect each other ; and how they
are influenced by the great seasonal changes which
sweep across the sea. Such a discussion of the
general economy of the sea is attempted in this
book.

It may seem to some readers who possess a first-
hand acquaintance with some of the subjects dealt
with, that many of the statements made are too
dogmatic. It is impossible to give all the evidence
that might be brought forward in support of them,
and I have therefore added some notes at the end
of the book, which indicate the main sources of
information. Finally one may admit that there is
difference of opinion with regard to some of the
views stated, but a good deal of evidence may be
adduced in their favour ; and at all events they
are permissible views in the present state of our
knowledge.

JAS. JOHNSTONE.

LIVERPOOL,

September, 1911.

CONTENTS

CHAP. PAGE

I. The Categories of Life 1

II. Rhythmic Change in the Sea .... 28

III. The Factors of Distribution .... 53

IV. Modes of Nutrition 87

V. The Sources of Food 118

Authorities 143

Index. . . 147

Frontispiece. Benthos and Nekton; a cora]
bank in the Red Sea. (From Haeckel.)

The Tailpieces represent various pieces of
oceanographic apparatus pp. 27, 52, 86,
117, 142

CHAPTER I

THE CATEGORIES OF LIFE

IT is probable that there are at least about
500,000 distinct species of plants and animals in the
sea. Each species is an assemblage of individuals
which differ from the individuals composing other
species or assemblages, in form, in life-history, and
to some extent in habits. Above all, the individuals
of each species differ from those of other species in
that their mode of reproduction has been so special-
ised, that the offspring born from them resemble the
parents more than they resemble the individuals of
any other species. Now it is the task — as yet an
incompletely performed one — of marine biology to
classify all these organisms so as to display their inter-
relationships to each other and to the species of
organisms which have inhabited the earth in past
geological periods. Even to indicate the main prin-
ciples of such classification would be beyond the scope
of this book, but if we assume a general knowledge
of the main types of plant and animal life it will not

j. i

L' LIFE IN THE SEA [CH.

be difficult to devise a grouping which will be at once
convenient and philosophical.

Let us suppose that it is possible to start from
some place on the west coast of Ireland and walk out
along the sea-bottom, underneath the North Atlantic
Ocean, until we reached the opposite shore of North
America. As we did so we should witness a re-
markable series of changes in the physical condition
of the sea-bottom, and of the water over our heads ;
and we shoujd also witness a similar change in the
nature of the life of the sea ; such a change as would
enable us to associate the variations in the physical
conditions with certain variations in the abundance
and nature of the animal and plant life of the ocean.
We will suppose that we were to start out from some
stretch of beach covered by stones and gravel: on
the higher parts of such a beach we should find a
particular kind of life — large sea- weeds, such as
the bladderwrack, clinging to the stones; mussels,
barnacles, periwinkles, dog- whelks and small Crustacea
such as the sandhoppers. The animals and plants of
which these are the most familiar examples inhabit
the sea beach between the upper and lower tide
marks and we may call them the Littoral Benthos.
Now descending the beach to near the limit of low
water of high spring tides we should find a more
varied and luxuriant fauna and flora, which would
consist of the large-fronded sea-weed Laminaria,

i] THE CATEGORIES OF LIFE 3

small fishes such as the blennies, gobies, and gunnells ;
many Crustacea such as lobsters, crabs, hermit-crabs,
and prawns; molluscs such as the whelks, horse-
mussels, clams, scallops, and brightly coloured sea-
snails (Nudibranchs) ; starfishes, sea-urchins and
brittle stars ; many kinds of crawling worms ; sea-
anemones, sponges, and numerous species of plant-like
zoophytes and polyzoa encrusting or growing on the
stones or larger algae. This fauna and flora inhabiting
the zone of shore only exposed by exceptional ebb-
tides we may call the Laminarian Benthos. If our
initial point of departure had been a rocky coast we
should still find much the same distinction, for the
upper parts of the rocks would be covered with the
barnacles, periwinkles, etc., while the lower rock-pools
would contain much the same organisms as we found
on the lower parts of the gravelly beach. If we had
started from a flat, sandy shore the nature of the
life would, however, be quite different, for the sand
between the tide marks would probably contain such
molluscs as the cockle, animals like the lugworm
burrowing beneath the surface, and small Crustacea
such as the Copepods. The larger rooted algae would
be absent, but their place would be taken by micro-
scopic plant life. Here and there on the surface of
the sand, on the ripple marks, or in the little gutters
left by the retreating tide there might be patches of
yellow-green slime, and on examining these we should

1—2

4 LIFE IN THE SEA [OH.

find them to consist of masses of Diatoms ; and almost
everywhere the sand would contain these though they
might not be visible to the naked eye. The impression
we should get would be that the sandy shore con-
tained less life than that found on the stones or
gravel, and this impression would probably be an
accurate one : nevertheless the sand would contain far
more life than one would at first imagine it to hold.

As we walked out to sea to greater depths we
should witness many changes in the nature of the
sea-bottom. Here and there it would consist of bare
rock, or of stones and gravel, and the nature of this
rock or gravel would depend on that of the adjacent
land. But as we went further the more common
deposit on the bottom would be sand, although there
might be patches of mud, particularly in the deeper
parts or channels. The general arrangement would
be first of all rock and stones or gravels, then sands,
and then muds; for all these materials result from
the erosion of the land, and the gravel is first deposited
or rolled down, then the sand, and lastly the mud.
Out beyond a depth of about 50 fathoms the muddy
deposits would be more abundant than closer inshore,
for the finer particles of which they are composed are
carried for a greater distance in the water, while
much of the fine particles carried in suspension by
the water of the rivers is precipitated by the salts
of the sea-water and only settles down very slowly.

i] THE CATEGORIES OF LIFE 5

This mud would form in the deeper depressions where
the run of tide is weakest, and we should never find
it on ridges of rock standing up from the sea bottom.
Now all these materials come from the wasting away
of the land, but we should also find sea-bottom deposits
which are formed by the agencies of living creatures.
There would be patches of hard, sandy substance
formed by the tubes of the worm Sabellaria; here
and there deposits of the coral-like skeletons of
calcareous algae, or Corallines; and perhaps also
extensive deposits of broken shell fragments. So
far we should have got down to a depth of perhaps
50 fathoms.

The life inhabiting the sea-bottom would not differ
much from that which we had already observed in
the Laminarian zone, but there would of course be
greater variety, especially in regard to the fishes.
The latter would consist of plaice, dabs, soles, gur-
nards, skate and ray, codling, whiting, dogfishes, and
others. There would be shrimps on the shallower
sandy bottoms, accompanied by hosts of invertebrates
such as various species of crabs, starfishes, ophiurids,
sea-urchins, hermit-crabs, polyzoa and many others.
In the deeper places near the 20-fathom line there
would be other species of fishes not found in the
shallower water, such as turbot and brill, and there
would also be a difference in the invertebrates : the
larger squids and cuttle-fishes and many others would

6 LIFE IN THE SEA [CH.

be found here. The rocky bottoms would harbour
many species of fishes not found on the sand — the
brilliantly-coloured wrasses for instance, with perhaps
pollack, ling or saithe. The muddy places would
contain many kinds of burrowing Crustacea and
worms, and perhaps the Norway lobster (Nephrops).
All this part of the sea we might call the shallow
water region, and the bottom life inhabiting it the
Shallow Water Benthos. Life here is richer and
more varied than in any other part of the sea, not
only because of its permanent fauna but also because
hosts of young fishes inhabit this region for the first
year or so of their lifetime. As we proceed further
out to sea there would be a progressive change both
in the nature and the abundance of life. The algae
would disappear entirely. The shallow water fishes
such as the plaice and sole would be replaced by the
witch and megrim, and the cod, whiting and haddock
by the hake. Invertebrate life would also become
rather less abundant. Between the 50-fathom and
the 100-fathom contour lines the region of sea may
be called the deep-sea area and its bottom life the
Deep-Sea Benthos. It would all be familiar to us.
So far we have been travelling under relatively
shallow water, over the narrow selvage of sea-bottom
which fringes the margins of the continents, and
extends out to sea for a variable distance from the
dry land. The British Islands themselves are situated

i] THE CATEGORIES OF LIFE 7

on this margin, which is called the ' Continental
Shelf/ The slope downwards would be a fairly
gradual one so far, but at depths of about perhaps
500 fathoms it would begin to become much more
steep, and in some parts of the ocean it would be
represented by a series of terraced precipices. Beyond
these, and at depths of over 1000 fathoms, would be
the permanent abysses of the ocean.

Very strange and unlike anything on the dry
land would be this ocean bottom. Beyond the edge
of the continental shelf there would be few hills,
or valleys, and the slopes downward to greater depths,
or upwards to oceanic islands, would be so gradual
as to be hardly perceptible to us. Sailors, visualising
the sea-bottom from their knowledge of it acquired by
soundings, speak of knolls, deeps, etc., but they usually
exaggerate the degree of slope actually existing.
It is said that there are some fissures on the ocean
bottom which resemble the deep river gorges of the
land, or bare pinnacles and ridges of primeval rock
uncovered by any deposits, but these must be quite
exceptional features, and the configuration of the
ocean floor probably resembles nothing so much as
that of the great prairie lands of the western conti-
nents. But nowhere on the land would there be
such immense tracts of level surface as those of the
beds of the Atlantic, and the greater part of the
Pacific, Oceans.

8 LIFE IN THE SEA [OH.

The materials forming the sea floor at shallow
depths would mostly be familiar to us, but near the
edge of the continental shelf they would become
quite unlike anything that we know on the land.
All the coarser detritus washed away from the land
surface would have settled down, and everywhere
the sea-bottom would be covered with soft, fine,
muds or oozes, varying in colour through tints of
blue, green, red or grey. These are the terrigenous
muds and they consist of the very finest particles
of mud borne in suspension by rivers and currents,
and floating in the water for immense distances.
Nearly everywhere they would be modified in com-
position by the action of decomposing organic matter
resulting from the dead bodies of marine organisms.
Here and there would be volcanic debris, scoriae,
fine dust, or even fragments of lava, either ejected
from submarine volcanic vents, or settling down on
the ocean floor after having been drifted about in
the sea or in the atmosphere. Beyond this area of
terrigenous muds we should encounter that of the
pelagic deposits ; and nearly everywhere at depths
of 1500 fathoms to rather over 3000 fathoms the sea
bottom deposits consist mainly of the shells or
skeletons of animals and plants which live in the
upper layers of the sea, and dying sink down to the
bottom to form the abyssal oozes.

In many parts of the world the characteristic

I]

THE CATEGORIES OF LIFE

deposit at depths of about 500 to 1500 fathoms is
that known as Pteropod ooze. This consists to a
large extent of the shells of the pelagic mollusca
known as Pteropoda. We should not encounter any
patches of this ooze in our traverse across the North

Diatom ooze

Pteropod ooze

Radiolarian ooze Globigerina ooze

Fig. 1. Deep- Sea Deposits.

Atlantic where the greater part of the sea-bottom,
outside the limit of distribution of the terrigenous
muds, consists of Globigerina ooze. This is a soft
white or grey mud containing gritty particles and
composed largely of carbonate of lime. The picture

10 LIFE IN THE SEA [CH.

on page 9 represents it after the finer amorphous
sediment which makes up the greater part of its
bulk is washed away. The rounded shells of which
it is composed are those of pelagic Foraminifera,
protozoa which live mainly in the upper parts of
the sea, and which dying there sink down to the
bottom, where their shells accumulate to form the
deposit. How deep it is we have no opportunity of
knowing, but telegraph engineers believe that it forms
at the rate pf about an inch in ten years in some
places. It is the characteristic deposit of the North
Atlantic ocean floor, and only here and there the
deeper parts of the latter are covered with material
of volcanic origin. At depths over 2500 fathoms the
Globigerina ooze tends to disappear, for the lime
becomes dissolved by the sea water at these depths.
The deposit where the sea is about 3000 fathoms in
depth elsewhere than in the Atlantic consists of
either volcanic material or the mud called Radiolarian
ooze. This latter formation consists largely of the
skeletons of the pelagic protozoa known as Radiolaria.
It contains very little carbonate of lime, for the
skeletons of the Radiolaria are composed of silica.
We should also find the skeletons of these organisms
in the deposits in shallower parts of the ocean, but
they are not so evident because of the greater mass
of the limy shells of the Foraminifera. The siliceous
^keleton of the Radiolarians is better able to withstand

i] THE CATEGORIES OF LIFE 11

the solvent action of the sea water. This ooze is
not the only one which is composed of siliceous ma-
terial, for in the Antarctic there is an area of ten
and a half millions of square miles covered with the
remains of diatoms. These organisms live in the
superficial layers of the sea in such enormous numbers
that their dead shells form the major part of the mud
covering the floor of the Antarctic.

In the greatest depths of all, 3500 fathoms and
over, we find no longer any obvious indications of
organic structures in the mud on the sea-bottom.
The deposit in these great depths is called the Red
Clay, and it is a very soft brown mud consisting of
exceedingly fine particles, which are the insoluble
remains of the shells of pelagic plants and animals,
and volcanic dust settling down from suspension in
the water, or from the atmosphere. Among it we
find cosmic dust — that which has originated in the
combustion of meteorites entering the atmosphere
of the earth from outer space. It accumulates with
exceeding slowness, for we ought to find meteoritic
dust in every part of the sea-bottom, but it can
usually be recognised only among the red clay, for
everywhere else it is hidden by the more abundant
materials. In the red clay we also find the earbones
of whales and the teeth of sharks. All the rest of
the skeletons of these great creatures have been
dissolved by the action of the sea water.

12 LIFE IN THE SEA [CH.

Nothing in our ordinary experience gives us any
adequate idea of the physical conditions at the bottom
of the ocean. As we descend deeper and deeper the
temperature falls more and more, and at a depth
of about 400 fathoms the annual variations in the
temperature of the sea due to the seasons fade away,
and uniformity reigns. From about 400 fathoms
downwards the temperature would fall very slowly,
and everywhere in the North Atlantic it would not
be much higher than that of the freezing point of
fresh water. In the circumpolar seas it would fall
still further and in some places would be about two
degrees below zero of the Centigrade scale.

The darkness would be greater than that of which
we have any ordinary experience. Even at a depth
of about ten fathoms the illumination would be that
of faint twilight only, and at fifty fathoms it would
be still less. The light there would be no longer
white, for sea water absorbs first the red, orange,
and yellow rays, and then the green, blue and violet.
At about 50 fathoms the violet light would still be
fairly strong, and it would be present even at a depth
of 200 fathoms, but there would be absolutely no
trace of the red rays. At about 400 to 500 fathoms
even the violet light would have disappeared entirely
and only the ultra-violet rays — light which is invisible
to our eyes, would remain. At a depth of about
900 fathoms the last traces of ultra-violet radiation

i] THE CATEGORIES OF LIFE 13

would have disappeared and a delicate photographic
plate exposed here for over two hours would not be
affected. Absolute darkness exceeding that of the
photographer's dark room would prevail.

We can easily imagine the cold, and the darkness^"?
and the great plains of semi-liquid mud, but not the
extreme pressure of the water. At the surface of
the sea an organism is exposed to a pressure of one
atmosphere, that is 15 pounds per square inch of its
surface, and a rise of even one atmosphere produces
very unpleasant effects in ourselves. But for every
ten metres (5 fathoms) that we descend, the pressure
due to the weight of the water rises by one atmo-
sphere, and at a depth of 3000 fathoms the pressure
on every square inch of the body of an animal
amounts to about three tons. If we did not kndw
that no part of the sea-bottom — however deep — is
utterly devoid of animal life we might conclude — as
the older naturalists did — that this enormous pressure
was inconsistent with a belief in the existence of
living organisms. Even now it is difficult to under-
stand how the tissues of an animal are able to
withstand it. As it is, the pressure exerts a profound
influence on the structure of the animals inhabiting
even such moderate depths as 200 fathoms. A deep-
sea fish is easily recognised by its large eyes, its long
and attenuated tail, and the softness of its flesh.
One's finger leaves a dent in the flesh of such a fish

14 LIFE IN THE SEA [OH.

as a hake if pressed against it. But the pressures
in the abyssal regions are very much greater than
those which a hake has to withstand. Even the thermo-
meters at first used in deep-sea exploration were
frequently shattered to fine dust by the pressure of
the water ; and this is the great difficulty in the
construction of all instruments for the investigation
of the oceanic depths.

Just as we recognise a littoral, a shallow water
and a deep-sea Benthos, so we may speak of an
Abyssal Benthos inhabiting the whole sea-bottom
deeper than 2000 fathoms. Before the time of the
great oceanographic expeditions there was much
speculation as to the nature of the animals which
might be found in the greatest depths of the sea.
It was thought that exploration might disclose
unfamiliar and bizarre forms of life, perhaps gigantic
animals, perhaps remnants of former geological faunas
persisting under the uniform conditions of the abysses,
while all the rest of the earth was undergoing physical
change. These expectations have not been realised,
for all the animals captured from the deep belong to
jwell-known groups of life ; and with the exception
of some stalked Crinoids, animals closely allied to
the Crinoids which formed so characteristic a feature
of the fauna of the Carboniferous period, most of the
species belong to recently evolved groups. One
cannot say that there are no monstrous or gigantic

i] THE CATEGORIES OF LIFE 15

animals still living in the abysses, for no form of
fishing gear yet used would enable us to capture a
very large abyssal animal, and one may perhaps
still attach some consideration to stories of sea-
serpents. Nevertheless the general experience is
that the animals obtained from the great depths
of the sea are as a rule smaller than their shallow
water allies. There is, of course, much that is strange
in the abyssal Benthos, but though a zoologist accus-
tomed only to shallow water animals would fail to
recognise most of the deep-sea forms, he would still be
able to refer them to their respective families or classes.
Deep-sea animals have certain general characters.
Most of the great invertebrate groups, the molluscs,
Crustacea, sponges, echinoderms, alcyonarians, etc., are
represented in the deep. The Crustacea are often
long-legged creatures coloured a uniform red. Most
of them are smaller, or at least no larger than are
the shallow water forms, but some are relatively
gigantic — one of the Isopods, for instance, a group
represented by the little i slaters/ is about nine
inches in length. Some of the crinoids, tunicates
and alcyonarians (seapens) are carried on long stalks
— apparently an adaptation to life in the soft oozes.
There are echinoderms, starfishes and sea-urchins
which do not differ in any essential respect from
those living in shallow water except that spines are
rather more pronounced in the abyssal forms.

16 LIFE IN THE SEA [OH.

The fishes are very unfamiliar to us though they all
/ belong to well-known groups, and it is very probable
/ that they have been evolved from shallow water
s. They have as a rule either very large eyes,
or very small ones, or they are quite blind. This is
what we might expect if the fishes had spread into
the deep from the shallow water, for evolution might
proceed in various directions : either an organ which
was inefficient under the new conditions would become
larger by the accumulation of small variations, or
being unused and useless it would disappear. In
most of them the tail is long and slender; the mouth
and teeth are very large so that the animal has an
eminently predatory character, which would be a
consequence of the difficulty in procuring food;
some abyssal fish are able to devour others almost
as large as themselves. Their bones are fragile
x^uadr have less than the usual proportion of lime ; a
character which is common to most abyssal animals
which have calcareous external or internal skeletons.
The flesh is soft and watery. There is a general
absence of the pronounced colour markings of the
shallow forms ; they are not silvery as a rule ; and
there are no indications of the obliterative counter-
shading of the fishes usually known to us whereby
the upper parts are darker than the lower ones so
as to destroy the appearance of solidity, and render
the animal nearly invisible to another in the water

i] THE CATEGORIES OF LIFE 17

at the same level as itself. The colour is usually a
monochrome — a black or dark red: in the deep, in
the absence of red light, such a coloured fish would
appear black ; if indeed one can speak at all of colour
in such surroundings.

The great majority of abyssal animals are phos-/
phorescent : they possess luminous organs similar to
eyes in structure, and often the fishes of moderately
deep water have rows of such ocelli along their sides.
Sometimes deep-sea animals secrete a sort of phos-
phorescent slime. Why they should produce light it
is hard to guess : we must remember that it is only
to our eyes that they appear luminous. Very strange
indeed would be the appearance of these animals if
we could see them in the deep. In the absolute
darkness of the abyss they would appear as ghostly
silver-blue shapes glimmering like an electric lamp
through dense fog on a dark moonless night. Of
all the characters of the deep-sea fauna this almost
universal phosphorescence is the strangest.

All the animals and plants which we have so far
examined belong to the category of life called the
Benthos — a collective term applied to those organisms
living at the sea-bottom attached to stones or
other fixed objects; or burrowing in the sand or
mud ; and which may also be extended so as to include
all those animals which, though free to move about,
are sluggish and inert, and which frequent only a

J. 2

18 LIFE IN THE SEA [OH.

comparatively small area of sea-bottom. Among the
attached forms we find the algae, the zoophytes and
polyzoa, the sponges, the reef-building and solitary
corals, the sea-anemones, the branching corals and
alcyonarians, the crinoids, some Crustacea of which
the barnacle is the most familiar example, many
molluscs such as the cockle, mussel and oyster and
most of the tunicates. Among the burrowing forms
are some protozoa, many worms, some sea-urchins,
and sea- cucumbers (Holothurians), some sea-anemones,
many Crustacea, and many molluscs such as the clams
or razor-shells. The sluggish, slowly moving, bottom
living animals include the starfishes and most of the
sea-urchins, crabs, lobsters and hosts of other Crustacea,
many worms, many molluscs such as the scallops,
periwinkles and whelks. Some fishes are also to be
placed in this category. Benthic animals thus include
representatives of all the great groups of life. When
we remember that distinct faunas are to be found
on sea-bottoms of different depths, and of different
physical conditions, we see that it is convenient to
speak of littoral, shallow water, deep water, and
abyssal benthic populations. In the wider economy
of the sea all these categories of life are distinct from
each other.

