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)LOGICAL LABORATORY

mapas , EDITED BY On Hoos

_ PROFESSOR H. N. MARTIN ae

nen AND |} s

PROFESSOR WILLIAM K. BROOKS

Pr aaat

IN ITS RELATION TO THE EVOLUTION

fOr ire Pea:

BALTIMORE i > met
THE JOHNS HOPKINS PRESS aN |
Sats ii _ May, 1893 __ ss ied

i) se y

Mis sa NR a [ Het

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Sl UDI S

FROM THE

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EDITED BY

PROFESSOR H. N. MARTIN

AND

PROFESSOR WILLIAM K. BROOKS

SALPA IN ITS RELATION TO THE EVOLUTION
OF LIFE/

By WILLIAM K, BROOKS, Pu. D.

APR 15 1988
LIBRARIES

BALTIMORE
THE JOHNS HOPKINS PRESS
May, 1893

ae
eon

PNT RODUCT OR YoNOs EE

(This paper consists of two chapters, VII and VIII, printed, in
advance, from my memoir on the Genus Salpa, which is now passing

through the press.)

In chapter VI of my memoir on the Genus Salpa I shall give, at
length, reasons which, briefly stated, are as follows, for believing
that, while salpa is remotely descended from a pelagic, appendicu-
laria-like ancestor, it is more immediately derived from a sessile
form, similar in its habit of life, and essentially, in structure also
to the ascidians.

Tn the first place, comparative anatomy forces us to believe that
the atrium of salpa is identical with the perithoracic and atrial
chamber of ordinary ascidians, and the facts of embryology show
beyond question that this is a real homology.

Writers on the embryology of salpa and allied animals have
involved the history of the atrial system in unnatural obscurity,
for its origin in the salpa embryo and in the aggregated salpa
is in perfect accordance with the teaching of comparative anatomy,
and quite irreconcilable with any view except the one which re-
gards these structures in salpa as strictly homologous with’ the
median and lateral atria of ordinary ascidians.

As Leuckart pointed out long ago, the atrial aperture of salpa
is much nearer the mouth when it first appears than it is later,
and in this respect the ontogeny of salpa exhibits evidence of an
ascidian-like stage in its ancestry. ‘The compactness of the ganglion
of salpa, as contrasted with the elongated central nervous system
of primitive chordata, and its position between the two apertures
of the body, are also features of resemblance to the ascidians ; and
while there are now no traces, at any stage of its development, of
numerous stigmatic gill-slits, like those of pyrosoma, there is ample

ili

iv INTRODUCTORY NOTE.

indirect evidence that they at one time existed in the ancestors of
salpa, which were in this respect, pyrosoma-like. _

The muscle bands of salpa are easily intelligible as modified oral
and atrial sphincters, and they are distinctly more irregular in the
young than they are in the adult. In the young aggregated Salpa
eylindrica, the fourth and fifth body muscles are clearly seen to
arise as branches from an atrial sphincter, and some of the body
muscles arise in the same way in the aggregated Salpa pinnata.

The peculiar anatomical relations of the pharynx and atrium of
ascidians are generally and justly regarded as modifications which
were gradually added on to the primitive tunicate type, as adapta-
tions to a sedentary life. If salpa has been evolved from a swim-
ming ancestor like appendicularia through an uninterrupted series
of free pelagic stages, we can give no explanation whatever of its
ascidian type of structure, while this is perfectly intelligible on the
view that it is a modified ascidian.

W. K. Brooks.

SALPA IN ITS RELATION TO THE EVOLUTION
OF LIFE. By W. K. BROOKS.

Salpa is distinctively a pelagic animal, adapted by its whole
structure for a free existence, and for life at the expense of the
micro-organisms in the water of the ocean.

To understand its position and significance in the economy of
nature, we must have before us the broad outlines, at least, of a
picture of the conditions under which oceanic life has been evolved.

I believe that the history of the evolution of salpa, as told by
its embryology, is most suggestive and important, and that it
contributes to the solution of some of the most profound ‘and
fundamental problems of biology, and brings us into conflict with
some of the most favorite dicta of modern morphology. I shall
therefore devote considerable space to a review of certain familiar
features of ocean life, in order that I may present in this way my
view of the significance of the phylogeny of salpa, in its bearing
upon the first principles of morphology.