Now the great majority of marine animals are
mthic in habit but there are a number which possess
>werful locomotory organs and which are thus able

i] THE CATEGORIES OF LIFE 19

to migrate over wide areas of sea. These are all
grouped together to form the category of life known
as the Nekton. Comparatively few marine animals
in addition to the fishes are nektic in habit. The
whales, seals, dolphins and porpoises are the next
largest group, and in addition to these and the fishes
only a few invertebrates, such as the squids, the
cuttle-fishes, and the giant calamaries are actively
moving animals; and perhaps some pelagic worms
and medusae may also so be described. According
to their habits the fishes may be further subdivided
into demersal and pelagic species. The demersal
fishes are those which live on the sea bottom and obtain
their food from animals which also live there. Most
of the commoner fishes belong to this category ; the
sole, plaice, whiting, haddock, cod, skate and ray,
turbot and brill and many others are examples. The
pelagic forms are the minority, and the best-known
examples are the herring, mackerel, sprat, pilchard,
and probably also the salmon while it inhabits the sea.
As a rule the distinction between demersal and
pelagic fishes is a very real one, and it is exceptional
for a fish which lives at the sea bottom to approach
the surface ; if it does the habit may be regarded as
an abnormal one. If we trawl demersal fish such as
the cod or whiting from even the moderate depth of
20 fathoms, and take care that the animals are not
injured by the action of the fishing gear, we see

2—2

20 LIFE IN THE SEA [OH.

that they have the greatest difficulty in again going
down to the sea-bottom, and for a time they struggle
helplessly at the surface belly upwards. This is
because of the expansion of the gas contained in
their swim-bladders, and it is only after this has
been absorbed by the blood stream that the fish
regains its former specific gravity and is able to go
below the surface. It used to be the habit of the
Grimsby trawlers, who often brought cod alive into
the markets, to prick the fish behind the shoulder
with a long needle and so allow the gas in the swim
bladder to escape. On the other hand fish like the
plaice and sole, which have no swim bladders, are able
to be transferred directly from water of moderate
depths to quite shallow tanks without any apparent
bodily disturbance, even although the difference in
pressure may be more than one atmosphere. Pelagic
fish are able to live both at the surface of the sea
and in the depths — say of ten to twenty fathoms;
thus the herring must go down to the sea-bottom in
order to spawn, and they are often caught by means
of the trawl-net in the same manner as the proper
demersal fishes are captured. But we may be sure
that these fishes are able by means of some control
over the blood vessels of the swim bladder to alter
their specific gravity.

In the case of the fishes of moderately shallow
water — say down to twenty fathoms — there is little,
if any essential difference in their form : a herring,

i] THE CATEGORIES OF LIFE 21

for instance, does not differ greatly in structure from
a whiting, though the one is a pelagic, surface-living
animal and the other is one which nearly always lives
at the sea-bottom, and both have swim bladders.
But in the case of the oceanic fishes the structure
may be strikingly different according to the depth of
water which the animal usually inhabits. A flying
fish !differs very notably from any of the abyssal
forms which we have already considered ; and those
species which live at intermediate depths are also
very remarkable in form. They are, as a rule, small,
very delicate in structure, with semi-transparent
bodies, often compressed from side to side. It is
quite easy to read ordinary print through the body
of the peculiar Leptocephalus larva of the eel. Many
of them are tinted in shades of violet or silver-blue,
and have a peculiar shimmering appearance. Often
their eyes telescope into sockets. As a rule these
are the characters of the pelagic fishes found in the
oceans, in the intermediate layers down to a depth
of about 200 fathoms. Below that level the colour
is usually black or dark red, and the form of the fish
is that which we have already recognised as charac-
teristic of the abyssal category.

All the animals and plants which we have dealt
with so far are such as can easily be seen with the
naked eye, or which can be captured by means of the
nets and lines of the fisherman. But everywhere the

22 LIFE IN THE SEA [CH.

sea water, no matter how clear it may be, contains
abundance of life in the form of organisms too small
to be seen without the aid of the microscope. We
can always capture these organisms by filtering even
a tumblerful of water through a sieve made of the
very fine silk cloth that millers use in separating the
various grades of flour. It would be rare indeed for
as much sea water as could be lifted in an ordinary
thimble to be without a dozen or more microscopic
organisms, and at the time of maximum abundance
of life in the sea such a quantity of water might
contain several thousands of living creatures. All
these organisms which can so be obtained are grouped
together to form our third great category of marine
life — the Plankton. The general character of all is
that they are unable to make migrations which can
materially influence their distribution. Many of
them do not possess any organs of locomotion, and
even if they do these organs are quite ineffective for
that purpose. Planktonic animals and plants are
simply drifted about passively in the sea by the
I agencies of tidal streams, currents, and winds.

The composition of the plankton is remarkably
\ variable. Looking at it from a general point of view
\we can recognise in its constitution two great classes
fof organisms. (1) Larval stages of all the great
groups of life : thus fishes which live either demersally
or pelagically spawn eggs which, as a rule, drift about

THE CATEGORIES OF LIFE

23

in the upper levels of the sea-water. Some fishes
indeed, are viviparous — thus some of the dogfishes,
rays, and sharks produce their young alive, and some
others lay large eggs which incubate while lying at
the sea-bottom in sheltered places. But the majority

8

Fig. 2. Plankton: Larvae. 1, Crab; 2, Fish-egg; 3, Sea-urchin;
4, Barnacle; 5, Fish; 6, Mussel; 7, Copepod; 8, Worm. All
magnified.

of fishes produce young which live in the plankton as
microscopic drifting animals, for a period of their lives.
Many of the Crustacea also breed in this way, and their
larvae are planktonic ; so are most of the very young

24

LIFE IN THE SEA

[CH.

mollusca, many of the starfishes and their allies, and
indeed most of the groups of invertebrata. All these
larval creatures form what we may call the transitory
plankton. They live drifting about in the sea for a
variable period of their lives, and then they settle

Fig. 3. Plankton. 1 and 2, pelagic worms; 3, a Medusa; 4, a
Copepod ; 5, a Ctenophore. All magnified.

down to their final habitat as members of either the
Benthos or the Nekton. (2) The second and larger
part of the plankton consists of organisms which live
there permanently.

I]

THE CATEGORIES OF LIFE

25

These latter creatures are far more important
from the point of view of the economy of the sea
than are the larvae. They constitute probably the

Fig. 4. Plankton: Unicellular organisms. 1, 2, 3, 7, 10, 11, 12, 13,
16, 17, 18, Diatoms; 4, 5, 6, 9, Peridinians; 8, an Algal repro-
ductive Cell, or Zoospore ; 14, Noctiluca ; 15, a Radiolarian. All
magnified.

largest of all the categories of life in the sea, for
though they are very minute in point of size, yet

26 LIFE IN THE SEA [CH.

they are present in extraordinary numbers. It is
quite probable that if we could suddenly dry up,
say, one square mile of sea, we should find that the
benthic and nektic animals and plants were really
less in bulk than were the invisible organisms of the
plankton. Now two great sub-kingdoms of life — the
Algae among the plants, and the Protozoa among
the animals — form the major part of the permanent
plankton. The Algae are represented principally by
the Diatoms, minute unicellular plants, possessing
an outer skeleton of silica. Generally speaking the
Diatoms constitute the bulk of the plankton during
the spring, and they are nearly always present at any
time of the year. Many sub-groups of Protozoa are
always present, among them we may mention the
Flagellates, Foraminifera and Radiolaria. These
latter groups, with the Diatoms, are responsible for
the formation of the abyssal sea-bottom deposits.
Among the Flagellates one group — the Peridinians —
is notable. It contains unicellular organisms which
we may regard either as animals or plants, for they
have the structure of the animal unicellular organism,
but they have the same method of nutrition as the
unicellular plant. They are creatures of fantastic
shapes, with outer skeletons of a substance resembling
excessively thin paper impregnated with silica ; and
possessing chlorophyll, the colouring matter of the
green plants.

i] THE CATEGORIES OF LIFE 27

Generally speaking the bulk of the plankton is
made up of larval invertebrates, Diatoms, Peridineans,
and some groups of micro-crustacea, the Copepods
mainly, which live permanently in the plankton.
There is this general relation between the abundance
of the Diatoms, or Peridinians, and that of the
Microcrustacea, such as the Copepods, that when one
is abundant, the other group is scarce. There are,
of course many other planktonic animals and plants.
The larger rooted algae are represented by their
zoospores. Many molluscs are permanently plank-
tonic — thus the Pteropods are abundant enough to
form the food of the whale, and to constitute a large
part of the sea-bottom deposits in some regions of
ocean. Many medusae and other jelly fishes are also
planktonic.

The Naturalist's Dredge.

CHAPTER II

RHYTHMICAL CHANGE IN THE SEA

THE ocean is regarded as the very type of muta-
bility and change, and indeed ordinary observation
shows us that its condition is never quite the same
from day to day. Many of the changes that occur
in it — storms and calms, fogs, exceptionally high
tides and the like, appear to us to be without order
or regularity, that is their causes are so many and
so complex that we cannot predict their occurrence.
On the other hand there are many changes which
are repeated so regularly as to constitute definite
rhythms of events. Experience shows that their
times can be calculated and that they recur over
and over again with the certainty of astronomical
phenomena. Such are the tides, the annual waves
of temperature, salinity and sunlight ; annual out-
bursts of animal and vegetable life ; animal and
plant migrations ; spawning periods ; fishery seasons
and the like. Of all these periodic changes in the
condition of the sea the purely physical ones are

CH.II] RHYTHMICAL CHANGE IN THE SEA 29

those which are repeated with the greatest regularity.
Organic changes depend on these physical ones, but
the inter-relationship is so complex that it is some-
times difficult to detect the periodicity.

About twice in every twenty-four hours the tide
encroaches on the land, converting what a few hours
ago were desolate expanses of sand and mud into
evanescent sheets of water ; and again laying them
bare to the atmosphere. Observing the order of the
tides a little more closely we notice that they do
not recur at precisely the same time, nor do they
rise to exactly the same height on two successive
days. Shortly after the time of full moon the water
rises to its maximum, and then for about a week the
flood tide culminates a little later every day — and
rises a little less high. About halfway between the
times of full moon and new moon the velocity of the
tidal stream is least, and so also is the height to
which the water rises. Then the tides begin to
gather force and shortly after the date of new moon
they again attain a maximum. Thus there is a
half-daily rhythm, for a rise and fall of the tides
occurs twice during each twenty-four hours ; but
there is also a fortnightly rhythm, for spring tides
and neap tides occur twice in each lunar month.
If we continue to observe these things throughout an
entire year we find also that every six months, about
the times of the vernal and autumnal equinoxes,

30 LIFE IN THE SEA [OH.

the spring tides rise to an unusual height. So there
is a semi-annual tidal period, and on this is super-
posed a semi-lunar one, and on this again a semi-
diurnal period.

Observation has shown that these rhythms are
almost absolutely regular. Variations do indeed
occur, for exceptionally strong winds blowing from
the land prevent the flood tide from rising so high
as it otherwise would, and cause the ebb tide to be
lower than is predicted ; while strong winds blowing
straight in from the sea have precisely the opposite
effect. If the barometer rises one twentieth of an
inch the flood tide will be an inch less than it would
be if the atmospheric pressure were constant ; and
if the barometer falls the reverse effect is produced.
But these are only apparent irregularities, for with
our present knowledge we cannot exactly foresee the
variations in the pressure of the atmosphere, and apart
from them almost everything is calculable or predict-
able. It is known for two or three years in advance,
and for a great number of places all over the world,
precisely how high the tide will rise, and when it
will be high water every day throughout the year ;
and so accurate are these tide-tables that sailors
have come to accept them almost as part of the
order of nature, thinking little of the wonderful
mathematical analysis that has made them possible.

Ordinary observation shows again that once a

n] RHYTHMICAL CHANGE IN THE SEA 31

year a wave of temperature change sweeps across
the sea. Sometime in our latitudes about February
or March the sea is coldest and then, day by day,
the temperature rises until it attains a maximum
in August, and then it falls until the time of the
next minimum. Just as surely as the earth revolves
round the sun does the sea experience this annual
temperature change ; but if we study the variation
day by day with a delicate thermometer we find
that the rhythm is not a simple one. There are, as
in the case of the tides, irregularities which are due
to storms and other unpredictable occurrences, but
there is also a daily rhythm. Just about sunrise the
sea is coldest and then it gradually rises in tempera-
ture until about 4 p.m., the time of the daily
maximum. Also there are rises and falls of tem-
perature synchronous with the tidal periods : a few
days after the time of full moon, or new moon, the
sea near the land is warmer during the summer than
it would be if there were no tidal streams ; and
during the winter it is a little colder. Thus there
is a daily temperature period superposed upon a
fortnightly one ; and this again is superposed upon
an annual period.

There are corresponding variations in the intensity
of the sunlight that falls upon the surface of the sea.
At about the end of the winter solstice this is at
a minimum, for the sun's height is then lowest and

32 LIFE IN THE SEA [CH.

its rays fall obliquely upon the surface of the sea.
But day by day the sun rises earlier and sets later,
and it also rises to a greater height, until the date
of the summer solstice when the amount of light
radiated on to the surface of the sea reaches a
maximum. The intensity of sunlight passes through
the same cycle of changes on the sea as on the land,
and the times of maximum and minimum are the
same in either case ; but this is not the case with
the temperature change, for the minimum and
maximum occur a little later in the sea than on the
land. There is also a fortnightly period of light
intensity in the sea, for once a month the moon
passes through all her phases, and for a certain
number of days reflected sunlight falls on the surface
of the sea during the night. The amount of energy
contained in the moonlight is not very great, but we
shall see that only a very small proportion of the
direct rays of the sun are used in the absorption
of energy by plants, and it is probably the case that
the light of the moon is strong enough to affect
the rate of growth of planktonic organisms. Thus
in the case of the intensity of sunlight there are
daily, fortnightly and annual periods.

Even if we depended on ordinary means of
observation, such as tasting the water, or observing
the height of the waterlines of ships, we should be
able to see that the salinity of the sea varied in

n] RHYTHMICAL CHANGE IN THE SEA 33

different parts of the ocean of the earth : it is saltest
in the Red Sea and freshest in some parts of the
Baltic. But if we used delicate hydrometers, or still
better chemical methods of estimating the proportion
of salts, we should find that the salinity varies at
the same place throughout the year. Now there
are many irregularities in the changes of salinity
which we cannot predict ; heavy rainfalls on the
surface of the sea, or floods in the rivers opening
into it, will produce local changes of salinity by
diluting the sea-water. Nevertheless there are
regularities as well. In some places we can detect
a fortnightly variation which is due to the tides,
where these run to and from the land. Thus, opposite
the mouth of an estuary the sea will be, on the
average, fresher some time shortly after the end of
the ebb tide, for the water coming down from
the river will have been slightly fresher than that
which would have been present in the sea had there
been no tidal streams, and this effect will be stronger
at the period of spring tides, when the streams run
with greater force.

There are also annual changes in the salinity, and
to understand these we must consider the general
scheme of oceanic circulation. The intense solar
radiation in the tropics heats the surface of the
sea, and the water becomes warmer and expands
so that it stands at a higher level. It therefore flows

j. 3

34 LIFE IN THE SEA [CH.

away to the north and south towards the poles, but
at about latitude 60° N. it has become cooled by
radiation of its heat to the atmosphere, and being
salter than the water normally present in the sea
at these latitudes it becomes heavier and sinks
towards the bottom. Part of the water thus sinking
flows back again towards the equator. Round the
poles the sea contains much ice, but when sea water
freezes most of the salt is squeezed out, and there-
fore when this ice melts it forms nearly fresh water.
The water of the circumpolar seas is therefore fresher
than normal sea-water and it accumulates at the
surface and then begins to flow towards the equator.
But it soon mixes with the other sea-water and
becomes much salter, and then being cold and heavy
it tends to sink to the bottom. This occurs at about
latitude 60° N. (considering the Atlantic alone) and
part of this sinking water flows along the sea-bottom
back again to the poles. Thus we have two main
systems of ocean currents, (1) warm and very salt
water flowing to the north and south from the
equator, and (2) cold and fresher water flowing also
to the south and north from the poles. Now these
drifts of water do not actually flow to the north and
south : they would do so if the earth were entirely
covered with sea and if it did not rotate ; but because
of the presence of the land the currents are diverted
in various ways ; and because of the rotation of the

ii] RHYTHMICAL CHANGE IN THE SEA 35

earth they are continually deflected to the east in
the northern hemisphere, and to the west in the
southern hemisphere. In the North Atlantic the
northerly flowing current is what we call the Gulf
Stream ; and the southerly flowing one consists of
the East Greenlandic and Icelandic currents, and
the Labrador current.

The Gulf Stream spreads out in the Atlantic in
the form of a gigantic whirlpool the northern margin
of which is never higher than the latitude of the
Azores. Looking at an astronomical diagram we
see that for six months in the year most heat falls
on the part of the sea immediately north of the
equator, while for other six months most heat falls
on the sea immediately south of the equator. There-
fore the equatorial stream shifts a little way north
and south of the equator in the course of the year,
and the whole area covered by the Gulf Stream
circulation (which is caused by the equatorial stream)
also shifts a little way north or south as the earth
swings round the sun in the course of the year.
There is just the same annual variation in the
amount of water flowing south from the north pole,
for more ice is melted during the summer than
during the winter and therefore more fresh water
forms and flows away to the south.

What we call the Gulf Stream in our latitudes
is really the drift of comparatively warm and salt

3—2

36 LIFE IN THE SEA [OH.

water proceeding to the north-east from the Gulf
Stream eddy in the subtropical Atlantic. When the
eddy has expanded farthest to the north the drift
is strongest, and when it has retracted furthest to
the south the drift is weakest. Now the water which
comes up to the north-east, to the British Islands and
coasts of Scandinavia, is always warmer and salter than
the water in the North Atlantic would be normally
if there were no Gulf Stream drift. Thus there is
an annual change of salinity in our seas which depends
mainly upon the strength of the Gulf Stream flow.
The water is saltest when the drift is strongest, in the
months of February to June, and it is less salt when
the drift is weakest, in the months of November to
February. The further north we go the later in the
year does the Gulf Stream drift culminate. The
amount of change that can be detected in the salt-
ness of the sea and which is due to these causes is
not much — the limits are about 34*5 to 35*5 per
thousand. Salinity is defined in this way — as the
total amount of salts weighed in grams and present
in 1000 cubic centimetres of sea- water at a tempera-
ture of 4° C.

Thus there is an annual period in the change of
salinity of the sea and superposed on this are smaller
periods which are due to various causes. But there
are also longer periods than the annual one. There
is a two-yearly period, for the sea is salter in the

ii] RHYTHMICAL CHANGE IN THE SEA 37

springs of the ' even ' years than it is in the springs
of the ' odd ' years. There is also a period of about
twelve years and we have reasons for believing
that this is to be associated with the twelve-yearly
period of sunspot change. The years when there are
the greatest numbers of sunspots are also the years
when the solar radiation is most intense, for the
sunspots are the indication of increased activity in
the sun. Both the temperature and salinity of the
sea are subject to these long-period changes.

All these rhythmic changes, the tides, sea tem-
perature and salinity, and intensity of sunlight are
of cosmic origin, and we can trace them back to
certain fundamental causes — the rotation and revolu-
tion of the sun, and the intensity of solar radiation.
In the case of the tides we can predict the rhythmic
change with great accuracy ; we can also predict the
annual temperature and salinity waves, but not the
exact dates of the maxima and minima ; and we can
predict with accuracy the times of sunrise and sunset,
and the daily declination of the sun, but not the
amount of cloud-covered sky. We know that the
sun is a variable star, but we do not know the exact
period of its variability, nor the range of variation
of its radiation.

All organic changes, that is, the periodic changes
in the habits of marine organisms, must ultimately
depend on these cosmic changes, but the inter-

38 LIFE IN THE SEA [OH.

relationships of the two series of events are so complex
that the organic changes are far less susceptible of
prediction than the physical ones. Nevertheless we
can easily show that very many changes in the nature
and abundance of life in the sea, and in the habits of
animals recur again and again with great regularity.
Let us take the case of the influence of the tides.
Nothing impresses one so much as the way in which
a fisherman regulates his practice according to the
tidal rhythms, so much so that he will associate the
latter with all sorts of variable occurrences with which
they can have nothing to do. The dependence, of
littoral animals on the ebb and flow of the tides is
absolute, for it is only when the foreshore is covered
with water that they can feed or move about. Many
animals such as the. cockle and mussel close their
shells when the tide ebbs off from the foreshore, and
others, such as the lug worm, burrow into the sand.
Convoluta, which is a little flat-worm living on the
sands in some parts of the coasts of France, also
burrows beneath the surface when the water leaves
it. These habits are easily explained, for a shell-fish,
such as a mussel, would become dried up if it were
to keep its shell open during the whole period of
ebb tide on a hot summer day, and the worms would
also become parched up by the heat of the sun. So
also with the spawning of Convoluta, which takes
place only once a fortnight at a certain phase of the

n] RHYTHMICAL CHANGE IN THE SEA 39

spring tide : the eggs are laid then because for a
longer period than usual the sands are covered with
water owing to the higher rise of the tide. But we
find that this tidal rhythm is very deeply impressed
on the conscious or unconscious memory of the
animals. If we attempt to keep cockles in an aquarium
for a long time it becomes difficult to keep the animals
in good health, and wre can improve the conditions by
running away the water of the tank for a time every
day in order to imitate the action of the tides. If we
keep Convoluta in vessels of water in the laboratory
under uniform conditions they will still burrow
beneath the surface of the sand at the time when the
tide is ebbing from off the beach, although the vessel
is kept continually full of water. There are similar
rhythms of habits in many other animals. When the
sea is phosphorescent in some parts of our waters the
luminosity is usually due to a protozoan called
Noctiluca, or to copepods, or to some peridinians.
All these animals become phosphorescent only when
it is dark, that is after sunset. But if we capture
them and keep them in a photographer's dark room
we find that they become phosphorescent there only
when it is dark outside under natural conditions:
that is the animals seem to remember the regular
alternation of day and night, even when they are kept
under strictly uniform conditions. The Chameleon
Shrimp, Hippolyte, shows a similar rhythmic habit.