CONTRAST BETWEEN TERRESTRIAL LIFE AND MARINE LIFE.

In a picture of the land, the mind calls up a vast expanse of
verdure, broken only by water, and stretching through forest and
meadow from high up on the mountains, over hills and valleys
and plains, down to the sea.

Our picture of the ocean is an empty waste, stretching on and
on with no break in the monotony, except, at long intervals, a
floating tuft of sargassum, or a flying fish, or a wandering sea-bird,
and we never think of the ocean as the home of vegetable life.

It contains plant-like animals, ‘“ zoophytes,” in abundance, but
while they resemble plants or flowers in form and color, and in
their mode of growth, they are true animals and not plants.

At Nassau, in the Bahama Islands, the visitor is taken in a
small boat, with windows of plate glass set in the bottom, to visit

129

130 W. K. BROOKS.

the “sea-gardens” at the inner end of a channel, through which
the pure water from the open sea flows between two coral islands,
into the lagoon. Here the true reef corals grow in quiet water
where they may be visited and examined. °

The bottom of the boat is below the surface ripples and reflec-
tions. When illuminated by the vertical sun of the tropics, and
by the light which is reflected back from the white bottom, the
pure transparent ocean water is as clear as air, and the smallest
object, forty or fifty feet down, is seen distinctly.

As the boat glides over the great mushroom-shaped coral domes
which arch up from the depths, the dark grottoes between them,
and the caves under their overhanging tops, are lighted up by the
sun far down among the flower-animals or anthozoa and the
animal plants or zoophytes which are seen through the waving
thickets of brown and purple sea fans and_sea feathers as they toss
before the swell from the ocean.

There are miles of these “sea-gardens”’ in the lagoons of the
Bahamas, and it has been my good fortune to spend many months
studying their wonders, but no description can convey any con-
ception of their beauty and luxuriance, and I never spent a day
among the reefs without longing, at every turn, for the skill to
copy with a brush the new beauties which never ceased to pre-
sent themselves.

The general effect is very garden-like, and the beautiful fishes
of black and golden yellow and iridescent cobalt blue hover like
birds among the thickets of yellow and lilac gorgonias. The
parrot fishes (Diodon and Ballistes) seem to be cropping the
plants like rabbits, but more careful examination shows that they
are biting off the tips of the gorgonias and branching madrepores,
or hunting for the small crustacea which hide in the thicket, and
that all the apparent plants are really animals. The delicate star-
like flowers are the vermilion heads of boring annelids, or the
scarlet tentacles of actinias, and the thicket is made up of pale
lavender bushes of branching madrepores and green and yellow
and olive masses of brain coral, of aleyonarians of all shades of
yellow and lilac and purplé and red, and of red and brown and
black sponges. Even the lichens which incrust the rocks are
hydroid corals, and the whole sea-garden is a dense jungle of

SALPA IN RELATION TO EVOLUTION OF LIFE. 131

animals where plant life is represented only by a few calcareous
algze, so strange in shape and texture that they are much less
plant-like than the true animals.

The scarcity of vegetation becomes still more noticeable when
we study the ocean as a whole.

On land, herbivorous animals are always much more abundant
and prolific than the carnivora, as they must be to keep up the
supply of food. Insectivorous birds are very abundant, but they
are not numerous enough to keep the plant-eating molluscs and
insects in check, and the devastation which is caused every year
by the armies of grasshoppers and locusts and herbivorous beetles
and by other less conspicuous insects, shows that their natural
enemies are not numerous enough to overtax their productive power.

The birds which feed upon grain and seeds and fruit are very
abundant indeed, and they sometimes gather at their breeding
grounds, or places of assembly, in innumerable multitudes, but
the hawks and owls which prey upon them are never numerous.

The small rodents, such as the rats, mice, squirrels, and rabbits,
are the most abundant and prolific of animals; but the small
carnivora are so rare that their very existence is known to few
except naturalists and trappers.

The homes of the wild sheep and goats, deer, antelopes, cattle and
horses support these large mammalia in incredible numbers, but
their carnivorous enemies are never abundant. It is clear that if
the destruction of the plant-eaters exceed their productive power,
both herbivora and carnivora would disappear, and terrestrial life
would come to an end.

The animal life of the ocean shows a most remarkable difference,
for marine animals are almost exclusively carnivorous.

The birds which live upon the ocean, the terns, gulls, petrels,
divers, cormorants, tropic birds and albatrosses, are very numerous
indeed ; so numerous that in many parts of the ocean some are
always visible in calm weather around the vessel, wherever it may
be. The only parallel to the pigeon-roosts and rookeries of the
land is found in the dense clouds of sea-birds around their breed-
ing places, but these sea-birds are all carnivorous ; most of them
are fishers, and others, such as the petrels, scoop up the copepods
and pteropods from the surface. Even the birds of the sea-shore

132 W. K. BROOKS.

subsist almost exclusively upon animals such as molluscs, crustacea
and annelids.

The seals pursue and destroy fishes ; the sea-elephants and wal-
ruses live upon lamellibranchs ; the whales, dolphins and porpoises,
and the marine reptiles, all feed upon animals, and most of them
are fierce beasts of prey. The manatee is a vegetable feeder, but
it is not strictly a marine animal, since its home is in the mouths
of great rivers.

There are a few fishes which pasture in the fringe of seaweed
which grows in the littoral zone of the ocean, and there are some
which browse among the floating tufts of algee upon its surface, but
most of them frequent these places in search of the small animals
which live among the plants. All the floating fishes whose home
is the floating sargassum ; the file fishes and trigger fishes (Ballistida) ;
the trunk fishes (Ostracion); the frog fishes (Antennarius) ; and
the puffing fishes (Tetradon and Diodon) are carnivorous, living
upon the barnacles and molluscs and hydroids which grow upon
the sargassum, or upon the crustacea, young fishes and the floating
larvee which seek its shelter.

In the Chesapeake Bay, the sheepshead (Diplodus probato-
cephalus) browses among the alge upon the submerged rocks and
piles like a marine sheep, but its food is exclusively animal, and I
have lain upon the edge of a wharf watching it crush the barnacles
and young oysters until the juices of their bodies streamed out of
the angles of its mouth and gathered a host of small fishes to
snatch the fragments as they drifted away with the tide.

Many important fishes, like the cod, pasture on the bottom, but
their pasturage consists of molluscs and annelids and crustacea,
instead of plants.

The vast majority of marine fishes are fierce hunters, pursuing
and destroying smaller fishes, and often exhibiting an insatiable
love of slaughter, as in the case of our own blue-fish and the
tropical albacore and barracuda. Others, such as the herring,
feed upon smaller fishes and the pelagic pteropods and copepods ;
and others, like the shad, upon the minute organisms of the ocean,
but all, with few exceptions, are carnivorous.

In the other great groups of marine animals we find some
scavengers, some which feed upon micro-organisms, and others

SALPA IN RELATION TO EVOLUTION OF LIFE. 133

which hunt and destroy each other, but there is no group of
marine animals which corresponds to the herbivora and rodents
and plant-eating birds and insects of the land. The pelagic
copepods are, of all the marine Metazoa, the ones whose place in
the economy of nature is most like that of the terrestrial plant-
eaters. They swarm in innumerable multitudes at the surface of
the ocean, and also below it down to a depth of a mile or more,
and they furnish the chief food for most young fishes, and for
great armies of herrings and pteropods and jelly-fishes and siphono-
phores, and for most pelagic larve.

There are plant-eating molluscs and echinoderms and annelids
in the ocean, but not in sufficient numbers to play any conspicuous
part in its economy, and the copepods are the only plant-eaters
which exist in sufficient numbers to be compared with those of the
land, and the food of the copepods is only partially vegetable, for
they devour microscopic animals as well as microscopic plants, and
probably to an equal amount.

The group crustacea as a whole is a carnivorous one, however,
for while a few subsist on alge, their number is inconsiderable.
Others chew the mud of the bottom and extract its organic matter,
but this is chiefly animal and consists of foraminifera and rhizo-
pods and infusoria. .