40 LIFE IN THE SEA [CH.

This little animal inhabits sea- weeds of various colours
and it nearly always varies in hue so as to match the
colour of the alga on which it rests. But in the dark,
after the sun has set, all varieties of Hippolyte,
whether brown or green or red, become a beautiful
transparent blue ; and this nocturnal blue is regularly
assumed in the conditions of the laboratory. This
periodicity of habit in Convoluta and Hippolyte is
fully described in Keeble's volume on 'Plant-Animals'
in this series of Manuals.

The annual changes in the temperature and
salinity of the sea are of much more importance in
their influence on marine life than are the changes of
tide. If we consider in the meantime only the higher
marine animals, the fishes, molluscs, and Crustacea,
we can easily make out three phases in their vital
condition throughout the year, A phase of partially
suspended animation coincides with the time when
the sea temperature is about its minimum; then
there is the reproductive phase which is assumed in
the spring, when the temperature is rising rapidly
from its lowest point; and then follows the growth
phase which coincides with the rise towards the
maximum and the period immediately following.
The dark and cold winter months, December to
February, are those during which the activities of
marine animals are at their lowest ebb. Many of
the fishes and crustaceans such as the plaice, flounder

ii] RHYTHMICAL CHANGE IN THE SEA 41

and shrimp hibernate, burying themselves beneath
the surface of the sand while the temperature of the
sea is at its lowest. Nearly all marine animals
become sluggish and inert at this time : they do not
migrate much; they do not feed so actively as at
other times, or perhaps they refuse to feed altogether.
Reproduction does not take place, or if it does occur
it is mimimal in degree.

What we call functional metabolism, that is the
assimilation and oxidation of food for the purposes of
keeping up the life-processes of the animal, is reduced
to a. minimum; the animal uses up as little of its
tissues as possible. But morphogenetic metabolism,
that is the formation of new substance in the shape
of the genital products — the eggs or spermatozoa —
increases to a maximum during the resting stage of
the cold winter months. During this time the animal
is using its own reserve food-stuff, that is, the fats,
carbohydrates, and proteids stored in its flesh, for the
purpose of manufacturing the generative substances.
The ovaries and testes thus increase largely in weight,
but the general tissues of the body lose in weight.
This of course only occurs in sexually mature animals :
if the fish is immature the whole weight of the body
falls off. If it is mature the ' condition' is worst of
all just after the time of spawning, and the fish is
then thinner than at any other time in the year.

The reproductive phase, that is the actual spawning

42 LIFE IN THE SEA [CH.

act in the case of an egg-laying animal, or the birth of
the young in the case of a viviparous animal, almost
always occurs during the months of February to July
in northern seas. The majority of fishes, the cod,
plaice, flounder, whiting, haddock, and many others
spawn during the months of March and April. The
sole spawns during May and June, and the turbot,
brill, and gurnards about the same time, or a little
later in the year. The majority of invertebrate
animals also spawn during March, April and May.
The herring spawns somewhere in British seas
throughout the year, that is we have ' winter' and
'summer' herring. But even here the reproductive
act is a strictly periodic one for the various races of
herring spawning throughout the year are local ones,
each with its own reproductive phase. The act of
spawning lasts for a variable time : it is prolonged in
the case of the flat fishes, such as the plaice or sole,
where the eggs ripen in successive batches, but it
takes place quickly in the case of the round fishes
like the cod, where all the eggs come to maturity at
about the same time. It may be prolonged over a
month or two in the case of the skates and rays where
two large eggs are laid at a time. In the case of the
fishes which lay a great number of eggs — that is in
the majority — the numbers laid at first are few, but
they soon increase to a maximum and then quickly
decrease. Both the time of the maximum spawning

n] RHYTHMICAL CHANGE IN THE SEA 43

and the duration of the spawning act are variable
from year to year. The date of liberation of the eggs
depends on the temperature of the water — until the
latter has attained a certain point it does not occur.
But it also depends on the temperature of the sea
during the month or two previous to the time of the
actual reproductive act, for the maturation of the
generative products proceeds slowly when the tem-
perature of the sea is low, and more quickly when it
is high. That is to say the temperature effect is partly
an integrative one.

The growth phase begins almost immediately
after the completion of the spawning, for in some
way or other the parental habit compels the animal
almost entirely to neglect its own nutrition. Thus
the male lumpsucker, a shore-living fish, attends to
the mass of eggs laid by the female, the latter having
deserted them immediately on depositing them on
the sea-shore. It is necessary for the male parent to
i stand by ' the eggs and prevent them from being
devoured by other small fishes or invertebrates,
and to keep them supplied with a current of water
which he sets up by movements of his fins. During
all this time he refuses to feed (even if food is sup-
plied to him) while he and the mass of eggs are
being kept in an aquarium. But immediately after
the eggs have mostly hatched out he begins to
feed greedily. This is the case with nearly all fishes

44 LIFE IN THE SEA [CH.

— their time of gorging is after the end of the spawn-
ing period. From then till about midway between
the time of the annual temperature maximum and
that of the minimum both functional and morpho-
genetic metabolism are most intense. The animal
lives at a more rapid pace during the summer months
and takes in more food and more oxygen and gives
off more carbonic acid during that time. But the
type of constructive metabolism undergoes change,
for instead of the development of the reproductive
organs we find that a rapid growth of the tissues takes
place. The fish increases in size, and assimilated
food reserves are stored up in its flesh. All the
growth of the year occurs during this phase, and
the greatest amount of growth takes place at the
time when the temperature is at its highest. Growth
in a marine animal is a strictly 'periodic function'
and is capable of representation by a mathematical
expression. In the case of most marine fishes the
rate of growth can often be represented by a ' damped
sine curve/ that is it is higher than the mean, and
maximal, during the warm months, and it is less
than the mean, and minimal, during the cold months.
The period is an annual one, and the amplitude
decreases as the fish becomes older, following an
exponential law. Such an expression for yearly growth
may also apply to most mollusca, but not to Crustacea,
for in the latter animals the growth takes place

ii] RHYTHMICAL CHANGE IN THE SEA 45

by jumps, immediately after the animal has cast its
shell. It has happened that a crab has crept into
an empty bottle just after casting, and then having
grown to its annual amount during the next week or
so it finds itself unable to leave the bottle. In the
case of animals growing in this way the growth
function is a ' discontinuous ' one.

Thus some time in the spring months a wave of
production of organic substance in the form of new
individuals sweeps across the sea, dying away in the
summer months. This wave is exhibited in the annual
spawning of almost all the higher animals and it
affects the composition of the plankton. Just as
certainly as the vernal equinox approaches and the
sea temperature rises, so does the mass of planktonic
animals and plants inhabiting a unit volume of sea-
water increase to its annual maximum, and then
decrease again. Now we must clearly distinguish
between the increase in the mass of the plankton due
to the myriads of eggs and larvae spawned by the
fishes and larger invertebrates, and that due to the
reproduction of the pelagic diatoms, algae, prqtozoa,
and micro-crustacea. The former have only a tran-
sitory planktonic phase in their life-histories, the
latter persist in the plankton throughout their entire
lives.

The transitory plankton includes a great number
of species each of which is strictly periodic in its

46 LIFE IN THE SEA [OH.

appearance. The succession of the planktonic larvae
in any sea area is generally the same from year to
year, and it can be predicted. Let us take the suc-
cession of organisms in the sea of Liverpool Bay as
an example. We should notice first of all the eggs
of fishes, chiefly cod, plaice, whiting, dab and some
others, and then the larvae of these species of fishes.
By the end of May all these would have disappeared.
The larvae of barnacles appear in the water during
the month of March, with the larvae of numerous
species of crabs, and all these persist for a time, but
soon the barnacle larvae become metamorphosed to
their final stages, and the crab larvae are also trans-
formed to further forms. About the end of May the
barnacle larvae would have disappeared entirely, but
the later crab larvae would still persist. About the
end of May the larvae of shrimps would appear in
quantity, but would very soon leave the plankton, as
they take up their permanent habitat on the sea-
bottom. At the end of May and the beginning of June
the later spawning fishes would contribute their eggs
to the plankton, but by the end of August these also
would mostly have gone down to the sea-bottom as
demersal organisms. June is the season for many other
planktonic larvae, such as those of many marine worms,
sea-urchins, or starfishes, and of these some would last
on till near the end of the autumn, but by that time
most of the larvae would have left the plankton.

n] RHYTHMICAL CHANGE IN THE SEA 47

We see then that a great number of fishes and
invertebrates reproduce in the spring and summer
months and that the plankton increases largely in
bulk because of the immense numbers of eggs and
larvae so produced. But the increase in the mass of
the plankton during the spring months — which is so
remarkable a phenomenon in the sea — is due far less
to the appearance of these larvae than to the repro-
duction of the pelagic unicellular organisms. Just
as each species of fish and invertebrate has its own
spawning time, so each species of diatom or protozoan
making up the permanent plankton also reproduces
at a particular time. The stimulus to the reproduc-
tion of the higher animals is the temperature change
in the spring of the year, and this has also something
to do with the reproduction of the diatoms and pro-
tozoa, but it appears that the change of sunlight and
salinity are also factors of great importance in the
latter cases. During the first two months of the
year there are very few diatoms in the sea in
Liverpool Bay, but sometimes about the beginning of
March certain spring species begin to appear, and
attain a maximum of abundance during that and the
next month, while at the same time the animal
plankton, which was characteristic of the sea before
this time becomes temporarily less. The spring
diatoms (which are best represented by the two
genera Biddulphia and Coscinodiscus) become very

48 LIFE IN THE SEA [CH.

much less in May and June and are succeeded by
others which we may regard as summer forms. In
this part of the sea the summer diatoms belong to
the genus Rhizosolenia and they are always accom-
panied by enormous numbers of the pelagic alga
Halosphera, a species which forms little mucilaginous
capsules which are easily visible to the naked eye.
During the month of June, and sometimes part of
July, the sea off the coasts of North Wales swarms
with these organisms, and no biological phenomenon
in the Irish Sea is more remarkable than this in-
vasion. The diatoms and the other algae can easily be
seen with the naked eye in a small quantity of water
from the beach — even so little as will lie in an empty
mussel shell ; and the nets of the fishermen are often
covered with a slimy mass produced by the bursting
of the capsules of Halosphera. Both species are intro-
duced by the inflowing Gulf Stream drift which is
strongest along the west coast of England and Wales,
and they reproduce when the temperature of the
water is rising rapidly. Some time in the month of
July both species disappear almost entirely and then
the sea is invaded by shoals of jelly-fishes, belonging
chiefly to the species Aurelia and Rhizostoma, but by
the end of the summer these too become much less
in number and during the autumn they sink to the
bottom and die. Shortly after they have attained
their maximum of abundance the sea becomes

n] RHYTHMICAL CHANGE IN THE SEA 49

invaded by the protozoan Noctiluca, and about this
time the water becomes brilliantly phosphorescent
because of the presence of the organism. It lasts
well on into the winter, when it disappears. During
the autumn, diatoms belonging to various species may
again become abundant, though this is not an in-
variable occurrence, and sometimes during the same
period peridinians belonging to the genus Ceratium
attain a maximum of abundance.

This series of changes in the planktonic life of
the sea is of course characteristic of this particular
locality alone and the sequence may be modified
in other parts of the northern seas. But almost
everywhere the general scheme will be: (l)a relative
scarcity of all kinds of life in the first two months
of the year ; (2) the appearance of various species
of diatoms which attain a maximum of temporary
abundance, and then become much less in number ;
(3) a short scarcity of planktonic life during the
early summer months ; and (4) a relative abundance
of various species of protozoa and micro-crustacea
during the autumn. As the winter approaches all
forms of life again become scarce in the sea.

Now associating these various changes with the
changes in the physical conditions of the sea we
notice that they are always to be regarded as de-
pendent on certain physical rhythms : the inflowing

J. 4

50 LIFE IN THE SEA [OH.

water of the Gulf Stream drift which attains its
maximum in the early summer ; the increase in the
intensity of the sunlight in the early spring, and the
rise of sea temperature in the early summer. They
are therefore strictly periodic in their occurrence and
if there are fluctuations in the time of appearance of
the inflowing Gulf Stream water, or in the rise of sea
temperature, or in the change in intensity of sunlight,
then the time of appearance of the various maxima of
planktonic life may be accelerated or delayed. If
there are long-period rhythms in the order of the
physical phenomena which we have considered then
there ought also to be long-period rhythms in the
abundance of life in the sea. This is of course difficult
to prove but where we have fishery statistics it is
quite evident that there are such periodicities. Only
two or three times during the nineteenth century
have great shoals of herring appeared oif the west
coasts of England and Wales, in the estuaries of the
Dee and Mersey, and these times no doubt coincided
with some unusual combinations of physical conditions.
There is a winter herring in the Skagerak, oif the
coast of Sweden, and this has been recorded since
the year 895 and has been found to recur at intervals
of about 111 years.

But the movements and reproductive habits of
a marine organism are not entirely dependent on

ii] RHYTHMICAL CHANGE IN THE SEA 51

the seasonal changes in the sea, for heredity has
stamped certain habits on its organisation, and even
if the conditions were to remain the same there
would still be periodicity in the life-processes. This
periodicity would ultimately be obliterated if the
conditions of life were always to remain strictly
uniform, but for a time it would manifest itself. If we
keep such a fish as an eel in an aquarium we shall find
that as the period of its life comes when it should
be migrating down to the sea in order to spawn in
the deep water off shore, various changes will occur
in spite of the fact that the animal is being kept in
water of uniform condition. When it reaches a
certain age it ought to be seeking water of great
depth, and if it is unable to do this its metabolism
becomes disturbed, since it is adjusting itself uncon-
sciously to inhabit water where the pressure is very
great, but it still is compelled to remain in the shallow
water of an aquarium tank. We find therefore that
bubbles of gas begin to form underneath the skin
because the pressure of the water is far less than
that to which the fish is adjusting itself by sheer
force of inheritance.

We must regard heredity as a kind of flywheel
which causes an organism to undergo periodic changes :
reproductive changes, migrations, growth, etc. Apart
altogether from external stimuli the flywheel would
still revolve and the organisms would still display

4—2

52

LIFE IN THE SEA

[CH. II

periodicity. But we cannot doubt that if the external
conditions were to become strictly uniform then
seasonal change would also largely or entirely dis-
appear from the life-habits of animals.

Water-sample bottle and thermometer.

CHAPTER III

THE FACTORS OF DISTRIBUTION

IF we were to compare lists of the species of
plants and animals known to occur in the English
Channel, the Irish Sea and the Firth of Clyde —
adjacent sea areas which have been fairly well
investigated, we should find that the majority of the
kinds of organisms were common to all three regions,
but also that each of the latter possessed a few species
which did not occur in one or more of the others.
We should also find that certain species were most
abundant in one of the areas and decreased in density
as we passed north or south. If again we were to
compare the fauna and flora of the Indian Ocean off
the coasts of Ceylon with those of the North Atlantic
between the Faroe Islands and the coasts of Scotland
we should find quite the opposite kind of contrast :
the great majority of the species would be different
in each case, yet there would be a few which were
common to the two areas. If, further, we had ex-
haustive lists of the animals and plants found in the

54 LIFE IN THE SEA [CH.

seas within the Arctic and Antarctic Circles we should
find still greater dissimilarity. There would indeed
be a general kind of resemblance between the two
faunas and floras, but only in the cases of one or two
migratory birds and whales would the same species of
organism occur in both circumpolar areas. Speaking
quite generally, the further apart on the face of
the earth two sea areas are, the greater will be
the difference between their contained life, because
the further apart they are the greater, generally,
will be the- difference of physical conditions. Very
few animals or plants are truly cosmopolitan.

It has been the province of systematic biology to
investigate and record the range of geographical
distribution of each species of animal and plant, and
so far the result has been the accumulation of a mass
of detail which possesses the very slightest interest,
and out of which it is not easy to extract general
laws or regularities. We may, however, try to indicate
a few prominent facts. Marine mammalia — the
whales, seals, and some other species of warm-blooded
animals, are most abundant within the two Polar
Circles, and the larger sea-birds inhabit the cold and
temperate seas, and are least abundant within the
tropics. The marine and anadromous fishes of the
sub-polar and temperate seas are far more abundant,
and are generally larger than those of the tropics,
but the variety of species is not so great as in

in] THE FACTORS OF DISTRIBUTION 55

the latter regions. So also with the invertebrates
generally : we find the greatest density in the colder
seas but there are more bizarre forms and richer
colouring in the warmer waters within the torrid
zone. Molluscs are much more abundant in the
temperate seas, and generally the shells of the cold-
water molluscs are less massive in proportion to the
size of the animal than in the warm-water species ;
thus the largest marine shellfish — the great Tridacnas
and the mother-of-pearl shells — are inhabitants of
tropical seas. The pelagic molluscs of the warm waters
have generally greatly reduced shells, while those of
the northern sub-arctic seas — the winged pteropods
on which the whalebone whales feed, have a more
massive shell. We find also that those animals which
secrete lime do so to a greater extent in the tropical
seas : thus the reef-building corals are practically
restricted to a belt of sea extending to about 20°
north and south from the equator, while even the
solitary corals are not very abundant in the colder
seas. The micro-crustacea are probably about equally
varied in the cold and warm seas but they are vastly
more abundant in the temperate and sub-arctic seas.
Marine plants are much more plentiful in the tem-
perate and polar seas : the larger rooted Algae are
represented by more species and are denser in the
temperate seas than anywhere else in the ocean.
The pelagic microscopic diatoms, though found

56 LIFE IN THE SEA [CH.

everywhere in the seas of the world are conspicuously
more abundant the further north or south we go.
But some pelagic algae — the small form Tricho-
desmium, and Sargassum, the gulf-weed, occur in
abundance only in tropical waters. The mangrove
swamps of the tropical sea-margins are also instances
of associations of brackish water plants which do not
occur in the temperate or polar seas.

All experience shows that the polar and temperate
seas are, generally speaking, far richer in life than are
the tropical "ones. There is a striking picture of the
relative density of life on land and sea in Darwin's
description of Tierra del Fuego. That desolate and
inhospitable land sustains with difficulty a sparse and
cannibalistic population, while the seas near the shore
contain such an abundance of life as Darwin could
only compare with that of a terrestrial forest in the
intertropical zone. Chlin, in his account of the voyage
of the German exploring ship 'Valdivia,' speaks of
the teeming plant and animal life of the ice-covered
Antarctic sea. The naturalist Kroyer found an abun-
dance of invertebrate life in the sea off Spitzbergen
such as he had never seen surpassed elsewhere. The
greatest and most productive fisheries of the world
are those of the seas round Britain, off the coasts of
northern Norway, on the Banks of Newfoundland, in
the seas off Iceland, and oft* north Europe close up to
the ice-packs. Nowhere else in the world has there

in] THE FACTORS OF DISTRIBUTION 57

been such extensive and ruthless destruction of
mammalian life as among the whales and seals of
Arctic and Antarctic seas. The account of every
Antarctic voyage contains striking pictures of the
enormous wealth of bird life in the penguin rookeries,
and among the petrels and gulls; and also of the
incredible abundance of diatoms, which visibly dis-
colour the sea- water and clog the nets used for fishing.
All plankton investigation bears witness to the paucity
of microscopic life in the tropical seas and to its
abundance in the north temperate and sub-polar
waters. The German exploring ship ' National'
obtained the richest plankton catches in the sea off
the south of Greenland, and the poorest in the warm
Sargasso Sea; and this has been the experience of
most investigators.

We see that there are some general facts of
distribution. The variety of species is greatest in
tropical seas, and least in the temperate and polar
seas; and it is greatest near the land, and least in
the oceanic areas far away from the influence of the
land: generally the density of both benthos, nektic
and planktonic life decreases as we pass out from
shallow into deep water. In the middle of the
greatest oceans planktonic life is scarcest, and at the
bottom of the oceans, in the abyssal regions, the benthos
is also scarcest. Perhaps no part of the ocean bed is
actually devoid of animal life, but the nearest approach

58 LIFE IN THE SEA [CH.

to the terrestrial deserts is to be found in the great
abyssal plains of the ocean beds. Plant life is most
abundant in the shallow water near the shore, and
least abundant at the surface in the mid-oceanic areas.
It is most abundant just a little way beneath the
surface of the sea everywhere and then it decreases
as we descend into the depths. It is absent entirely
at the bottom of the deep sea.

We have seen also that there are cyclical changes
in the abundance of life of all kinds ; changes which
are seasonal ones. Each species is normally restricted
to some particular area of sea or sea-bottom, but
this area expands or contracts with the seasons
and so the range of geographical distribution of the
organisms of the species also expands and contracts ;
that is, they are able to migrate. We have now to
consider in what way these general facts of distribution
and distributional changes are to be associated with
the physical conditions in the sea, and how fluctua-
tions of life are to be associated with fluctuations in
the physical conditions.

We consider first of all the physical conditions of
temperature, salinity, and sunlight; and then the
nature and abundance of food in the sea. These are
the factors of distribution. The surface temperature
of the sea is highest a little way north from the equator,
and then it decreases towards the poles, falling at a
nearly equal rate as we pass north or south. This

in] THE FACTORS OF DISTRIBUTION 59

regular decrease of temperature is modified by the
influence of the land and by ocean currents : thus the
temperature of the Red Sea, and of the Atlantic Ocean
to the west of Ireland are higher than they ought to
be if the latitude were the only factor, because of
the influence of the land in the first case, and that of
the Gulf Stream in the second. The temperature
of the Atlantic off the coast of North America is lower
than it ought to be because of the influence of the
Labrador Current. The temperature of the sea is
very generally highest at the surface and then falls
towards the bottom, quickly at first and then more
slowly. Towards the bottom in very deep seas it may
rise a very little, probably because of the generation
of heat from radio-active substances in the bottom
deposits, but it is a general rule that almost every-
where the temperature at the ocean bottom lies
between 2° C. and zero, and in places it may fall
below the freezing point of fresh water. Occasionally
the deeper or the intermediate layers may be warmer
than those of the surface, but these conditions are
anomalous and are to be traced to the action of
currents.

The salinity of the sea is rather low along a belt
of ocean between the equator and 5° N., that is in the
region of tropical calms and torrential rainfalls. It
is highest along two belts of ocean situated between
25° K and 30° N. ; and between 20° S. and 25° S.,

60 LIFE IN THE SEA [OH.

that is, in the regions of high evaporation due to the
trade winds. On each side of the trade-wind belts
the salinity falls — rapidly towards the north, and
less rapidly towards the south. In the Arctic and
Antarctic seas the salinity is low because of the
melting of ice which contains less salt than sea-water.
Here and there we find marked deviations from this
general distribution of salinity — the Red Sea, for
instance is salter than it ought to be because of the
great evaporation due to the high temperature ; and
the sea off the coasts of Denmark at some parts of
the year is fresher than it ought to be because of the
influence of the outflowing Baltic fresh current.

The intensity of sunlight is greatest on the surface
of the tropical seas, and least on that of the circum-
polar seas. But the amount of solar radiation falling
on the surface of the sea is greatly affected by the
amount of fog and cloud, and variations due to these
causes affect the abundance of plant life at the surface
of the sea.

The physical conditions over the oceans are far
more uniform than over the land. Thus the extremes
of temperature on the land are — 90° C. and 65° C., a
range of 155°C., while the extremes in the sea are
only -2'8C C. and 31° C., a range of 33'8° C. There
are no changes in the sea corresponding to changes
of humidity, or to rain, hail, snow, etc., on the land.
As we ascend the high mountains the temperature

in] THE FACTORS OF DISTRIBUTION 61

falls to a much greater extent than it does as we
descend into the depths of the ocean abysses. If we
ascend to the summit of a mountain 24,000 feet high
the pressure does not decrease by nearly so much as one
atmosphere, but as we descend to a corresponding
depth in the sea the pressure rises to about 700
atmospheres. The extremes of salinity variation in
the sea are about 30 and 36 except in very exceptional
cases. The annual change of temperature is much
less on the land than in the sea. The annual change
of sunlight intensity due to the seasons is of course
the same in each case, and so also is -its variation
from place to place since this depends on the latitude.
The distribution of plants and animals on the land
is restricted by mechanical barriers, by mountain
chains, deserts, seas, great rivers, etc. In many cases
we find that these barriers are insurmountable so
that the great continents and the oceanic islands are
characterised by the possession of species peculiar to
themselves. Birds and insects may fly, or be blown,
for great distances, and plant-seeds may be transported
in several ways ; but we find that the barriers have
operated in producing the present distribution and
diversity of life ; and that apart from human agencies
very few species of plants or animals would be truly
cosmopolitan. In the sea, however, there are no such
mechanical barriers to distribution, for the watery
medium is everywhere continuous : on the other hand

62 LIFE IN THE SEA [OH.

the oceanic currents afford a powerful means of
transport of both animals and plants such as no agency
provides on the land.

So we find that relatively large areas of sea or
sea-bottom may possess a fairly uniform fauna or
flora — the Sargasso Sea in the tropical Atlantic, the
intermediate layers of the North Atlantic, or the
bottom of the same ocean at over two thousand
fathoms depth are examples. Even apart from such
cases of wide distribution of a nearly similar life there
are cases of the range of isolated species over very
large tracts of ocean. Thus some copepods found in
the sea off the coast of Ceylon or in the Gulf of Guinea
are also found in the waters of the Faroe-Shetland
Channel ; and some diatoms inhabiting Chinese seas
are now known to occur over many parts of the North
Atlantic. Peridinians occurring in the Pacific off the
coasts of California are also found off the coasts of
Norway. Some species of tapeworms inhabiting the
intestines of fishes in the sea off Ceylon also infest
British fishes. In these cases the agency of the wide
limits of distribution is the oceanic circulation — the
Chinese and Ceylonese diatoms and copepods have
been taken into the Equatorial Stream circulation
and so into the Gulf Stream system; the Pacific
peridinians have discovered the north-east passage,
and have drifted across the North Polar basin ; while
the tapeworms probably in their early larval stages

in] THE FACTORS OF DISTRIBUTION 63

inhabit some pelagic animal which is drifted for great
distances.

The barriers to the universal distribution of marine
organisms are to be found in the different types of
structure evolved by them, and in corresponding
specialisation in the modes of metabolism acquired
during the course of their evolution. Most organisms
have developed types of bodily conformation which
necessarily restrict them to definite areas of sea or
sea-bottom. A shore alga lives attached to a stone
on the beach and cannot exist floating permanently
at the surface of the sea ; on the other hand a pelagic
alga or diatom lives drifting at the sea-surface and
cannot always inhabit the sand or mud of the shore.
A pelagic fish may roam over extensive tracts of sea-
surface, but if it attempts to descend towards the
bottom the increasing pressure will be fatal to it;
and conversely an abyssal fish might migrate over
the entire ocean bottom of the world sea, but it cannot
approach the surface without great distension of its
swim-bladder, protrusion of its stomach through its
mouth, and disintegration of the muscles and other
tissues — such injuries as would be fatal to it. Sessile
benthic animals like zoophytes, barnacles, or sea-
anemones must live on the beach or at the sea-bottom
attached to stones, and cannot drift about at the
surface; while littoral animals such as the cockle
or lugworm must restrict themselves to the shore

64 LIFE IN THE SEA [OH.

between the tide marks. On the other hand plank-
tonic organisms like medusae or ctenophores must
drift about at the surface, or just beneath the
latter in clear water. If they are stranded by tides
or gales — a thing that often happens — they perish.
In the course of their evolution of certain types of
structure and habit most marine organisms have
surrendered the capacity of existing except under
very restricted conditions.

Yet this limitation of habitat is partially com-
pensated for by the evolution of larval stages in the
life-history of an organism. We nearly always find
that a sessile benthic animal has evolved a free-
swimming larval stage : or the primitive pelagic form
has evolved a sessile habit during the latter period
of its life-history. Whichever view we take it is the
case that the great majority of animals which live
attached to the sea-bottom, or burrowing in the sand
or mud, or crawling or swimming along the sea-
bottom, begin life as eggs which drift about in the sea,
or which may be attached to the parent during their
period of incubation. In either case the creature
which hatches out from the egg-shell is a larva, and
it lives pelagically among the plankton drifting over
relatively wide sea areas. This is the meaning of
a larval stage in the life-history of an animal —
it is an interruption of the course of the develop-
ment to afford the incompletely formed embryo the

in] THE FACTORS OF DISTRIBUTION 65

opportunity of feeding and being drifted about by
the tides, winds and currents over as wide an area as
possible. After a pelagic phase of variable duration
the larva undergoes one or more metamorphoses and
assumes the form of the parent, and then settles
down on the sea-bottom. The larva is usually loco-
motory though the adult is not : thus the rooted
immoveable algae produce motile zoospores and the
fixed zoophytes free-swimming medusoid larvae.
But whether or not the larva is locomotory it is so
small, and its specific gravity is so near that of sea-
water, that it is carried for great distances by tidal
streams and currents, and it may settle down very far
away from the place where it was born. Its life as
a larva is usually a very limited one — if the free-
swimming stage lasted long enough there would not
appear to be any limit to the normal distribution of
the species to which it belongs. The object of the
development of the larval stage — if we may speak of
an 'object ' in an evolutionary process — is obviously
the enlargement of the area of distribution of the
species.

This distribution of a sessile, or semi-sedentary,
species by means of free-swimming larvae must be,
to some extent, a fortuitous process. For though
the larvae may appear to be very active creatures
when examined in a few drops of water under a
microscope, yet their powers of locomotion are very

J. 5

66 LIFE IN THE SEA [OH.

feeble when compared with the distances to which
they may be carried by wind-drifts and currents.
They can have no control over the direction in which
they are being carried, and when the metamorphosis
occurs — a process over which also the larva has no
control — it must assume the habitat of the parent ;
and if it is a benthic form it must settle down on the
sea-bottom. If it is a littoral form — a cockle or
barnacle for instance — and if the eggs or larvae have
been transported into deep water, when the meta-
morphosis occurs the newly developed adult will
settle down to the sea-bottom and will be destroyed.
Or the larvae of a pelagic fish inhabiting deep water
may be carried into the brackish water at the mouth
of an estuary and will so be lost. It is easy to see
that very great numbers of planktonic larvae must
therefore fail to find their proper habitat at the time
when the transformation occurs, and this is one of
the reasons why most marine organisms produce
such large numbers of pelagic eggs — the turbot,
for instance, may spawn as many as nine millions of
eggs each year ; even the relatively slowly repro-
ducing barnacle spawns about five to nine thousand
eggs.

Yet the manner of distribution is not such a
hap-hazard process as it might appear to be, for
though the majority of larvae may fail to enter
adult life at the proper time or place, enough may be

in] THE FACTORS OF DISTRIBUTION 67

successfully ' planted ' to carry on the species in un-
diminished numbers. Moreover the eggs are usually
so liberated by the parent that the currents or tides
will carry them to suitable places. If the parent is
sessile in habit it is usually found in shallow water
near the shore, and the tidal streams flow, as a rule,
along the land, or surge backwards and forwards to
and from the land ; therefore the larvae may inhabit
shallow water during the whole of their free-swim-
ming period. If the parent animal is a nektic one it
usually performs a spawning migration, that is to say
it migrates when about to spawn to such a place as
will ensure that its eggs will not all be lost. The
majority of fishes produce buoyant eggs which float
at or near to the surface of the sea, but the larvae
hatched from these eggs prefer to inhabit shallow
water near the shore. When the fish is about to
spawn it migrates out to sea to some distance from
the land and then spawns, and the spawning place is
so selected that the currents at that time of year, or
the prevailing winds will drift the eggs towards the
land. The cod, plaice, flounder, sole and many other
fishes behave in this way — they migrate offshore to
spawn, and then the eggs drift inshore, completing
their embryonic development during the transporta-
tion, so that, barring accidents, the larvae are just
about to undergo metamorphosis by the time they
reach shallow water. The spawning takes place

5—2

68 LIFE IN THE SEA [OH.

during the spring when there is abundant food,
and when the temperature is rising rapidly in the
shallow water near the land ; therefore the young
larvae are carried to a land of plenty. The egg-
laying skates and rays behave in an analogous manner,
migrating towards parts of the sea where the bottom
is rough and stony and contains abundance of sea-
weed, and where there is little run of tide. The eggs
are laid on the sea-bottom where they develop with-
out much risk of being washed away by the tide, or
stranded on muddy beaches, or taken out into deeper
water. The herring makes a somewhat similar migra-
tion for the same purpose. The salmon was probably
originally a marine fish which adopted the habit of
migrating into rivers in order that its eggs might not
be exposed to the same risks of destruction by such
enemies as they would encounter in the sea. Thus
the spawning migrations of fishes are directed move-
ments carried out to achieve a definite purpose. We
need not suppose that they are conscious actions ;
and we may regard them as ' instinctive acts.' The
stimulus to the performance of the spawning migra-
tion is the development and ripening of the generative
organs, and the elaboration of an internal secretion
from the ovary or testis which produces an intoxi-
cation, and impels the fish to seek water of definite
physical conditions. What these conditions may be
depends on the former history of the species — the

in] THE FACTORS OF DISTRIBUTION 69

' historical basis of acting' being the determining
factor in this choice.

Thus the evolutionary history of a species has
determined to some degree its mode and extent of
distribution ; for the development of definite struc-
tures and habits restricts the animal to a particular
range of habitats. Nevertheless it is easy to see that
most species of marine animals would be distributed
over a much wider area than they actually are if
there were not some factors which limit this distri-
bution. These factors are those of temperature,
salinity, pressure of water, and intensity of sunlight.
We have next to consider in what way these affect
marine organisms.

The temperature is probably the most important
of all these factors, and it is easy to see that there are
many ways in which a marine animal, such as a fish,
can be affected by changes in it. There is no reason
to suppose that a fish cannot feel changes of temp-
erature in the way we do, that is by its sense-organs.
It appears that sudden changes can produce patho-
logical effects, for fresh-water fish are known to be
affected by a form of dermal catarrh when they are
suddenly moved from water of high to water of
much lower temperature ; and there is a record of
the destruction of millions of tile-fish off the coast
of North America by a rapid and extensive chilling
of the water stratum in which they were living.

70 LIFE IN THE SEA [OH.

Such sudden temperature changes must however be
very exceptional in the sea, and a nektic animal, such
as a fish, would usually be able to respond to them
by means of a migration into water of more favour-
able conditions. Temperature changes must have
for their usual effects changes in the intensity and
mode of metabolism of the animal ; and there is
plenty of experimental evidence that this is the case.
We have to regard the life-processes of an animal as
the result in some way of a regulated series of
chemical reactions, and we know that the velocity of
a chemical reaction increases as the temperature rises.
The amount of oxygen absorbed in respiration and
that of the carbonic acid excreted are greater or less
according as the temperature is greater or less ; and
the rapidity of respiration varies in the same way.
In cold water the respiratory movements of the
mouth and gills of a fish slow down greatly and
often assume the Cheyne-Stokes type, that is there
are relatively long pauses followed by groups of re-
spirations. This means that the rate of metabolism,
that is the amount of chemical change going on in the
tissues of the animal, decreases as the temperature
falls. Towards the end of the year the quantity of
food taken decreases and many fishes such as the
plaice and flounder cease altogether to feed during
the months of December to February. The stomach
and intestine are usually empty during this period ;

in] THE FACTORS OF DISTRIBUTION 71

tissue waste is reduced to the minimum ; and the
animal hibernates by burying itself in the sand at the
sea-bottom. The tissues are however wasted away
to a greater extent than they are renewed, and the
' condition ' of the fish is at its worst at the period of
minimal sea temperature.

The rate of development of the embryo of a
marine animal varies also with the temperature ;
thus a flounder embryo will hatch out from the egg
in about ten days at the beginning of the spawning
period, when the water is coldest ; but it will hatch
out in about six days at the end of the spawning
period when the water has become much warmer.
This relation holds for all developing eggs of cold-
blooded animals which have been investigated, and
it has been stated as follows ; for a difference of
temperature of 10° C.

Average duration of development at n C. _ 2*85
Average duration of development at (n — 10)° " 1

which means that when the temperature is reduced
by 10° C. the time required for the hatching out of
the embryo of a marine animal is increased about
threefold.

The duration of life also varies with the tempera-
ture in such a way that the lower the temperature of
its life-medium the longer an animal lives. Probably
this also holds good for warm-blooded animals — for

72 LIFE IN THE SEA [OH.

instance cats which live in cold-storage warehouses
have sleeker coats and are apparently healthier than
are animals living under normal conditions ; and it
is possible that the connection between length of life
and temperature might also be proved by comparative
vital statistics. However this may be, the following
experiment affords direct evidence of the probability
of the statement : sea-urchin eggs are fertilised and
are kept at a constant temperature, and then small
quantities are taken and are put into water of
different temperatures, and the higher the latter the
shorter is the life of the egg. If we call the duration
of life of an animal at temperature t° at, then the
duration of life at temperature (t - n)° is a&n. This
means, supposing of course that the experiments
referred to should be confirmed, and prove to be
of general application, that if the temperature of the
water in which a cold-blooded animal is living is
reduced by 10° C. its duration of life will be increased
a thousand-fold. It seems too good to be true !

It would seem, however, that some such relation
between temperature and length of life of the kind
stated does actually exist. Now let us apply it to
the case of the observed density of plant and animal
life in warm and cold seas and we have a possible
explanation — not the only one as we shall see — but
still perhaps the most probable one. When the
temperature is reduced by 10°C. — not so great a

in] THE FACTORS OF DISTRIBUTION 73

difference as we find between tropical and polar seas,
the duration of life is increased about a thousand-fold,
but the rate of development is reduced to only a third.
This means that a marine organism will reproduce
about three times less actively in cold seas than it
will in warm seas, but it will live a thousand times
longer ; and therefore it must be much more prolific
and there will be many more overlapping generations
in the polar than in the tropical seas. If the relation
holds good we must have a greater density of life
in the former waters than in the latter. We should
not, indeed, expect to find such a difference as would
be expected from the experiment described, for the
abundance of food would limit the development, and
so also would other factors — the less intensity of
sunlight, for instance, in the case of a marine plant,
such as a diatom. But it seems clear that the
difference between rate of development and duration
of life is a factor. How exactly the temperature
affects the duration of life we do not know : it
may be that some substances are produced, and are
accumulated in the tissues, and that these substances
are detrimental to life. If these toxic substances
are produced at a greater rate the higher is the
temperature, then the duration of life will be
• shorter at high than at low temperatures.

How does a change in the salinity of the sea-water
affect a marine animal ? If a fish, say, moves from

74 LIFE IN THE SEA [CH.

water containing a relatively high concentration of
salt to water containing a low concentration, can it
perceive the change by means of its receptor taste
organs ? It does not seem likely that we could
perceive by taste alone the difference between water
containing 3 '45 % of salt and water containing 3*40 %,
nevertheless this is a change towards which a fish
might react. Now the atmosphere is to us very
much what the sea-water is to a fish, and we can
detect by means of our sense-organs changes less
than that instanced. Let us suppose that the tem-
perature falls rather suddenly from 20° C. to 15*6° C. :
we should certainly be able to detect such a change.
It would be a change of 1*5 % of the total possible
range of temperature, regarding 20° C. as the upper
limit; and the salinity change is also 1*5% of the
total range, regarding 3'45 % as the upper limit.
It seems quite probable then that the fish would be
able to detect, by means of its taste-buds, the change
in the saltness of the water, especially as we have
reason to believe that the olfactory receptor organs
in a fish have a lower threshold than in our own case,
and smell is only ' taste at a distance.'

But we are not restricted to the supposition that
the fish can only be affected by salinity changes if
it can appreciate those by its organs of taste. Its
blood and lymph are watery fluids containing certain
salts of sodium, potassium, lime and magnesium in

in] THE FACTORS OF DISTRIBUTION 75

solution, and these liquids are separated from the
surrounding sea- water only by a membrane which is
very thin in certain places, over the gills for instance.
Now the sea also contains these same salts in greater
concentration than they are contained in the blood ;
and if the membrane covering the gills were a dead
one, then water would diffuse out from the blood and
lymph of the fish into the surrounding sea-water.
It does so diffuse but only to a very slight extent, and
not nearly so much as would be expected if the
process of diffusion in the case of a living membrane
were the same as that which holds good for an
inorganic one. Diffusion does not take place to the
full extent possible, for among all the chemical and
physical reactions that may occur in the tissues of
an animal some do not happen because of the power
of regulation possessed by the living organism. But
if the concentration of the sea- water is higher than
that of the blood, and if diffusion does not take place,
then work must be done to prevent it. If the fish
moves from a region of high to one of low concen-
tration less work must be done, and this is a reaction
on the part of the organism towards the change of
salinity.

It is easy to showr that changes in the intensity
of sunlight must directly affect a marine organism.
They do, of course, profoundly affect a green plant,
as we shall see later ; but animals which do not

76 LIFE IN THE SEA [OH.

contain chlorophyll in the tissues are also affected.
Those which have eyes will probably perceive light
changes in the way we do, but even if eyes are
absent there is always the possibility that light
changes can be perceived by means of the skin.
Intense light radiation may even kill some of the
lower organisms, and in ourselves certain well-known
curative effects depend on some such reaction on
the part of diseased tissues. Almost all animals
which can be kept in aquaria respond by directed
movements to changes in the intensity of the light
falling on them. In the cases of the fishes and
higher Crustacea these movements are usually away
from the source of light, but in the cases of the lower
organisms they may be either towards or away from
the light. Plaice and flounders in aquaria usually
prefer the darker parts ; and in the sea they lie on
the bottom during the day, but at night they may
swim up towards the surface. Crustacean larvae
usually swim to the source of light when the other
conditions are normal. The abundance of animal
plankton in the sea is affected by the intensity of
light in that the organisms move up and down from
stratum to stratum of water. We usually get a
different kind of plankton during the day from that
we get at night, and even the amount of cloud in
the sky will make a difference.