The molluses as a whole are carnivorous, and while there are
many exceptions, such as the nudibranchs for example, many
nudibranchs feed on hydroids.

The cephalopods and pteropods and heteropods and many of
the gasteropods pursue and destroy their prey, and other gastero-
pods are scavengers, while the lamellibranchs gather up the micro-
scopic organisms which are drawn into their gills with the water.

The majority of the worms and echinoderms are animal-feeders.
Some of them, like the common starfish, are actively predaceous ;
others, like the erinoids, gather up microscopic organisms from
the water ; others, such as most holothurians, eat the mud of the
bottom and digest out of it the foraminifera and small molluscs
and annelids and crustacea which it contains, while others, such as
the sea-urchins of the coral reefs, grind away and swallow the
living coral. The universal presence of a poisoning apparatus in
the ccelenterates shows that the food of this great and important

134 W. K. BROOKS.

1

group of marine animals must consist, in the main, of animals
which are able to resist or to escape, and observation shows that
this is true. Floating jelly-fishes and siphonophores are often
found fastened to the half-digested carcasses of sagittas or hetero-
pods or fishes larger than their captors, and they consume enor-
mous numbers of copepods, pteropods, young fish, and _ pelagic
larvee of all sorts. So far as we know, all the sea-anemones and
coral polyps and aleyonarians and hydroids are carnivorous. Some
of the discomedusz, the rhizostomes, feed upon microscopic organ-
isms, but this mode of life is exceptional, and some recent observa-
tions, as yet unpublished, by Dr. R. P. Bigelow, show that the
food of the rhizostomes consists of copepods.

Except for a few plant-eating fishes and molluscs and worms
and echinoderms, all the animals of the ocean fall into two classes,
those which subsist on microscopic organisms, and those which
prey upon each other and correspond to the rapacious animals of
the land.

There is practically nothing in the ocean corresponding to the
terrestrial herbivora, and nothing like terrestrial vegetation, except
the fringe of seaweeds in the shallow water along the coast, and a
few floating islands of alge like the Sargasso Sea.

While these tracts of vegetation are pretty extensive, they are
totally inadequate to support the animal life of the ocean, and as
the whole auimal world is dependent directly or indirectly upon
plants, we must ask what takes the place of terrestrial vegetation.

THE FAauNA OF MID-OCEAN.

There is so much room in the vast spaces of the ocean, and the
part which is open to our direct observation is such an inconsider-
able part of the whole, that it is only when great multitudes of
pelagic animals are gathered together at the surface that the abun-
dance of marine life becomes visible and impressive ; but some faint
conception of the boundless wealth of the ocean may be gained by
observing the quickness with which marine animals become crowded
at the surface in favorable weather.

On a cruise of more than two weeks from Cape Hatteras to the
Bahama Islands I was surrounded continually, night and day, by

SALPA IN RELATION TO EVOLUTION OF LIFE. 135

a vast army of dark-brown jelly-fishes (Linerges mercutia), whose
dark color made them very conspicuous in the clear water. They
were not densely crowded, although they were so abundant that
nearly every bucketful of water we dipped up contained some of
them. We could see them at a distance from the vessel, and at
noon, when the sun was overhead, we could look down into the
water to a great depth through a well in the middle of the vessel
where the centreboard hung, and as far down as the eye could pene-
trate, fifty or sixty feet at least, we could see the brown spots drift-
ing by like motes in the sunbeam. We cruised through them for
more than five hundred miles, and we tacked back and forth over
a breadth of almost a hundred miles, and they were everywhere in
equal abundance.

The recent literature of pelagic exploration, which has been sum-
marized by Haeckel (Plankton Studien: von Ernst Haeckel, Jena,
1890), is full of references to great accumulations of pelagic animals,
from which I have selected those which follow.

Chiercha says that during a cruise of forty days between Peru
and Hawaii the net brought in from the surface and from all depths
down to about two miles, a multitude of pelagic animals which
would be incredible to those who have not witnessed it.

The naturalists of the Challenger found the waters of the equa-
torial Pacific swarming with life, not at the surface alone, but in
its deeper layers, and the ship often sailed through great banks of
pelagic animals.