If a species of plant or animal can possibly

in] THE FACTORS OF DISTRIBUTION 77

inhabit a wide area of sea, but is restricted to only
a part of that area it is because it finds in the area
which it has come to inhabit its optimal conditions
of life. In the course of its evolution not only has
a definite form been developed, but also an adapta-
tion to certain conditions of temperature, depth of
water, salinity, and sunlight. It does not follow that
because the optimal temperature for a marine diatom,
say, is 10° C. it cannot exist at 0°C. or 15° C., but
its vitality is greatest at 10°C., and it reproduces
most accurately at that temperature. If we take
some beef tea and inoculate this with some species
of bacillus we shall probably find that the most rapid
reproduction takes place at a certain temperature—
37° C. may be a common optimum. Far below this
temperature the bacillus would still be able to live,
but it will not reproduce when the temperature
approaches the zero of the Centigrade scale, and at a
temperature not very far above the optimum it will die.
Most species of unicellular organisms are able to live
at any temperature found in the sea and some are
said to be able to withstand even the cold of liquid
air ; almost any salinity found in the sea is fit for
marine life, and any degree of ordinary illumination.
But the rate of reproduction of any organism is
greatest for a certain combination of all the physical
factors, and if these are varied, or even if one of
them is varied greatly, the rate of reproduction is

78 LIFE IN THE SEA [OH.

affected favourably or unfavourably according to
the direction of the variation. If the latter is great
enough the species will not reproduce at all even
if it should still be able to live. At all times the
organisms of a species are subject to destruction by
their natural enemies, and the species is only able
to maintain itself if its powers of reproduction are
at least equal to the rate at which it is being
destroyed. It will not usually be the case that a
species lives under optimal conditions : generally it
will just be able to hold its own against its enemies
but at certain times and places the conditions become
optimal and then the species attains a temporary
maximum of abundance. Such a maximum occurs
during the spring in the case of the diatoms, for the
temperature, the food, and the intensity of sunlight
have then become optimal.

Where in the sea the conditions are optimal there
the species has its centre of distribution. From this
centre it spreads out in all directions that are possible,
for the tendency is for it to enlarge its area of distri-
bution, and the further we go from the centre the less
abundant it will be, and at a certain distance it will
disappear entirely. Near an imaginary contour line
drawn round the centre of distribution the rate of
reproduction is just equal to the rate of destruction ;
beyond this line the latter becomes the more powerful
of the two, and the species becomes rarer and rarer

in] THE FACTORS OF DISTRIBUTION 79

until it can no longer be found. The area within the
contour line we may call the area of productive
distribution, that without it, and including all parts
of the sea where the species has occurred, we may call
the area of nominal distribution. It is unable to
maintain itself within the latter area unless it is
recruited from the former one.

The individuals of a species may be carried far
beyond their area of optimal distribution, for if pelagic
larvae are produced they may be transported for
considerable distances by currents. Thus the pelagic
larvae of the crustacean Squilla may be found in
St George's Channel but we have no record of the
occurrence of the adult there. It probably reproduces
only in the Biscay-Mediterranean waters and the
larvae are carried to the north in the Gulf Stream
drift. So also with the copepods which are occasion-
ally found in the Faroe-Shetland Channel: their
productive area is in tropical or sub-tropical seas and
the larvae are simply drifted into northern waters,
growing but not reproducing there. If such pelagic
larvae were to live long enough there would probably
be no limit to their area of nominal distribution.
Powerfully swimming nektic animals or migratory
birds are also able to migrate far beyond their area
of optimal conditions ; thus sturgeon, breeding only in
the great rivers flowing into the Baltic may never-
theless be found over practically the whole north

80 LIFE IN THE SEA [OH.

European seas; and the Arctic tern, which breeds
only in the Arctic Circle may yet be found in Antarctic
seas.

But the area of optimal conditions varies with the
seasons and we therefore find that the productive
distribution of most species fluctuates within certain
limits. Flounders live under optimal conditions in
the brackish waters of the estuaries when the tem-
perature of the water there is higher than it is in the
open sea offshore ; but whenever the first winter snows
begin to melt and form freshets in the rivers then the
fish begin to migrate offshore, where the temperature
is now higher than it is in the estuarine areas. The
cod which come into British waters during the winter
have migrated down from more northerly latitudes
because when the temperature of the sea was falling
there it was a little warmer to the south. The sea-perch,
mackerel and gar-fish (Belone) are species which have
their area of productive distribution to the south-east
of the British Isles, in the Mediterranean, and off
the coasts of southern Europe; they migrate into
northern waters because the flooding of the latter
with Atlantic water in the spring and the seasonal
rise of temperature have enlarged their area of distri-
bution. Plaice from more southern latitudes invade
the Barentz Sea to the north of Europe in the autumn
because the temperature of the bottom water there
has been raised by the inpouring of water from the

in] THE FACTORS OF DISTRIBUTION 81

Gulf Stream drift. If we study the migration paths
of some species of fishes we can see they move so as
generally to cross the isohalsine or isothermal lines in
the sea. If in any place the salinity and temperature
alter, the fish seem often to move so as to regain the
former conditions. Many fishes inhabit, by preference,
not the inflowing Atlantic water of high salinity, nor
the fresher coastal water, but rather the mixture of
the two. The herring which are caught during the
summer off the east coasts of Britain first appear off
the Western Hebrides and are then found further
and further to the south as the season advances.
This used to be explained by supposing that the fish
migrated from somewhere in the north and then
moved to the south in the course of the season. But
the herrings which appear at each station spawn, and
it can hardly be the case that they all belong to the
same shoals. It is far more probable that there are
local races of fish which inhabit the North Sea
throughout the year and that they shoal and spawn
when the conditions become optimal. This optimum
of conditions is the result of the inflowing Atlantic
water from the Gulf Stream drift, and this flows
further to the south as the year advances, while the
herring cling to the borders of the salt water where
it is mixing with the fresher water coming from
the land. Halibut and ling inhabit the bed of the
North Sea where it slopes down into the depths

J. 6

82 LIFE IN THE SEA [CH.

of the Norwegian Sea. At the bottom of the latter
is a stratum of cold Arctic water and overlying
this is the warmer and salter water from the Gulf
Stream, while between these strata is a layer inter-
mediate between the two in character, and this is
the region of optimal conditions for the fish. Each
of the principal strata of water shifts periodically with
the rhythmic movements of the Gulf Stream and
Arctic current, and the fish migrate so as always to
remain in the mixture of water from the two sources.
A bottom-living, sessile animal is unable to respond
to changes in the physical conditions by migrations ;
it can only do so by changes in its own mode of
metabolism. It can, however, distribute itself over a
wide area by reason of the fact that it produces free-
swimming larvae which are carried in the currents of
the sea. But a nektic organism which usually also
produces free-swimming larvae must generally have
a wider range of distribution than a bottom-living
fixed one, since it can migrate so that it will always
remain within the area of optimal conditions. It is
usual nowadays to describe the movement of an animal
as a 'tropism' or ' taxis/ The tropistic theory is the
result of the modern tendency to look upon all the
activities of an organism as the results of known
chemical or physical laws. A taxis is some movement
on the part of an organism which follows inevitably
on some external stimulus, unless it is prevented by

in] THE FACTORS OF DISTRIBUTION 83

some other stimulus, or by some regulatory action
effected by the nervous system of the organism. The
flight of a moth into a candle flame is an example of
phototaxis, and so is the action of a bird when it dashes
itself against the glass of a lighthouse lantern. Now
probably these are acts which can fairly well be
described as inevitable taxic movements which the
animal must perform under the irresistible stimulus of
the light. So also the movement of an inebriate
towards a public-house door, when he had just been
given a sixpence, would fittingly be described as an
inevitable taxis or tropism. But it seems to be rather
straining after generality to attempt to describe all
the movements and migrations of organisms as
tropisms. This, for instance, is the kind of physiology
that is often taught. A certain caterpillar feeds on
the tender shoots at the upper extremities of a plant,
and to reach these it must migrate upwards towards
the light, for the plant grows so that it always bends
towards the source of light. The caterpillar is said
to be ' orientated' by the light falling on its bodily
surface and ifc must place itself so that the light strikes
equally on both sides of its body. Since it moves
upwards it is said to be 'positively phototactic.' But
having fed it now moves downwards again and it is
now said to be 'negatively phototactic/ the act of
feeding having reversed its reaction to light. Obviously
one might describe the movements of a hungry man

6—2

84 LIFE IN THE SEA [OH.

towards and then away from a fried-fish shop in just
the same way. Now there is another manner of
description which will appeal with greater force to
some minds, and which makes use of effective con-
sciousness as an agency of regulation of the move-
ments of an organism. An animal in good health has
superabundant vitality, and it moves about continually
in all directions, it may be in search of food, it may
be quite capriciously. It is adapted to live best in
certain optimal conditions and if it happens to move
into a region where the conditions are unfavourable
it will endeavour to avoid this region. Its method is
that of ' trial and error/ For instance, plaice of over
one and under two years of age have for their optimal
conditions those of the sea in shallow water where
the temperature is rather high, where there is plenty
of light, and where there is plenty of food in the shape
of small shell-fish. As the fish moves about it must
often enter the region of darker, colder, and deeper
water offshore, but having done so both the decrease
in the intensity of light, and either the increasing
pressure of the water or the increase of salinity will
affect it in some of the ways suggested, and then it
will turn this way and that, testing the conditions all
round it by means of its receptor organs, until finally
it reverses its former path and re-enters the former
region of optimal conditions. If a rapidly growing
bed of young mussels forms anywhere within the area

in] THE FACTORS OF DISTRIBUTION 85

of optimal conditions of such young plaice, it will very
soon be inhabited by a multitude of fish feeding
greedily on the nutritious shell-fish. Now we must
assume that they actually 'like' the mussels, for it is
the case that even if other food is abundant the plaice
of certain regions will take the young mussels to the
exclusion of other shell-fish — there must therefore be
choice on the part of the fish. A fisherman will
doubtless say that the bed of mussels has ' attracted '
the plaice, and a physiologist of the mechanistic school
will explain the aggregation of the fish on the shell-fish
bed in terms of his ingenuous theory of tropisms, and
may call it an example of 'trophotaxis,' or some such
term. What doubtless happens is that the fish,
moving about in all directions within their optimal
area, accidentally encounter the shell-fish bed and
finding food which is to their liking they will stay
there. While feeding upon the mussels they still
move about and it must often happen that a fish will
leave the bed and so will not obtain so much food.
But by turning about it will again, after a number of
trials and failures, re-enter the area of abundant food.
In attempting to explain the movements of a marine
animal we assume the existence of consciousness in
it, for the same reasons as we have to assume it in
other individuals of our species ; and it seems sound
enough to assume that in the^fish as in ourselves con-
sciousness has become an effective factor of evolution

86

LIFE IN THE SEA

[CH. Ill

and behaviour. We must assume the quality of
memory on the part of the animal — it may be some
kind of unconscious memory. Then it follows that
an animal will react not entirely in response to the
stimulus which reaches it from without, but also in
response to all the stimuli which have affected it in
the past ; that is to say its behaviour at any time is
modified by its experience. Now if one attentively
studies the facts of migration it seems that this way
of looking at them is more in accord with what one
sees than simply to suppose that the behaviour is
a series of inevitable responses to changes in the
environment

The quantitative plankton net.

CHAPTER IV

MODES OF NUTRITION

IT is thus easy to show that the life-processes of
marine organisms are directly affected by changes in
the external conditions under which they live : that
is to say in the chemical composition of the sea-
water ; in tKe temperature of the sea ; in the intensity
of solar radiation other than heat ; in the pressure
of the sea- water and so on. But each species of
organism feeds in some manner and its density and
range of geographical distribution must be affected
by changes in the abundance of its food. Now most
marine animals nourish themselves by eating other
animals or plants ; and most marine plants by
assimilating certain substances which are contained
in solution in the water of the sea. We have there-
fore to consider what are the precise ways in which
these nutritive processes are carried out, and how
the food-stuffs are distributed in the sea.

The majority of marine animals are predatory, and
the history of the evolution of their bodily form is to

88 LIFE IN THE SEA [CH.

a great extent that of the development of organs for
the capture of food. Such organs are the jaws, teeth
and some other structures in fishes ; the suctorial
mouths and rasping tongues of lampreys ; the suckers
and horny beaks of the squids and cuttle-fishes, the
pincers, maxillae and mandibles of the Crustacea ;
the blood-sucking mouths of leeches and some other
worms ; the scraping tongues, or radulae, of whelks ;
the tentacles and stinging cells of sea-anemones and
jelly-fish ; the suckers and protrusible stomachs of
starfish and sea-urchins, etc. These are examples
which illustrate the great diversity of methods by
which the predatory marine animals capture their
prey. The majority of nektic and benthic animals
hunt for the organisms which serve them as food,
even the microscopic amoebae and infusorians appear
to roam about incessantly in search of food. Many
marine animals are cannibalistic, and not a few of
them are quite promiscuous in their choice of food,
but as a rule each species eats by preference one or
a few species of food organisms. A small minority
of the nektic animals, chiefly the whalebone whales,
and some pelagic fishes like the herring and mackerel,
are plankton feeders. The whales take in mouthfuls
of water, and then on closing the jaws this water is
forced out through a sieve formed by the frayed-out
margins of the whalebone plates. The herring and
mackerel take water into the mouth and then the

iv] MODES OF NUTRITION 89

latter is closed, but because of the presence of
mandibular valves this water is forced out through
the gill-apertures and is compelled to pass through a
sieve formed by delicate bones on the hinder margins
of the gills — the gill-rakers — and the particles of food
are thus intercepted. In each case, that of the whale
and the pelagic fish, the food is scraped off by means
of the tongue and is then swallowed. Sedentary
shell-fish like the cockle or mussel cause a current
of water to flow through the shell cavity by means
of ciliary action. This water passes through a fine
filter formed by the gills and the sediment and
plankton contained in it are thus intercepted and are
carried into the mouth by means of another system
of cilia. Sedentary tunicates have a similar manner
of feeding. Sponges act in the same way, by causing
a current of water to flow in through the pores and
then out again through the osculum. In traversing
the system of canals which permeates the body of the
sponge the food substance is removed from the water
and is taken up directly by the cells lining these
cavities or canals. Barnacles open their shells at
regular intervals and protrude a bunch of appendages
known as the ' glass hand ' : these are finely fringed
by hairs so that when they open out they form an
apparatus like the ' cast-net7 of a fisherman, and
this, as it sweeps through the water, captures any
small organisms contained therein. When the hand

90 LIFE IN THE SEA [CH.

is withdrawn into the shell the food particles are
scraped off by other appendages and are then
swallowed. Some microscopic infusorians, such as
the Vorticellids, which are attached to fixed objects,
cause a current of water to flow towards the oral
funnel or mouth of the creature, and this current
carries with it planktonic food organisms or food in
some other form. Other marine animals have more
complex methods of capturing their food, but these
examples will illustrate the variety of means em-
ployed.

The majority of the larger marine animals take
the food into their alimentary canals and digest
it in essentially the same way as does the warm-
blooded mammal. Many fishes crush the shells of the
molluscs or Crustacea which they use as food between
the hard roof of the mouth and the bones which form
the skeleton of the gills ; Crustacea, as a rule, hold
the food animal in their pincers and then pull it to
pieces by means of the masticatory appendages ;
squids and cuttle-fishes crush the food in their
tentacles and then disintegrate it by the horny beaks ;
and many molluscs scrape away the food by their
rough tongues or radulae. The teeth and jaws of the
fishes are organs for the prehension of the food
rather than for its trituration, and these animals, as
a rule, 'bolt' their food. Starfishes ingest and
digest their food outside their bodies, for they can

iv] MODES OF NUTRITION 91

hold a mussel, say, and then by means of long-
continued traction they cause the latter to open its
shell when the starfish protrudes its stomach and
sucks away the soft parts of the mollusc. In the
larger animals there is however very little trituration
of the food in the mouth and hardly at all any
insalivation.

The processes of digestion and assimilation are
probably the same in the fishes and invertebrates as
in the mammals, but one must not be too dogmatic
on this point for very little exhaustive investigation
has been made with regard to the digestive processes
of the larger invertebrata. The broken-down food
contains, in addition to its indigestible skeleton the
' proximate food-stuffs ' of the dietetic text-books ;
that is proteid (which is the substance of flesh), fats,
and carbohydrates (starches, sugars and gums). The
general composition of the food of the marine animals
is therefore similar to that of the terrestrial carnivores
but it must usually contain a larger proportion of
non-assimilable substance, in the shape of the
calcareous or horny shells and carapaces of molluscs
and Crustacea. The herbivores of the land are
represented in the sea by a few fishes such as the
mullets which ' graze' on the alga-covered stones
of the sea-bottom, and by the plankton feeders. The
food of the pasture-feeding or frugivorous land
animals contains a relatively large proportion of

92 LIFE IN THE SEA [OH.

difficultly assimilable cellulose, and that of the
herbivorous marine animals contains in place of
cellulose the calcareous or siliceous shells or skeletons
of the food animals. Generally speaking the food
of the marine carnivores and herbivores corresponds
to that* of their counterparts on the land, but bulk
for bulk their food is not so rich in the tissue-forming
constituents, nor in the heat-producing fats or carbo-
hydrates.

The processes of digestion must be clearly
distinguished from those of assimilation — an im-
portant distinction which is made far more clear by
the vegetable physiologists than by writers on marine
biology. The tissues which compose the bodies of
plants and animals are made up of cells modified in
innumerable ways to carry on vital processes.
Organic cells form about them many other structures :
fibrous structures, calcareous and siliceous structures
in the form of shells, etc., and other hard parts ;
they also lay down in themselves substances such as
fats, sugars, oils and starch, which can be used for
the production of heat or other forms of energy by
being oxidised or burned. The living substance of
the organic cell may be called bioplasm : we do not
know what this is, but when it is killed we recognise
it as protoplasm. The bioplasm of the cells is
continually dying or wasting away and it must be
renewed ; under its control the hard parts of the

iv] MODES OF NUTRITION 93

body are being formed and the reserve food substances
are being oxidised, or are being decomposed, to
supply heat or energy to the organism. Now the
living cell must be supplied with food to compensate
for its waste ; to form new structures or tissues, or
new individuals during reproduction ; or to form the
skeletal structures ; or to be oxidised for heat and
energy production. The substances which are brought
to the cells for these purposes are the foods, and
their incorporation in the cell substance is the process
of assimilation.

We recognise in the tissues of an animal substances
called proteids, fats and carbohydrates. The fats are
relatively few in number, and when not incorporated
with the proteids are fairly simple in composition.
So are the carbohydrates, and we know the chemical
structure of most of these substances. But the
proteids are exceedingly complex, more so than any
other chemicals known to us, and in spite of long-
continued research we do not know what is their
constitution. But we do know that the proteids of
every species of organism must be different from
those of any other species, and it is possible that this
holds true even for individuals. The facts of heredity
can hardly be explained in any other way, and indeed
there is a certain amount of experimental proof in
support of this assertion. We also know that the
proteids can be split up, or decomposed in various

94 LIFE IN THE SEA [CH.

ways, and that the products of these decompositions
are always substances called ammo-acids, which
though very much simpler in composition than the
proteids are still very complex chemical compounds.
We also know that the amino-acids can be re-
constituted to form proteids, but this reconstitution
can only be effected by the agency of a living cell,
and all attempts to make this synthesis outside the
living substance have hitherto failed.

The food-stuffs of an animal are the proteids, fats,
and carbohydrates of some other animal or plant and
they are always different from those of its own body,
and cannot be directly utilised by it as food. Before
these substances can be assimilated they must be
broken down into their constituents and then the
latter must be resynthesised to form the specific
proteids, fats, or carbohydrates of the animal. This
decomposition and recomposition make up the
processes of digestion in the body of an animal. In
our own case the proteids may be those of lean
meat, of white of egg, of cheese, of the gluten of
wheat bread, milk-albumen, etc. The carbohydrates
may be the starches or sugars of various grains, and
the fats are also usually different from those of our
own body.

Digestion of the food is carried out by substances
called ferments, or enzymes, elaborated by the
digestive glands. The proteolytic enzymes are those

iv] MODES OF NUTRITION 95

which act on proteids ; they are the pepsin of the
gastric juice, the trypsin from the pancreas, and the
erepsin and enterokinase secreted by the wall of the
intestine. They decompose the proteids first of all
to peptone and then to amino-acids, but what
proportion of the proteid taken into the intestine
is broken down so profoundly as the stage of amino-
acids is still uncertain. The lipolytic enzymes are
those which split up the fats into fatty acids and
glycerine : the principal one is the lipase from the
secretion of the pancreas. The amylolytic enzymes
are those which resolve the starches into sugar or
rearrange the sugars into forms which are capable
of assimilation. They are the ptyalin of the saliva,
and the amylase from the pancreatic juice.

Thus the proteids are converted into amino-acids,
the fats to fatty acids and glycerine, and the starches
to sugars — all this by the activity of the enzymes
of the alimentary canal. These products of digestion
— the food-stuffs we may call them — are soluble in
water and they are absorbed by the cells lining the
wall of the intestine. But these cells also contain
the enzymes and the action of the latter is reversible,
that is they can not only split up the proteids to
amino-acids but they can recombine these substances
back again into proteid. So in the process of
digestion the amino-acids are reconverted into proteid,
the fatty acids and glycerine are reconverted to fats,

96 LIFE IN THE SEA [CH.

and the sugars either pass directly into the blood
stream without further change or they are transformed
into compounds which are capable of assimilation.
But — and this is the aim of the whole series of
digestive actions — the reconverted proteid and fats
are different from those which were taken into the
alimentary canal and are now similar to those which
make up the tissues of the animal. • They are the
true foods, and as such they are taken into the blood
stream and are carried over the body to be assimilated
by the cells of the tissues.

The nutritive processes of plants are profoundly
different from those of animals. The proximate food-
stuffs of the plant organism (or the raw materials of
the food-stuffs, as they are sometimes called) are
salts of nitric acid or nitrous acid, salts of ammonia,
and carbonic acid. These are exceedingly simple
substances, and though the plant is able to make use
of more complex nitrogenous stuffs than the salts of
nitric acid or ammonia, yet the latter are a sufficient
source of food. The mineral salts are taken up from
solution in the water of the soil by the root hairs, and
the carbonic acid is taken in from the atmosphere
through the stomata of the leaves. If we are dealing
with a marine plant these substances are simply
absorbed from solution in the sea-water over the
entire surface of the organism. The carbonic acid
and water taken into the plant tissues are combined

iv] MODES OF NUTRITION 97

to form starch, a synthesis which cannot be carried
out in the laboratory by any agency available to
chemists apart from the activity of the living cell.
The latter effects the combination by making use of
the energy of sunlight, and it appears that a very
small proportion of the total light falling on the sur-
face of the plant may be sufficient for this purpose,
nevertheless this fraction is quite essential. In spite
of the most careful and protracted investigation the
details of this process of the photo-synthesis of starch
are imperfectly known ; but carbohydrate in the form
of starch is elaborated by the plant organism from
inorganic material, and is immediately converted into
soluble sugar which is the carbohydrate food of the
plant.