The equatorial Atlantic is like the Pacific, and Chiercha says
that its zone of equatorial calms is rich beyond all measure in animal
life, and that the water often looks and feels like coagulated jelly.

Of the Indian Ocean, Haeckel says that in his voyage to and
from Ceylon he was wonderstruck with the wealth of pelagic life
day after day on the mirror-like surface. At night it was an un-
broken sheet of sparkling light as far as the eye could reach, and
the water which was dipped up at random held such a thick swarm
of densely crowded luminous animals (Ostracods, Salpz, Pyrosomas,
and Meduse) that a printed book could be read distinctly in a dark
night by this pelagic light.

In temperate and arctic waters there is less diversity, but, as
Haeckel shows, there is no evidence of any decrease in individuals,

136 W. K. BROOKS.

and banks of pteropods (Clio and Limacina), so dense that they
seem almost solid, are met even beyond the arctic circle.

Haeckel says that in a cruise to the northwest of Scotland he met
with such enormous masses of Limacina that each bucket of water
which was dipped up contained thousands.

The tendency to gather in crowds is not restricted to the smaller
pelagic animals, and many species of raptorial fishes are found in
densely packed banks.

The fishes in a school of mackerel are as numerous as the birds
in a flight of wild pigeons. Goode, in his History of Aquatic
Animals, tells of one school of mackeral which was estimated to
contain a million barrels, and of another which was a windrow of
fish half a mile wide and at least twenty miles long ; but while the
pigeons are plant-eaters, the mackerel are rapacious hunters, pur-
suing and devouring the herrings, as well as the pteropods and
pelagic crustacea.

Herring swarm like locusts, and a herring bank is almost a solid
wall. In 1879 three hundred thousand river herring were landed
by a single haul of the seine in Albemarle Sound ; but the herrings
are also carnivorous, each one consuming myriads of copepods every
day. In spite of this destruction and the ravages of armies of me-
dusze and siphonophores and pteropods, the fertility of the copepods is
so great that they are abundant in all parts of the ocean, and they are
met with in numbers which exceed our powers of comprehension.

On one occasion the Challenger steamed for two days through a
dense cloud formed of a single species, and they are found in all
latitudes from the arctic regions to the equator, in masses which
discolor the water for miles. We know, too, that they are not
restricted to the surface, and that the banks of copepods are some-
times a mile thick. When we reflect that thousands would find
ample room and food in a pint of water, we can form some faint
conception of their universal abundance.

Tue Prrutary Foop-supPiy.

As the result of our review, we find that the organisms which
are visible without a microscope in the water of the ocean and on
the sea bottom are almost universally engaged in devouring each

SALPA IN RELATION 10 EVOLUTION OF LIFE. 137

other, and many of them, like the blue-fish and the albacore, are
never satisfied with slaughter, but kill from mere sport.

Insatiable rapacity must end in extermination unless there is
some unfailing supply, and as we find no visible supply in the
water of the ocean we must seek it with a microscope. By its aid
we find a wonderfully rich and diversified fauna made up of
innumerable larvee of all sorts of marine animals, together with a
few minute and simple metazoa, but these things cannot form the
food-supply of the ocean. It is clear that a single carnivorous
animal could not exist very long by devouring its own children,
and the result must be the same however great the number of
individuals or species.

The total amount of these organisms is inconsiderable, however,
when compared with the abundance of a few forms of protozoa
and protophytes, and both observation and deduction force us to
recognize that the most important element in the total amount of
marine life consists of some half-a-dozen types of protozoa and
unicellular plants, of globigerinze and radiolarians, and of tricho-
desmium, pyrocystis, protococcus, and the coccospheres, rhabdo-
spheres and diatomes.

Modern microscopic research has shown that these simple
plants, and the globigerinee and radiolarians which feed upon
them, are so abundant and prolific that they meet all the
demands made upon them and supply the food for all the
animals of the ocean.

This is the fundamental conception of marine biology. The
basis of all the life in the modern ocean is to be sought in the
micro-organisms of the surface.

This is not all. The simplicity and abundance of the micro-
scopic forms and their importance in the economy of nature show
that the organic world has gradually shaped itself around and has
been controlled by them.