The simple compounds of nitric acid and ammonia
are taken into the tissues of the plant and are then
combined with the carbohydrate to form amino-acids,
and these food-stuffs are further elaborated into the
form of proteid. The sugars and proteids pass into
circulation in the vascular tissues of the plant and
are assimilated by the cells — either stored up in the
form of proteid, oils, or starch, or oxidised to supply
energy.

We can exhibit this contrast between the nutritive
processes of the plant and animal organism as
follows.

98 LIFE IN THE SEA [OH.

The proximate food-stuffs are :

/Proteid, splitting to form amino-acids say,

' leucine, amino-iso-butyl-acetic acid,
In the ) CH3\rH rn „„/ NH2

animal I CH3/ H*C] \COOH

Fat, say olein ... C^OC^CO),,

\Cane sugar C^H^On

(Nitrate, say sodium nitrate ... NaN03

,ne J Carbon dioxide ... C02

Plant (Water OH2

The proximate food-stuffs, the amino-acids, the
fats, and carbohydrates are the same in each case.
The foods are also the same, that is they are proteids,
fats, and soluble carbohydrates. The process of
assimilation, that is the incorporation of these foods
by the bioplasm of the cells is also the same. The
difference is in the preparation of the proximate food-
stuffs, for in order to obtain these the animal must
break down material which it can only obtain from
the tissues of other animals or plants — materials
which are highly complex in structure, and it must
then synthesise these to form its food-stuffs. The
plant builds up its food-stuffs from inorganic material
possessing almost the simplest chemical structure
known to us. The difference is expressed by saying
that the functions of the plant organism are in the
main synthetic, or constructive ones, while those of
the animal are, in the main, analytic, or destructive

iv] MODES OF NUTRITION 99

ones. This is true generally but we must not forget
that many of the processes of the animal are also
synthetic.

Now there are few conceivable forms of structure
that have not been evolved among animals and plants,
and we may assume that there are also few possibilities
of nutrition that have not also been evolved. We
have to consider, then, what other modes of nutrition
besides the purely plant one (holophytic) and the
purely animal one (holozoic) exist among marine
organisms. There is obviously a combination of the
two, and this occurs among some molluscs, some
polyzoa, at least one starfish, many coelenterates,
including most corals, many worms, possibly in some
sponges, and in a great number of protozoa. In all
these cases the animal which exhibits the plant-like
mode of nutrition is coloured green, and this is due
to the presence of chlorophyll — the colouring matter
of plants, by reason of which they are able to intercept
the energy of sunlight and make use of it in the
process of photo-synthesis. The best known case
of this kind is that of the worm Convoluta, a full
account of which is given in the volume Plant- Animals
in this series of Manuals. Convoluta is green because
of the presence in its tissues of cells containing
chlorophyll ; but it does not necessarily begin life as
a green animal, for the green cells are the degenerate
remains of algae which have infected the tissues of

7—2

100 LIFE IN THE SEA [OH.

the worm almost as soon as it was born. It is quite
possible to rear the worm apart from the algae as a
colourless animal, and it is also possible to rear the
algae apart from the worm. When it is young the
worm feeds by ingesting diatoms, etc., but after a
time it ceases to take in any solid food. It obtains
its carbohydrate from the activity of the green cells,
which are able to synthesise starch from the carbonic
acid absorbed from solution in the sea-water, and
this starch is converted into sugar and is assimilated
by the other tissues of the worm. The alga obtains
its nitrogenous food from the products of excretion
of the worm, and for a time the association of the
plant and animal in one compound organism is of
advantage to both partners. But since the animal
has lost the power of feeding it becomes unable to
supply itself with proteid food, although it receives
abundance of carbohydrate food from the photo-
synthetic activity of the alga. Towards the end of
its life it therefore begins to digest the green cells in
order to obtain proteid food, and thus by cutting off
its supplies of carbohydrate it dies of starvation.
The earlier stage of the association of the two animals
is one of symbiosis, and it is of advantage to either
partner, but the later stage is one of parasitism on
the part of the worm.

In this and some other cases the association
together of a plant and animal is an obligatory one,

tv] MODES OF NUTRITION - -itfi

neither organism being able to exist without the
other. This, however, is not always the rule, and it
is probable that the association may be one which
can be lost and regained : that is, the animal may be
able to live well enough in some circumstances with-
out the aid of the algal cells, while it is certain that
the latter may be able to live as independent
organisms in the water, procuring their own proteid
food as well as their carbohydrate. Alcyonaria,
which are colonial animals, are often coloured green
in tropical seas, but they are colourless in British
waters. Corals in warm seas are often also coloured
because they contain associated algal cells, and some
species of the protozoan Noctiluca which are colour-
less in British seas are green in the tropical waters.
We have seen that the tropical seas are poor in
plankton, and it is probably the difficulty of procuring
sufficient carbohydrate food that has induced the
association of chlorophyll-containing algae with the
animal organism, for whenever this partnership is set
up the difficulty disappears because of the abundance
of carbohydrate resulting from the photo-synthetic
activity of the algae.

In all these cases which we have considered the
association of the two organisms establishes a com-
bination of the two modes of nutrition — the holophy tic
and holozoic — and this is termed heterotrophic.
There is no doubt in such cases that we have to deal

102 LIFE IN THE SEA [CH.

with plant-animals, compound forms brought together
by mutual necessity. But there are also the cases of
many planktonic unicellular organisms, such as the
peridinians, and some others which are more difficult
to understand ; for although these organisms possess
chlorophyll corpuscles in their cells so that their
mode of nutrition is truly holophytic, yet their general
characters may suggest those of the animal rather
than the plant. The difficulty, however, is not one
that need concern us, for it may be the case that many
groups of the protozoa have evolved from chlorophyll-
containing organisms which we need not regard as
either plants or animals; and that some of them
while developing along the animal line have still
retained their plant-like mode of nutrition.

Parasitism, to which we have already referred, is
essentially a mode of nutrition in which the process
of digestion is either eliminated entirely, or is greatly
abbreviated. In this condition two animals or two
plants are associated together, or an animal may be
associated with a plant. The association is often said
to be an obligatory one in that one of the associates,
the parasite, must live within the body, or on the body
of another, the host; although in few cases is the
actual contingency of the associates strictly necessary
for the life of the parasite. The parasitic habit is
very common among marine animals and we are able
to distinguish between two main groups of animals

iv] MODES OF NUTRITION 103

pursuing this mode of life: (1) animals which live
attached to the outer surfaces of their host — the skin,
the gills, the branchial cavities, and the cavities of
the nose or mouth. Examples of this class of parasites
are the Copepods (fish-lice), which are attached to
the external surfaces of many species of fishes ; some
leeches ; and some Trematodes (flukes). (2) Internal
parasites which live in the alimentary canals of their
hosts, either holding on by means of suckers or hooks,
or lying quite free in these cavities (tapeworms,
internal trematodes, and thread-worms); or which
live in the blood-stream or lymph channels, or in the
peritoneal or serous cavities; or embedded in the
substance of the muscles or other tissues (trematodes,
thread-worms, larvae of tapeworms, trypanosomes,
etc.). The external parasites usually pass through a
life-history which includes a free-swimming stage in
which the organism lives among the plankton as an
independent animal, and then at a certain phase,
coming in contact accidentally with its host, it attaches
itself thereto and undergoes metamorphosis to its
sexually mature form. Some of the internal parasites
also pass through a free-swimming stage in the course
of which they are taken into the alimentary canal,
or into some other part of the body of an animal
which acts as their larval host. They live in the
tissues of this host and do not undergo development
beyond a certain stage, and finally degenerate unless

104 LIFE IN THE SEA [CH.

the larval host is eaten by another animal which acts
as the adult host. When this occurs the parasite under-
goes full development and attains sexual maturity,
and its eggs pass out of the body of the host and
infect further larval hosts. In other cases still the
parasite never lives in the open except as an egg
which is eaten by the larval host, and this in turn is
eaten by the adult host.

The parasitic habit is advantageous to the parasite
but is detrimental to the host, since substances may
be excreted by the parasite which may be to some
extent poisonous. The association is also harmful to
the host since the parasite uses up the food-stuffs of
the latter. A typical parasite lives in the alimentary
canal of another animal and it is bathed in the food
matter which is undergoing the process of digestion.
It may or may not possess an alimentary canal and
mouth: no tapeworm does but most trematodes do.
If an alimentary canal is absent the parasite simply
absorbs the soluble food matter of the host through
its skin; if it is present the food matter may be
absorbed through the skin or taken into the mouth.
In whatever way the food-stuffs are absorbed they
always consist of soluble peptones or amino-acids, of
soluble fatty acids and glycerine, and of soluble
sugars ; and all these substances have been prepared
by the digestive enzymes of the host. This is the
complete elimination of the process of digestion and

iv] MODES OF NUTRITION 105

all the parasite has to do is to recombine the food-
stuffs into the specific proteids, fats, and carbohy-
drates of its own tissues. The process of digestion is
abbreviated in the case of those parasites which live
on the external surfaces of their hosts, for there the
food consists of mucus which is probably easier to
digest than the proteid of a captured animal. In the
most profound parasitic conditions, as when such a
form as a trypanosome inhabits the blood-stream or
the cerebro-spinal fluid; or when a blood-sucking
parasite lives on the external surface of its host with
its suctorial mouth embedded in one of the larger
blood-vessels, it simply absorbs the ready-made
proteids of the host, possibly in the same way as the
cells of the latter also absorb them. Essentially then
the parasitic mode of nutrition is one in which the
digestion of food-stuff is performed outside the body
of the organism and by the agency of some other
organism. It may be called the saprophytic mode of
nutrition when we speak of a plant parasite, and the
saprozoic mode when we speak of an animal parasite.
Now let us consider parasitism in the open — that
is a parasitic mode of life practised by an animal
which does not live in the cavities, or on the body of
another animal. It seems paradoxical so to speak,
but we must remember our definition of the habit —
that it consists in essence of the utilisation of already
digested food ; and further that it is possible to rear

106 LIFE IN THE SEA [CH.

many parasites outside the body of other animals in
artificially prepared juices or jellies; thus many
species of bacteria may live either in the body of a
host or in the sea- water or mud, but their mode of
nutrition is the same in each case.

Are there animals which live in the open and
which do not ingest solid food in the shape of the
bodies of other animals and plants, but live by
ingesting matter which is dissolved in the sea-water ;
in short are there marine animals which do not eat
visible food ? Now it is not at all remarkable that
zoologists, whose attention is usually concentrated
on the study of form rather than function — who
look on an organism not so much as ' something
happening/ but as a structure of skeleton, muscles,
alimentary canal, glands, nervous system, etc. —
should have regarded such a mode of nutrition as
improbable. Why should an animal possess an ali-
mentary canal and glands if these structures are
not for the purpose of the digestion of food ? Yet
the study of parasitism and its modifications should
have led us to postulate the existence of a wide-
spread saprozoic mode of nutrition, at all events
among the lower marine animals. We might not
have expected to find it among the terrestrial
animals but where an organism lives continually
bathed in a liquid medium, the absorption of food
matter through its outer surface — apart altogether

iv] MODES OF NUTRITION 107

from an alimentary canal — should always be regarded
as a possibility of nutrition.

If we assume that marine animals living in the
open nourish themselves exclusively by capturing
and digesting solid food we immediately get into
difficulties. For it is often difficult to convince
oneself, by examination of the contents of the ali-
mentary canal, that sufficient food organisms for the
apparent needs of an animal have been ingested.
We hardly find anything in the intestine of a lug-
worm except sand which does not differ from that
in which the animal lives. It burrows by eating
the sand in the same way as an earthworm burrows
by eating the soil, and we suppose that its food is
contained in the sand that passes through its body.
But if its food consists of microscopic organisms there
are very few such in the sand beneath the superficial
layers. Many organisms have indeed been identified
from the contents of the intestine of the lugworm
but it is always possible to regard these merely as
residues which are contained in the sand, and it does
not appear to be probable that the worm can obtain
enough food-stuff in this shape from the sand which
it ingests. Just the same difficulty meets us in the
cases of cockles and mussels which live on the sea-
bottom and which have been supposed to obtain
their food from the plankton contained in the water
which passes through their shell cavities. If we

108 LIFE IN THE SEA [CH.

examine the contents of the alimentary canal of
these animals we nearly always find that it contains
hardly anything but fine sand and mud particles.
There are never many food organisms, such as diatoms.
It has been said that these animals may feed on such
organisms as infusoria which possess no shells and
which could, therefore, hardly be identified in the
contents of the intestine, but we do not find many
such if we examine the water in which the molluscs
live. Sometimes indeed we do find that the intestine
contains large quantities of green matter which
probably consists of algal spores, but this is ex-
ceptional. If, however, we remember that it is
essential for the good condition of such molluscs as
these that they should live in a part of the sea
where there is plenty of fresh water flowing down
from rivers, then we receive the suggestion that
probably the food may be in solution, for the water
of rivers flowing into the sea contains more of
dissolved organic compounds than does sea-water.

Then it can be determined in many cases that
the quantity of planktonic organisms contained in
the plankton is too small to account for the observed
metabolism of an animal. A mussel, for instance,
may very easily grow from two inches in length to
two and a half inches in the course of a single
summer, and it was easy to show in one such series
of cases that this growth represented an average

iv] MODES OF NUTRITION 109

gain in dry organic substance of 713 milligrams, say
356 milligrams of proteid, since analyses of mussel
flesh give about half the dry weight as consisting
of this constituent. Now let us suppose that all
this proteid came from the plankton captured by
the shell-fish. A good estimate of the amount of
plankton contained in northern waters is that 1000
litres of water contain, on the average, 168 milligrams
of dry organic substance, and since we know the
composition of various forms of plankton we can
easily calculate the amount of proteid contained in
the water : it is 25*2 milligrams. If the solid matter
filtered from the water by the mussel were as rich
as this — and it certainly is not, for the greater part
of it consists of fine mud, and if the animal captured
all the solid matter contained in the water sucked
into its shell — and again it certainly does not — then
it would have to filter 14,200 litres of sea- water in
order to obtain this amount of proteid. But even
then it must have digested and assimilated all the
food matter taken into its intestine — and the digestive
machinery of no animal is as efficient as this. Above
all it must have built up all the assimilated proteid
into the form of tissue, and it certainly could only
have so disposed of a small fraction of it for the
largest part by far must have been oxidised in the
production of energy. This example gives us an
idea of the difficulties we saddle ourselves with when

110 LIFE IN THE SEA [CH.

we try to show that such an animal as we have been
discussing nourishes itself in the same way as a
mammal does. Again a sponge causes a current of
water to flow through the system of cavities per-
meating its body, and it nourishes itself by removing
the food matter contained in this water of circulation.
If we estimate the quantity of carbonic acid excreted
by the sponge in the course of an hour, say, we can
find what was the quantity of carbon contained
in the parts of its body which were oxidised during
that time — that is to say we find what was the
quantity of carbon contained in the food which it
must have taken in order to keep itself from losing
weight. Now we know what quantity of carbon is
contained in the water in the form of plankton and
it is therefore easy to calculate what quantity of sea-
water must be filtered by the sponge in order to get
enough food, on the assumption that all its food is
contained in the plankton. The volume of water so
found is ridiculously large — far too large to make it
possible for the sponge to have filtered it in the time
required.

We can show also, by actual experiment, that
many marine animals, even fishes, can be kept for
many months in water which is filtered so that it
does not contain any plankton. The fish or other
animal may live in good health for months in such
circumstances provided the water is kept cool, and is

iv] MODES OF NUTRITION 111

frequently changed ; and we therefore say that it
must receive abundant supplies of oxygen for the
purposes of respiration. But the whole object of
respiration is the oxidation of the substance of the
body in order to yield the animal the energy it
requires, and if respiration continues there must be
a continual wastage of the tissues or reserve sub-
stances of the body. This waste is either replenished
by the assimilation of food, or the animal must lose in
weight, having used up its own body substance. Now
the loss of tissue due to respiration can be calculated
from the amount of carbonic acid excreted and it
may be greater than can be accounted for by the
loss of weight of the animal. In such a case as this
the animal must have nourished itself in the manner
of a parasite — saprozoically, by the absorption of
dissolved food matters.

Why then should an alimentary canal have been
evolved among marine animals if they can nourish
themselves by absorbing food from solution in the
sea- water? In attempting to answer this question
we will assume that primitive marine organisms were
microscopic in size and that they nourished them-
selves by obtaining their food-stuff from solution in
the water, just as a peridinian does at present obtain
its nitrogenous food-stuff. It gets its carbon food
by building it up from the carbonic acid which it
also absorbs from solution, but it cannot do this

112 LIFE IN THE SEA [OH.

unless it also obtains nitrogenous food, and it must
get this from the amino-acids or analogous substances
which it finds in solution in the sea. Myriads of
marine unicellular animals so obtain their proteid
food-stuff from soluble substances and they can do
so easily because they are small. If they are very
small their surface is large compared with their
bulk, and they absorb food-stuff in proportion to
their surface. Now let the organism increase greatly
in size and its surface must diminish relatively to its
volume ; for the surface with increasing size is
proportional to the square of the radius, while the
volume is proportional to the cube of the radius. As
the organism grows in size it must therefore find
increasing difficulty in absorbing food-stuff from
solution in the sea- water ; for its requirements are
proportional to its volume while its powers of ob-
taining food are proportional only to its surface.
Further, the absorptive surface must become reduced
as the animal increases in size, since part of it at
least may become thickened to form an integument,
so as to afford protection from mechanical injury,
and the powers of absorption must therefore be
reduced.

The surface must therefore be increased during
the evolution of increasing size, and this is effected
by the infolding of a part of the body-wall so as to
create an internal cavity, the wall of which may

iv] MODES OF NUTRITION 113

remain thin because it is protected by its situation.
Such an internal cavity we find in Hydra and in coelen-
terates generally. Let the cavity open at both ends
— a mouth and an anus — and we have an alimentary
canal. Even then the surface may be insufficient,
for as the animal increases in size its movements
through the water become slower when compared
with its size ; but if it should begin to swallow water
and pass this through its alimentary canal the ab-
sorptive power would still further be increased. If
it should still be insufficient, a further increase in
absorptive surface may be attained by the outfolding
of the body- wall so as to form tentacles, gills, plumes,
etc., structures which have a large surface compared
with their volume, and the walls of which may remain
thin if they are protected from mechanical injury in
some way — as in the case of the gills of a fish. Now
let the animal still further increase in size and two
new devices may come into operation : (1) a circulation
of blood or other internal fluid is initiated, and this
is most abundant in the outfoldings of the body- walls
and round the wall of the internal cavity. Since the
absorbing fluid is thus continually being changed,
obviously more food-stuff can be taken in. (2) a
circulation of water is established in relation to the
gills, or outfoldings, by means of cilia, or other
mechanisms, and thus the sea-water is caused to
flow more rapidly over the absorptive surface. Since

J. 8

114 LIFE IN THE SEA [OH.

the source of soluble food-stuff is thus continually
being changed more of the latter can be taken up
than if the water were at rest.

Thus we can account for the evolution of an
alimentary canal, mouth and anus, gills, blood cir-
culation, and respiratory movements and mechanism,
solely by reference to the necessity for the increased
absorption of dissolved food-stuff, and without
reference at all to the digestion of solid food
matter ; and it is the increase in size of organisms
which has led to the necessity for this develop-
ment.

But nevertheless there is a digestive mechanism
in many animals, and we have also to account for the
development of this even if the absorption of soluble
food remains a factor in their nutritive process. Now
the key to this is the establishment of an internal
cavity. Some insectivorous plants have such an
internal cavity, evolved by leaf modifications. It
happens that small animals enter these cavities and
they are utilised as sources of food-stuffs by the
plants, but they are not necessarily digested. Utri-
cularia does not digest entrapped flies, but the
latter die in the bladders and undergo bacterial
decomposition. Drosera does possess a digestive
fluid but this only carries on the process as far as
the formation of peptones. Nepenthes forms a true
proteolytic enzyme. We can easily conceive of the

iv] MODES OF NUTRITION 115

evolution of such a digestive fluid in the internal
cavities evolved for the purpose of increasing the
absorptive surface of animals. Small particles taken
into them would be ingested by the cells lining their
walls just as small particles are ingested by the
amoeboid cells of the alimentary canals of many
worms, and just as an amoeba ingests its prey. But
if the food particles were too big to be ingested in
this way? Even then we need not postulate the
existence of a digestive fluid, for autolysis of the
food organism would occur when it died. Every
cell of the body of an animal contains proteolytic,
lipolytic, and amylolytic enzymes, and when a tissue
dies self-digestion, or autolysis, begins. It is by
autolysis that the exudation producing consolidation
of the lung disappears in cases of recovery from lobar
pneumonia. Or bacterial decomposition of the body
of the entrapped food organism would occur. The
same result would be attained either by autolysis,
bacterial decomposition, or by digestion — that is the
resolution of the proteids to the stage of amino-acids,
that of the fats to fatty acids, and that of the carbo-
hydrates to soluble sugars. All these substances
would then be absorbed by the walls of the internal
cavity in which they lay.

We may however reasonably suppose that a cell
in contact with an eatable organism will be stimulated

8—2

116 LIFE IN THE SEA [CH.

to produce enzymes in greater quantity when it is
in contact with a trace of the decomposition products
of a food organism ; that is a signalling substance,
or hormone, would be produced just as it is in the
mammalian intestine, and this would stimulate the
cells to elaborate the enzymes. But if the food
organisms were too big for the cells to ingest then
we may suppose that their contained enzymes would
diffuse out into the cavity, that is to say, a ' secre-
tion ' would take place. This is indeed the manner
of digestion in many invertebrates. Molluscs, for
example, possess a ' digestive gland,' ' hepato-
pancreas ' or ' liver ' as it is called, and there are
accounts of the ' secretion' from this organ and of
the reactions of the fluid said to be so produced.
There is really no evidence of the production of a
secretion in the sense in which we employ the word
in the physiology of the mammal. The tests made
involve the pulping of the gland and the examina-
tion of the product, and the latter certainly contains
the intra-cellular enzymes of the cells of the gland.
The latter is only an extension of the cavity of the
intestine of the mollusc.

There is of course no reason why the preparation
of the food by the digestion of captured organisms,
and its preparation by the absorption of dissolved
food-stuff should not proceed simultaneously in an

MODES OF NUTRITION

117

animal, just as the preparation of food by the digestion
of the body of a captured insect, and its preparation
by the process of photo-synthesis go on simultaneously
in insectivorous plants.

The Ekman Current-Meter.

CHAPTER V

THE SOURCES OF FOOD

WE have not yet considered all the known modes
of nutrition for it will be more convenient to discuss
some of them when dealing with the question of the
circulation of food-stuff in the sea. We shall not be
able to understand the meaning of the more obscure
modes of nutrition unless we have clear ideas as to
the assimilation of food. All organisms, whatever be
their mode of life, have this in common, that their
bioplasm continually wastes away by its own activity
and must be renewed ; and further, to obtain energy all
must oxidise or transform some substance which the
bioplasm associates with itself. The substance which
they use in order to renew their bioplasm is proteid,
and that which they use to obtain energy is called
the respirable material. The latter may be one of
several substances.

In green plants and animals the respirable
material is some form of fat or carbohydrate, and

OH. v] THE SOURCES OF FOOD 119

both forms of organisms oxidise this to carbonic
acid and water, which are then excreted into the
surrounding air or water. In oxidising this carbona-
ceous food heat, or some other form of energy, is
produced. The carbonaceous food consists of carbon
and hydrogen, or of these elements and oxygen as
well. All forms of carbohydrate and fat are built up
of chains of compact little groups of atoms ; thus
grape sugar has the following composition :

COH.(CH.OH)4.CH2.OH.

The proteid is much more complex than any other
substance known to us. It is built up from amino-
acids, which are chains or rings of little groups of
carbon and hydrogen atoms as in the case of the
carbohydrates, but in addition to these the amino-
acid contains a group of nitrogen and hydrogen
atoms — the amino group, NH2 — and this is associated
in a certain position with another group, the carboxyl
one, COOH. Amino-acids are piled on amino-acids
to form the giant proteid molecule, and this differs
from the other food-stuffs, not only in its greater
complexity, but also in the fact that it contains
nitrogen. In order to manufacture their proteid
food all organisms require nitrogenous food-stuff.

Two sharply contrasted modes of nutrition have
forced themselves on our attention — the holophytic
and the holozoic. In the former the carbonaceous

120 LIFE IN THE SEA [CH.

food-stuff is the carbonic acid of the atmosphere, and
the nitrogenous food-stuff is some salt of nitric acid
or ammonia. The respirable material is the carbohy-
drate built up from the carbonic acid and water, and
this is then combined with the amino-acid formed
from it and the nitrogenous food-stuff to form the
proteid. These processes are to us chemical miracles
which we cannot — as yet — imitate in the laboratory.
In holozoic" nutrition the carbonaceous food-stuff is
fat and carbohydrate, and these are converted in the
tissues into sugars and fats which are the respirable
materials ; and the nitrogenous food-stuff is proteid
which is converted into the specific proteid of the
animal after having been digested. The mode of
nutrition of the plant-animals is a combination of
these two primary modes ; and that of the parasites
and other saprozoic organisms is simply the holozoic
mode abbreviated in some way.

All animals, whether they live in the open, or are
parasites, or whatever be their manner of feeding,
must nourish themselves by ingesting the dead or
living bodies of other animals or plants, or proteid or
amino-acid derived from these sources ; and they can
only obtain their carbonaceous and nitrogenous food-
stuffs in this way, and not from inorganic materials.
Animals must eat other animals or plants. Let us
think of an ocean containing only animals and we see
that the larger must eat the smaller ones in order to

v] THE SOURCES OF FOOD 121

live, until in the end there would only be one animal
left and it would die of starvation. The animals are
consumers, and their life-process may be regarded as
that of a clock which is always running down and
cannot be wound up except from the outside of
itself.

The plants are producers for they can convert
inorganic into organic living substance. Carbonic
acid, salts of ammonia, and nitric acid exist in
nature and can come into existence apart from the
agency of life, and these very simple substances can
be synthesised to form proteid and carbohydrate. All
living substance therefore arises from the carbonic
acid of the atmosphere or the sea-water, and from
simple mineral nitrogen compounds, mainly by means
of the agency of plants. The pastures of the land
are the main sources of terrestrial animal life, and the
diatoms, peridinians and other vegetable planktonic
organisms are the pastures of the sea.

All animal life is destructive and its continuance
implies the degradation of chemical compounds.
Proteid, carbohydrate and fat are compounds pos-
sessing high potential energy, that is they can be
oxidised or burned, as coal is burned, giving a large
amount of heat or energy ; or they can be decomposed,
as an explosive is decomposed, also yielding heat and
energy. When taken into the tissues they are
oxidised or decomposed and energy is obtained, and

122 LIFE IN THE SEA [CH.

the carbon and hydrogen which they contain leave
the body in the form of carbonic acid and water,
mainly from the lungs and kidneys. The proteid is
also oxidised or decomposed and most of its carbon
and hydrogen also leave the body in the form of
carbonic acid and water, but the nitrogen part of the
molecule is not completely degraded. Carbonic acid
and water are compounds which are fully oxidised
and can no longer be used to yield energy but the
product of the nitrogenous metabolism is either urea,
or uric acid, or some other analogous substance which
is not fully oxidised and can still supply energy.
With the exception of these nitrogen compounds the
clock has run down.

When an animal or plant dies the substance of its
body becomes degraded so that the ultimate products
of decomposition are water, carbonic acid, nitric acid,
sulphuretted hydrogen and a few other substances.
This resolution of the organic body is effected by the
agency of a number of species of bacteria. The
proteids are attacked by the putrefactive micro-
organisms which first of all reduce them to the form
of amino-acids just as in the case of digestion. The
fats and carbohydrates are attacked by the fermenta-
tive micro-organisms which reduce them ultimately
to the form of carbonic acid and water. Some of the
microbes of fermentation and putrefaction act with
the assistance of oxygen (aerobic bacteria), while

v] THE SOURCES OF FOOD 123

others (the anaerobic bacteria) can act in the absence
of oxygen except that which is contained in the sub-
stance which they are attacking. When putrefaction
takes place in the absence of oxygen the breakdown
is much more complex and evil-smelling compounds
are produced : in the presence of oxygen the process
of putrefaction is more rapid. The bacteria of putre-
faction and fermentation have a mode of nutrition
which is very similar to that of the saprozoic animals,
but they are living among abundance of food and
their metabolism is very wasteful. That is to say
they break down a large quantity of proteid or
carbohydrate in order to obtain a small quantity of
energy; and the total amount of bacterial life in a
fermenting or putrefying mixture is far less than
could be sustained if the latter were used to the
greatest advantage. Now we cannot say with con-
fidence that the bacteria are either plants or animals
so we hedge, and speak of their mode of nutrition
as metatrophic.

In the long run the products of bacterial action
on organic matter are mainly ammonia, carbonic acid,
water, sulphuretted and phosphoretted hydrogen.
Now the urea excreted by animals undergoes change
to ammonia and carbonic acid, either by means of an
enzyme, or by bacterial action, so that the excretory
products formed by an animal during its life are in
the long run the same as those which are formed

124 LIFE IN THE SEA [OH.

after death by the decomposition of the substance of
its body: that is, they are carbonic acid, water,
ammonia with traces of sulphuretted hydrogen and
phosphoretted hydrogen. The nitrogen which was in
the proteid is not however completely degraded for
it can be oxidised and can still yield energy. But
we do not find ammonia in nature except where it is
being produced, as in the emanations from volcanoes,
or in decomposing matter, such as farmyard manure,
for example. It is never stored in nature, and the
only natural nitrogen compounds are nitrates, chiefly
Chili saltpetre (sodium nitrate). The conversion of
ammonia to nitric acid is a process of oxidation and
it is accomplished by the agency of bacteria.

These nitrifying bacteria introduce us to another
frnode of nutrition of extraordinary interest. They
pxist wherever there is plenty of oxygen and ammonia.
There are various species — the same apparently in all
parts of the world and sea where they have been
investigated. One species oxidises ammonia (NH3)
to nitrous acid (HN02) and another species further
oxidises the latter compound to nitric acid (HN03).
They do not require organic matter as a source of
food, and are indeed inhibited by the presence of
such; and like green plants they can form proteid
and carbohydrate from simple inorganic compounds
of nitrogen, and from carbonic acid and water. But
a green plant can only do this in the presence of

v] THE SOURCES OF FOOD 125

sunlight, which is employed by the chlorophyll it con-
tains; while the nitrifying bacteria can do all that
a green plant can do, in the absence of chlorophyll,
and in the dark. Of all organisms theirs is the
simplest and most profound mode of nutrition. We
have to invent a new term to describe it and we call
it prototrophic.

There remains the sulphuretted hydrogen and the
phosphoretted hydrogen which are produced during
decomposition or during excretion. These also are
compounds which are not entirely degraded and still
possess energy in that they can be oxidised. The
phosphoretted hydrogen is at once oxidised in the air
to phosphoric acid, and the sulphuretted hydrogen
may also be directly oxidised. But wherever in the
sea, or in fresh water, there is sufficient of this gas
there are also red or colourless sulphur bacteria, and
both of these forms of microbes are able to use the
gas, which is poisonous to the higher organisms, as a
source of food-stuff. A mere trace of carbonaceous
food in the form of some simple compound, such as
formic acid for instance, and a trace of nitrogenous
substance in the form of ammonia are sufficient for
the renewal of their bioplasm, and in place of the
soluble sugar, which is assimilated as respirable
material by the higher organisms, these bacteria can
assimilate the sulphuretted hydrogen, oxidising its
hydrogen to water, and setting free the sulphur in

126 LIFE IN THE SEA [OH.

their cells. This process it will be seen is analogous
to the assimilation of carbon dioxide by the green
plant and the storage of starch or sugar in the cells.
The sulphur is then oxidised to produce energy, and
ultimately forms sulphuric acid, when it cannot be
oxidised further. The sulphur bacteria are, like the
nitrifying bacteria and the green plant, prototrophic
in their mode of nutrition, and can form proteid from
inorganic materials, but they can accomplish this in
the absence of chlorophyll and sunlight.

Thus we have accounted for the degradation pro-
ducts of the animal body whether these result from
the waste during life or the decomposition after
death. The nitrogen finally becomes nitric acid, the
hydrogen water, the carbon carbonic acid, the sulphur
sulphuric acid, and the phosphorus phosphoric acid.
The trace of iron also present in the body becomes
altered by the action of the iron bacteria to ferric
oxide. Plant tissues may also undergo decomposition
in such a way as to form masses of nearly pure
carbon, as in the formation of coal and anthracite,
and there is now evidence that even this amorphous
carbon may be attacked by certain bacteria which
can oxidise it to carbonic acid with the production of
heat. Thus all the products of destructive metabolism
(katabolism) tend to pass into the state of fully
oxidised compounds so that they can no longer be
made use of by the animal as sources of energy.

v] THE SOURCES OF FOOD 127

Their elements are the same as those which make up
the greater mass of the animal body, but they have
passed out of circulation so far as the nutrition of
the latter is concerned.

But the degradation products of the animal body
are the sources of food-stuff for the plant organism,
and just where the destructive processes of the animal
end, there the constructive processes of the plant
begin. The simple compounds, carbonic acid, nitrate
or ammonia, are utilised by the algae, the diatoms,
the peridinians and other protozoa which possess
chlorophyll, and by all the higher animals which also
possess green cells in their tissues. These substances
are those on which all the life in the sea depends for
they are employed in the constructive work of the
plants. They are the raw materials for the production
of living substance — the ultimate food-stuffs of the sea.

It is important then that we should have a know-
ledge of the proportions in which these substances
exist in sea- water, and it must be confessed that our
knowledge on this point is not so full as is desirable.
Now it is to be noted first of all that not all the
proteid and carbohydrate and fat of animal or plant
tissues suffer, at once, the profound degradation
which we have mentioned above. Many organic
carbon and nitrogen compounds enter the sea in
the water of the rivers, or pass into solution from
organisms living and dying in the sea. These carbon

128 LIFE IN THE SEA [OH.

compounds are generally similar to those found in
the soil and known as humus — they consist of fatty
acids and soluble carbohydrates ; the nitrogen com-
pounds are such as are described by the water
analysts as 'albuminoid ammonia/ that is amino-acids
and analogous compounds. They would finally be
reduced to their simplest terms by the action of
bacteria, but before they become so oxidised part of
them is made use of as food-stuff by the saprozoic
animals and the saprophytic plants.

Trustworthy analyses made from the water of the
North Atlantic show that these substances are present
in the following proportions :

Carbon compounds other than carbonates dis-
solved in sea-water — 9*2 milligrams per litre (9*2
parts per million).

Nitrogen compounds other than ammonia or
nitrate dissolved in sea-water — 0*126 milligram per
litre (0*126 part per million).

The amount of these substances at the disposal of
saprozoic organisms is therefore very small, but it is
greater than the amount of proteid or carbohy-
drate contained in similar volumes of sea-water in
the form of plankton. Thus some recent trustworthy
estimations of the food value of the plankton give
the following results (northern seas) :

Carbon present in the form of organisms — 0*083
milligram per litre (0*083 part per million).

v] THE SOURCES OF FOOD 129

Mtrogen present in the form of organisms — 0*008
milligram per litre (0*008 part per million).

Thus there are apparently about 110 times as
much organic carbon, and about 16 times as much
organic nitrogen present in solution in sea-water as
is contained in the form of the organised substance
of plants and animals of the plankton. These sub-
stances are capable of assimilation by saprozoic
animals and we may take it that there are creatures
in the sea which do feed on them. Nevertheless this
does not prevent the final destruction of organic
carbon and nitrogen compounds, for of all such
resulting from the waste of the animal metabolism
only a part will be directly utilised in this way by
the saprozoic animals, the rest being broken down by
bacteria. And the part thus built up into animal
tissue is again broken down in metabolism and part
of it becomes the prey of the bacteria. Finally all
must undergo this fate.

We know a great deal more with reference to the
amounts of the ultimate food-stuffs present in the
sea. The amount of carbonic acid varies very greatly
according to the salinity, the temperature and the
depth. It may be stated in round numbers to
amount to 50 milligrams per litre of sea-water.
There are also a good many facts with regard to the
amount of inorganic compounds present in the sea.
Analyses by modern methods have been made from

130

LIFE IN THE SEA

[CH.

samples of water taken from the North Atlantic,
the equatorial seas, and the Antarctic Ocean. The
following figures apply to northern seas, and each is
an average based on from 15 to 30 samples made in
each month of each of the years mentioned.

Inorganic nitrogen compounds present per litre of water
from the North Sea and Baltic in the years 1904-6.
The figures represent milligrams. The proportions are
therefore parts per million of sea-water.

Month

Nitrogen from
ammonia

Nitrogen from
nitrites and
nitrates

Total
inorganic
nitrogen

February

0-061

0-149

0-210

May

0-073

0-136

0-209

August

0-065

0-087

0-152

November

0-073

0-088

0-161

We may consider along with these results those
of the recent Antarctic voyage of the German ship
' Gauss/ and also some observations made in tropical
seas. Combining the results and ' rounding off' the
numbers we find that :—

Water from the Antarctic contained about 0-5 part per million
,, ,, Equatorial seas ,, 0*1 ,, ,,

,, ,, North Atlantic ,, 0-15 ,, ,,

of inorganic nitrogen compounds.

All these results apply to water taken from the

v] THE SOURCES OF FOOD 131

surface of the sea at some distance from the land.
Many analyses have also been made of coastal water
and water from the deep, and without quoting these
we may remark that the coastal water contains more
inorganic nitrogen and more carbonic acid, and
also more organic dissolved carbon and nitrogen
compounds than does the water at some distance
from the land. The amounts of these substances
also increase regularly the deeper in the sea we go,
and the difference between the surface and the
bottom is very like the extreme difference found in
the surface waters of the Antarctic and tropics.

Considering the results a little more closely we
notice that : (1) The amount of inorganic nitrogen in
sea-water is very small, varying from one-tenth to
about one-half per million of water ; nevertheless
upon this minute trace depends all the life of the
sea : it is so small because it is continually being
absorbed by plant organisms. (2) It is greatest in
the Antarctic where the temperature was nearly that
of freezing water ; much less in the North Atlantic,
where the temperature varied from about 5° to 10° C. ;
and least in the tropics, where the temperature was
about 28° C. (3) The proportion of inorganic nitrogen
was least in August, when the temperature was
highest ; and greatest in February, when the tem-
perature was lowest. (4) The proportion was greatest
at the bottom of the sea and least at the surface. It

9—2

132 LIFE IN THE SEA [CH.

is greatest in the oozes at the bottom. It is large
near the land.

In considering the distribution of marine animals
and plants we noticed that the abundance of food
was a factor of importance and we see now that the
abundance of food, that is, the abundance of the
plants — the producers of animal food — depends on the
nitrogenous and other ultimate food-stuffs present in
the sea- water. We need only consider the distribution
of the nitrogen for this is the substance which rules
the production in the sea. It is true that carbonic
acid, mineral salts and oxygen are also required, and
that lime and silica are necessary for the shells or
skeletons of marine organisms. But when a number
of indispensable food-stuffs are required and when
one of these is present in small proportions, then the
production depends on the substance which is present
in minimal quantity, just as the strength of a chain
depends on that of its weakest link. The silica and
phosphates are present in the sea in very small traces,
but they are usually more abundant than is necessary
when the amount of nitrogen present is considered.
The other materials, and oxygen, are present in
relatively large amount.

Let us return now to some of the physical
questions discussed in former chapters. We have
seen that light penetrates to a depth of about
400 fathoms at the most, and probably effective light

v] THE SOURCES OF FOOD 133

disappears far above that level. It is only in the
upper strata of the sea that plants can reproduce
and grow, for only there do they receive the energy
of sunlight necessary for the photo-synthesis of
carbohydrate food-stuff. It does not help them that
practically unlimited quantities of carbon dioxide
can be obtained from the atmosphere, and unlimited
energy from solar radiation, for their production can
only keep pace with the limited amount of nitrogen
compounds present in solution in the water. On the
other hand it is of no avail that more nitrogen is
present in the depths, for in the absence of the power
of photo-synthesis of starch, no proteid can be formed.,
and unlimited nitrogenous food-stuff would be of no
use to them. Production goes on only in the upper
well-lighted layers of the sea and the dead bodies of
plants and animals living there fall to the sea-bottom
and accumulate in the oozes, the globigerina and
diatom deposits for instance. Now it is doubtful
how far bacterial life can flourish at the sea-bottom
where the temperature is very low, and some samples
of oozes examined seem to have been sterile in the
media employed for cultures. Some amount of
bacterial action may go on, but what it is exactly
we do not know — research in this direction is urgently
needed. We may however conclude that the processes
of putrefaction proceed much more slowly at the
bottom of the ocean than on the land, or in shallow

134 LIFE IN THE SEA [OH.

waters, or at the surface of the sea ; and that the
proteid, carbohydrate, and fat of the bodies of the
organisms falling to the bottom may remain for some
time not much altered in composition. They thus
form the food of the abyssal animals, and the latter
must be, to some extent, carrion-eating creatures.
In the end, however, the carbon and nitrogen
compounds of the bodies of organisms at the sea-
bottom will be transformed to products like urea,
and, of course, carbonic acid ; and the urea, or similar
substance probably passes into the form of ammonia
by the action of enzymes. Then these substances
must be removed from the sea-bottom, for otherwise
the bottom water would be richer in organic matter
than it actually is.

This removal is effected by the agency of currents.
In the main the system of oceanic circulation consists
of a bottom-drift from about the region of the
temperate seas to the equator. This cold bottom
water rises up to the surface at the equator, and
then becoming heated flows away to the north and
south. There is a similar bottom-drift from the
temperate regions towards the poles, and then this
water rises to the surface to take the place of that
which flows to the south or north after it has become
lighter by freezing. Thus there is a continual
streaming of water along the sea-bottom and this
water rises to the surface in the equatorial and polar

v] THE SOURCES OF FOOD 135

seas. It contains an excess of organic matter which
it has removed from the abyssal levels of water.
Even in shallow seas, near the land, or at the
junction of warm and cold currents, there are also
these vertical currents from the bottom towards the
surface. Seasonal changes of temperature must also
produce them, for in the winter the surface waters
must cool and sink to the bottom. Wherever there
is a vertical current coming up from the sea-bottom
there should be an increase in the abundance of
plant life, for this vertical current should contain an
excess of ultimate food-stuffs. For the same reason
there should be a greater amount of life in the
shallow water near the land.

We do actually find this abundance of plant life
near the land, and it is pretty certain that local
abundances of plankton are sometimes due to up-
welling currents bringing abundant food-stuff from
the bottom. The richness of plant life in the
plankton of the circumpolar seas is also due in part
to the up welling currents. There are great differences
from year to year in the abundance of fish life in the
Norwegian seas, and also in the time of spawning, and
in the condition of the fish ; and it is remarkable
that it is the years when the temperature of the sea
is lowest which yield the greatest abundance of life.
Now this has been explained by attributing the
lower temperature of those years to a stronger drift

136 LIFE IN THE SEA [OH.

of water from the Arctic seas, and this stronger drift
would bring with it a greater supply of food-stuff.

But why do we not get a corresponding abundance
of plant life in equatorial seas where the upwelling
of bottom water is maximal, and where nitrogen and
carbon food-stuff must be transported from the bottom
to the surface ? As we have seen, the opposite is the
case, and the tropical seas are relatively poor in
plant life. This poverty might be explained in more
than one way, by the temperature hypothesis for
instance, which we have already noticed in Chapter III.
But if this were true we should still have an
abundance of nitrogen compounds produced by the
wasteful metabolism of the organisms there, and it is
the case that nitrogen compounds are scarcer in
equatorial seas than elsewhere in the ocean. A
further possible explanation is that these substances
are destroyed by bacteria.

Nitrogen compounds must be removed from the
sea in some way for they are continually being added
to it in the water flowing down from the land in the
rivers, and it has been estimated that about 39
millions of tons of this element in the form of
dissolved inorganic and organic compounds are added
to the seas of the world every year. The amount
that flows into the North Sea itself is said to be at
least about 390 millions of kilograms annually, while
the amount of nitrogen in the form of fish and other

v] THE SOURCES OF FOOD 137

economic products that is taken from the North Sea
in the same time is only about 30 millions of kilograms.
The excess must therefore be removed for the sea
does not appear to be getting richer in these
compounds. No deposits containing nitrogen are
formed in the same way as calcareous deposits are
laid down, but it has been suggested that ammonia
is given off from the surface of the sea into the
atmosphere, and that it is then redeposited on the
land in the rainfall. But this is unlikely and it is far
more probable that the excess is removed from the
sea by the agency of bacteria.

Denitrifying bacteria have long been known from
cultivated land, and they have also been discovered
in the sea near to the land. They are organisms
which appear to live under a variety of conditions :
some requiring simple carbonaceous food while
others must receive more complex substances. Some
of them can act in the absence of oxygen while others
require this gas. They obtain their nitrogen food
from nitrates, nitrites and ammonia, and their life-
process with respect to these substances is one of
deoxidation, for they can reduce the nitrate to nitrite,
the nitrite to ammonia, and the latter substance to
free nitrogen. The free nitrogen so formed passes
into solution in the sea- water, but the latter already
contains as much as it can hold, so the nitrogen then
passes back again into the atmosphere. Now it has

138 LIFE IN THE SEA [CH.

been found that the activity of the denitrifying
bacteria is greatest at about 25° C., that is ap-
proximately the temperature of the tropical seas,
and it is practically arrested at freezing point.
Therefore if these bacteria are universally distributed
over the seas of the world they must be most active
in equatorial regions and least active at the bottom
of the ocean, and in circumpolar seas. It is possible
to account- for the destruction of the excess of
nitrogen compounds entering the sea by assuming
their activity. If they are present in tropical waters
we may also account for the poverty of these seas in
nitrates and nitrites, and we should have a convincing
explanation of the poverty of tropical seas in vegetable
plankton.

One is tempted to follow up these hints by a
speculation as to past modes of nutrition of marine
animals, and as to the kinds of food-stuff which they
had at their disposal. No fact of biological chemistry
is stranger than this — that nitrogen should be the
element which is so intimately associated with the
processes of life. For of nearly all chemical elements
it is this one which is the most sluggish and inert.
Yet its compounds with carbon, hydrogen and oxygen
are those which exhibit the greatest degree of
complexity known to us, and the reactions of these sub-
stances with each other, and with simpler compounds
constitute what we recognise as the phenomena of life.

v] THE SOURCES OF FOOD 139

All writers who have considered the question of the
circulation of nitrogenous food-stuff in nature have
pointed out the remarkable scarcity of these com-
pounds. Everywhere in the land and sea organisms
suffer from a state of chronic nitrogen hunger, and
this incessant demand for more nitrogen food than
the sea or soil affords has led to a host of adaptations
of modes of nutrition. It has been suggested that life
first developed, or sprang into existence on the earth
in the presence of accumulations of amino-acids
formed after the crust of the earth first began to
solidify permanently. Ammonia is still given off in
the emanations from volcanoes and it may be that
plutonic activity, in the presence of conditions that
can never recur while the earth remains a planet,
gave rise to extensive accumulations of nitrogen
compounds capable of acting as the soil on which the
germs of life, introduced to the earth from outer
space, developed. One must suppose that such
nitrogen compounds were at one time more abundant
than they are now.

If they were, then it is probable that the metabolism
of organisms was more wasteful than it is at the
present, just as the metabolism of putrefactive and
fermentative bacteria, organisms which live in a
greater plenitude of food than any others, are of all
the most wasteful. Such primitive organisms were
probably saprophytic in habit, that is they lived by

140 LIFE IN THE SEA [OH.

assimilating the rich supply of food substance through
their surface. The result of their metabolism would
be the degradation of both carbon and nitrogen food
substance to the form of ammonia and carbon dioxide,
both substances on which they could no longer subsist.
It is also probable that degradation of the nitrogen
compounds would be carried on further than this
stage, and that free nitrogen would be formed, as
it is at present, by denitrifying organisms. For we
cannot but be struck by the fact that while there is
so little of nitrogen compounds in nature there is
nevertheless an enormous quantity of the element in
the atmosphere. We are tempted to regard this as
a residue formed by the degradation of compounds
of nitrogen by the agency of living organisms.

There would thus arise a scarcity of both forms
of food-stuff, carbonaceous and nitrogenous. By
developing the power of photo-synthesis plant organ-
isms solved the question of getting sufficient carbon
food from carbon dioxide and water, under the
influence of light; and they became able to utilise
ammonia and nitrate — that is they became proto-
trophic in habit. Organisms developing along the
animal line continued to be saprozoic, and many
still remain so ; but this became a difficult and
precarious mode of nutrition, as it still is ; and so the
two savage methods of procuring food — by parasitism,
and by hunting and devouring other organisms — were

v] THE SOURCES OF FOOD 141

evolved. Even among the plants the scarcity of
nitrogen food has led to the adoption of the parasitic
habit, to their symbiotic association with animals,
and to the insectivorous method of feeding. One
must think of modes of nutrition as being just as
adaptable to changed conditions as are the structures,
habits, and life-histories of organisms.

On this view then the 79% of nitrogen which
exists in the atmosphere, and the relatively small
quantities of nitrogen compounds which are present in
the soil and sea are the results of denitrification
carried on in the past at a greater rate than at the
present time ; and the tendencies of vital processes
as we know them are towards a still greater amount
of denitrification. Yet we need not be alarmed, for
living processes are essentially regulatory, and both
nature and art are combining to solve the nitrogen
question ; art, in that it is now possible by the
advance of modern chemistry to combine atmospheric
nitrogen with oxygen on a large scale ; and nature by
reason of the adaptability of organisms. We know
that a number of species of plants, the Legumin-
osae (peas, beans, clover, etc.), are able to take up
elementary nitrogen from the atmosphere by the
assistance of associated bacteria contained in their
root tissues. Such nitrogen-fixing bacteria have also
been discovered to be widely distributed in the sea
and on the land. They can assimilate the free gas,

142

LIFE IN THE SEA

[CH. V

and in the presence of carbon food-stuff they can
synthesise it to proteid. We may then speculate
further and think of a continued evolution of this
mode of nutrition; that by-and-by the majority of
plants and other holophytic plankton organisms may
become freed from the shackles of inherited modes
of nutrition; and become able to assimilate atmo-
spheric nitrogen, just as their ancestors, who felt the
pinch of carbon hunger became able to assimilate
atmospheric carbon dioxide, when that gas was a
residue as nitrogen is at the present. In this way
we can imagine the nitrogen hunger of marine or-
ganisms becoming at last satisfied.

The Lucas Sounding Lead and Snapper.

AUTHOEITIES

1. General Marine Biology.

Works dealing with this subject are very numerous but we
may mention the volumes of the Cambridge Natural History
which treat of marine groups of animals. Oltman's Morphologic
und Biologic der Algen, Vols. I and II, Jena. 1904-5, gives a very
good account of the Algae. Cunningham's Marketable Marine
Fishes (Macmillan) is a good account of the habits and life-history
of the commoner fishes. Hickson's Fauna of the Deep Sea (Kegan
Paul, 1894) ; and Hjort's account of the recent voyage of the
* Michael Sars,' in Nature, Vol. LXXXV, 1910-11, deal with abyssal
animals. Part of Chapter IV of the volume on Deep-Sea Deposits
in the 'Challenger' Reports, and from page 1431 in the General
Summary, 2nd part, should also be read.

2. Plankton.

The series, Nordisches Plankton, Kiel and Leipzig, now being
published is a very complete account of the organisms occurring in
northern seas. Steuer's Planktonkunde (Leipzig, 1910) is a general
treatise on all questions connected with plankton studies. Schiitt's
Analytische Plank ton-Studien (Kiel and Leipzig, 1892) deals
with oceanic plankton, but Victor Henson has just published a
summary of the results of the German Plankton Expedition. The
most recent and exhaustive account of methods is that given by
Lohmann in the Kiel Kommission's Wissenschaftliche Meeres-
untersuchungen, Vols. 7 (1903) and 9 (1908). Detailed tables giving
the actual numbers of plankton organisms estimated as being

144 AUTHORITIES

present per square metre of certain parts of the North Sea and
Baltic are given by Apstein in the Wissenschaftliche Meeres-
untersuchungen, Vol. 9, 1906, and later vols. Tables of the
occurrence and distribution of plankton organisms are published
by the International Council for the Exploration of the Sea, in
the Bulletin Trimestriel. Short accounts with bibliographies of
the methods and results of plankton investigations are given by
Jenkins and Dakin in the Transactions of the Liverpool Biological
Society, Vol. 15, 1901, and Vol. 22, 1908 ; and by Johnstone in
Conditions of Life in the Sea (Cambridge University Press, 1908).

3. Oceanography.

There is no English text-book, but Mill gives a good account
of the principal facts in The Realm of Nature (Murray, 1895).
The only exhaustive book on the subject is Kriimmel's Handbuch
der Ozeanographie (Stuttgart, 2 vols. 1907 and 1911). This book
is not translated.

4. Ultimate food-stuffs in the sea.

The main original sources are Raben's papers in the Wissen-
schaftliche Meeresuntersuchungen, Vol. 8, 1905, and Vol. 11, 1910.
Gran and Nathansohn also give some analytical results in the
Internationale Revue Gesamten Hydrobiologie, Vol. 1, 1908. An
account of the results obtained during the cruise of the German
ship 'Gauss' in the Antarctic is given by Gebbing in the Internat.
Rev. Hydrobiol, Vol. 3, 1910.

5. Circulation of food-stuffs in the sea.

See Nathansohn, Bulletin de Flnstitut Oceanographique,
Monaco, May, 1909. See also the work of Helland-Hansen and
Nansen on the Gulf Stream in high northern latitudes. This
memoir is very important. Reviewed in Science Progress for
Jany. 1910 and April 1910 by Johnstone.

6. Nutrition of marine animals.

The hypothesis that a saprozoic mode of nutrition is very

AUTHORITIES 145

general among the lower marine organisms is not generally
accepted by English zoologists. It has been developed by Flitter
in Ernahrung der Wassertiere (Fischer, Jena, 1909). There
is an account of Putter's earlier work in Science Progress for
July, 1908, with references to the literature.

7. Plant-Animals.

See Keeble's book in this series. There is a bibliography, and
the general question of nitrogen hunger is discussed.

8. Denitrification in the sea.

The suggestion that this occurs was first made by Brandt, see the
Wissenschaftliche Meeresuntersuchungen, Vol. 4, 1899, and Vol. 6,
1 902 — * Stoffwechsel im Meeres.' These papers are very important
but they have not been translated. They are reviewed and
full references are given in Science Progress, Oct. 1907. The
hypothesis that the distribution of plant-life in the sea is
conditioned by denitrification is probable, but investigation of
the occurrence of these bacteria in all seas is still wanting.

9. Temperature and the duration of life.

The hypothesis which is mentioned in this book is due to Loeb.
See Archiv fur die Gesammten Physiologic, Bonn, Bd. 124, 1908.
The hypothesis, which is nevertheless a very probable one, is
supported by some very inadequate experiments : these must
be extended and confirmed before the 'coefficients' can be
accepted.

10. The theory of tropisms.

This is the subject of a very large literature, but see Loeb,
Dynamics of Living Matter, Columbia University Biological
Series, 1906.

11. Jennings' book, Behaviour of the Lower Organisms, Columbia
University Biological Series, 1906, describes numerous experimental
results. It and Driesch's Science and Philosophy of the Organism
(A. and C. Black, 1907-8) should be read. They take a view of
animal behaviour directly opposed to that of the tropistic theory.

J. 10

146 AUTHORITIES

12. General physiology.

Very much has been written on this subject, but see Verworn,
General Physiology (Macmillan, 1899). The best recent book on
the chemistry of the animal tissues is Wells' Chemical Pathology
(Saunders, New York, 1907).

13. .The origin of life.

The suggestion that life originated in accumulations of amino-
acids due to plutonic activity is made by Snyder, Science Progress,
April, 1909, and July, 1911. These papers contain full references.
The suggestion that life reached the earth from cosmic space is
an old one, but it is now based on an argument derived from
recent work on the radiation pressure of light, by Arrhenius ; see
Worlds in the Making (Harper Bros. 1908).

14. The theory of migration by Intoxications.

This is due to Gurley ; see American Journal of Psychology,
Vol. 20, p. 102, 1909.

INDEX

Absorption of food-stuff, 112
Abysses, oceanic, 7 ; organisms

of, 14, 58, 63
Algae, calcareous, 5 ; distribution,

55 ; pelagic, 48, 56 ; symbiotic,

100

Alimentary canal, 111
Amino-acids, 94, 98, 119
Ammonia in the sea, 130
Amoeba, 115
Antarctic, sea-bottom, 11 ; fauna

and flora, 57

Assimilation, 91, 96, 118, 126
Atlantic, sea- bottom, 9
Autolysis, 115

Bacteria, aerobic, 123 ; anaerobic,
123 ; carbon, 126 ; denitrifying,
137 ; fermentative, 123 ; nitri-
fying, 124 ; nitrogen-fixing,
141 ; putrefactive, 123, sul-
phur, 125

Barnacles, feeding of, 89

Benthos, 17 ; abyssal, 14 ; deep-
sea, 6 ; laminarian, 3 ; shallow-
water, 6

Bioplasm, 92, 118

Burrowing animals, 3, 6, 38

Carbohydrates, 93, 119 ; syn-
thesis of, 97, 100

Carbon compound in sea, 128
Carbonaceous food-stuff, 119
Carbon dioxide in sea, 129
Calcareous organisms, 55
Carnivores, marine, 91
Chlorophyll, 99
Circulation in ocean, 33
Classification of organisms, 1
Cod, spawning of, 42
Consumers, 121
Continental shelf, 7
Convoluta, burrowing of, 38 ;

nutrition, 99 ; spawning, 37
Copepods, 27
Corallines, 5

Corals, 55 ; nutrition of, 101
Crustacea, feeding of, 90
Currents, ocean, 34
Cuttle-fishes, feeding of, 90

Degradation products, 126
Density of marine life, 51
Development and temperature,

71
Diatoms, 25 ; in oceanic oozes,

9; distribution phases, 47; on

shore, 4
Digestion, 91 ; bacterial, 115 ;

digestive glands, 94, 116 ;

intracellular, 116
Distribution of marine organ-

148

INDEX

isms, 54, 61, 67, 78 ; distribu-
tion barriers, 61, 63 ; productive,
79 ; nominal, 79
Diffusion, 75

Eel, migrations of, 51
Enzymes, 94
Evolution of habits, 69
Excretory products, 123

Fat as food- stuff, 93

Faunas contrasted, 52

Fecundity of marine organisms,
66

Fermentation, 122

Ferments, 94

Fishes, abyssal, 16; cold-water,
56 ; deep-sea, 13 ; demersal,
19; feeding of, 89; maturity
of, 41 ; migrations, 79 ; paren-
tal habits, 43 ; pelagic, 19, 21 ;
shallow-water, 5 ; shore, 3 ;
spawning of, 42 ; specific
gravity of, 20 ; reproduction
of, 23, 41

Food-stuffs, indispensable, 132 ;
nitrogenous, 96, 119 ; plant,
96 ; proximate, 91 ; ultimate,
127

Foraminifera, 10

Greenlandic current, 35
Growth of marine animals, 43
Gulf Stream, 35, 59, 82 ; and

plankton, 48 ; and fisheries,

50
Gulf Stream drift, 36

Halibut and salinity, 81
Herbivorous marine animals, 91
Heredity of habit, 51
Herring fisheries, 50, 81

Hibernation of fishes, 41
Hippolyte, 39
Hormones, 116

Icelandic current, 35

Infusorians, 90

Invertebrates of shallow water, 5 ;
deep water, 15 ; phosphores-
cent, 17

Insectivorous plants, 114

Katabolism, 126

Labrador current, 35, 59

Larvae, 23 ; distribution of, 67 ;
locomotion of, 65 ; metamor-
phoses of, 66 ; periodicity of,
46

Larval stages, 64

Leguminous plants, 141

Life, duration of, 71

Light and organisms, 76

Littoral fauna and flora, 2

Locomotion of marine organisms,
19

Lugworm, food of, 107

Marine animals, food of, 91 •

Medusae, 27

Metabolism, 41, 44 ; destructive,
126 ; nitrogenous, 122 ; syn-
thetic, 98

Migrations of copepods, 62;
diatoms, 62 ; fishes, 67, 79 ;
peridinians, 62

Mollusca, feeding of, 90

Moonlight and plankton, 32

Mussel, feeding of, 108

Nekton, 19

Nitrates and nitrites, 130

Nitrogen compounds, 128 ; in

INDEX

149

Atlantic, 131 ; destruction of,
136, 140; in deep water, 131;
storage of, 124 ; in polar seas,
130 ; in tropical seas, 131 ;
seasonal variation of, 131

Nitrogen hunger, 139

Nutrition, holozoic and holophy-
tic, 99, 101 ; heterotrophic,
101 ; metatrophic, 123 ; proto-
trophic, 125 ; saprophytic, 105

Ocean circulation, 134

Ocean currents, vertical, 135

Oceanic fishes, 21

Oozes, deep-sea, 8 ; abyssal, 8 ;

diatom, 9 ; Globigerina, 9 ;

pteropod, 9; radiolarian, 9;

Red clay, 11

Ooze-eating animals, 134
Optimal conditions, 77

Parasites, 102 ; life-history of,
103 ; nutrition of, 104

Peridinians, 25 ; feeding of, 111

Pelagic fishes, 88

Periodicity of habit, 51

Phosphates in sea, 132

Phosphorescence, 17

Photosynthesis, 97, 99, 133

Phototaxis, 83

Physical conditions and life, 58

Plant-animals, 120

Plant nutrition, 96

Plaice, spawning of, 42

Plankton, 22 ; plankton-feeders,
88; as food, 101, 108; and
physical conditions, 49 ; larvae
of, 23 ; succession of, 46, 49 ;
transitory, 45,

Predatory animals, 87

Prehensile organs, 88

Pressure and marine life, 13, 20

Production in sea, 45

Producers, 121

Proteids, 93; constitution of,

119 ; digestion of, 95
Protozoa, 26 ; nutrition of, 102
Proximate food- stuffs, 96, 98
Putrefaction, 122

Radioactivity in sea, 59
Radiolaria, 10, 25
Reproduction and temperature, 78
Respiration of fishes, 70
Respiratory organs, 113
Respirable material, 118

Salinity of sea, 32, 59 ; per-
ception of, 74 ; and migrations,
75 ; and tides, 33 ; range of,
60 ; variations of, 36

Sand, organisms of, 4

Sargasso Sea, 62

Sea-bottom, illumination of, 12 ;
pressure at, 13 ; temperature
at, 12 ; uniformity of, 7

Sea-bottom deposits, 8 ; cal-
careous, 9 ; meteoritic, 11 ;
shelly, 5 ; siliceous, 11 ; vol-
canic, 8 ; sharks' teeth in, 11 ;
whale earbones in, 11

Sessile organisms, 63 ; distribu-
tion of, 65

Shellfish, food of, 89, 107

Shore invertebrates, 2

Silica in sea, 132

Skate, spawning of, 42

Spawning and temperature, 43

Spawning migrations, 67

Sponges, feeding of, 9, 110

Starch, synthesis of, 97

Starfishes, feeding of, 90

Sunlight and diatoms, 75 ; and
plant metabolism, 97

150 INDEX

Taxis, 82 Tropisms, 82

Temperature of sea, 31 ; and life- Tropical life, 55

phases, 40 ; perception of, 69 ; Tunicates, feeding of, 89

optimal, 77 ; range of, 60

Tides, 29 ; and cockle, 38 ; and Whales, feeding of, 88

Convoluta, 38 ; and fisheries,

38 ; irregularities of, 30 ; and Zones of life, 5

littoral animals, 38

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Hamilton Thompson, M.A., F.S.A.

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Discovery in Greek Lands. By F. H. Marshall, M.A.

The Poetry of Burns. By H. J. C. Grierson, M.A.

The Crusades. By Rev. Prof. J. P. Whitney, B.D.

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Cambridge University Press
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