They are not only the fundamental food-supply, but the primeval
supply, which has determined the whole course of the evolution of
marine life.

The pelagic plant-life of the ocean has retained its primitive
simplicity on account of the very favorable character of its environ-

138 W. K. BROOKS.

ment, and the higher rank of the littoral vegetation and that of the
land is the result of hardship.

On the land the mineral elements of plant-food are slowly sup-
plied as the rains dissolve them ; limited space brings crowding
and competition for this scanty supply ; growth is arrested for a
great part of each year by drought or cold ; the diversity of the
earth’s surface demands diversity of structure and habit, and the
great size and complicated structure of terrestrial plants are adapta-
tions to these conditions of hardship.

The conditions of the surface of the ocean ; the abundance and
uniform distribution of mineral food in solution ; the area which
is available for plants ; the volume of sunlight and the uniformity
of the temperature are all favorable to the growth of plants, and
as each plant is bathed on all sides by a nutritive fluid, it is
advantageous for the new plant-cells which are formed by cell
multiplication to separate from each other as soon as possible in
order to expose the whole of their surface to the water. Cell
aggregation, the first step towards higher evolution, is therefore
disadvantageous to the pelagic plants, and as the environment is
so homogeneous at the surface of the ocean that there is little
opportunity for an aggregation of cells to gain a compensating
advantage by seizing upon a more favorable habitat, the pelagic
plants have retained their primitive simplicity.

The list of pelagic micro-organisms is a long one, but a few
forms are so predominant that the others have little significance
at the present day in comparison, and we may regard the great
primary food-supply as made up of two simple protozoa, globi-
gerina and the radiolarians, and some five or six unicellular
plants.

Of these only two, the radiolarians and the diatomes, show
any great diversity of species, and while the radiolarians are so
diversified that the Challenger collection alone furnished more
than four thousand species, this variety does not obscure the
primitive simplicity of the type, and the most distinctive pecu-
liarity of the microscopic food-supply of the ocean is the very
small number of the forms which go to make up the enormous
mass of individuals.

SALPA IN RELATION TO EVOLUTION OF LIFE. 139

THE ORIGIN OF PELAGIC ANIMALS.

All the animals of the ocean are dependent upon the micro-
scopic food-supply, and many of them are adapted for preying
upon it directly. Among these Salpa is one of the most conspicu-
ous examples. It passes its whole life in the open water, and it
has no sessile stage in its ontogeny, as many floating animals have.
It abounds in all parts of the ocean, and over some great seas it is
always present at the surface. As the result of three years’ obser-
vation, Schminkewitch says that the salpas are perennial pelagic
animals, and Chun has shown that they are also found in abun-
dance at great depths. :

As long as it is alive and breathing a steady stream of micro-
organisms is slipping along its pharynx and down through its
cesophagus into its stomach, and sections of the intestine of salpa
afford most beautiful preparations of radiolarians and diatomes.

The pelagic food-supply is very ancient, and we have, in salpa,
an animal which has been especially evolved to pass its life swim-
ming through the living broth of the mid-ocean.

If we were to select the typical pelagic animal we should probably
choose salpa, and it is therefore most surprising to find that salpa
itself has not been produced at the surface of the ocean by gradual
evolution from a simple pelagic ancestor.

The structure which fits it so well for its mode of life has come
to it by the inheritance of peculiarities which were originally ac-
quired by bottom animals in adaptation to the needs of a sessile life.

This is all the more remarkable since both salpa and its fixed
allies show by their embryology that still more remotely they are
descended from a pelagic form like appendicularia.

The place in the pelagic world which Salpa fills so well has been
ready for it from primeval times.

Why then has not the simple pelagic appendicularia given rise,
in the open sea, to series of more and more perfected pelagic de-
scendants culminating in salpa ?

Why should the descendants of a pelagic ancestor have passed
through a sessile stage before they acquired their improved pelagic
structure ?

140 W. K. BROOKS.

If this were a solitary case it would not deserve notice ; but ex-
amination will show that no highly organized animal has ever been
evolved at the surface, although all depend on the pelagic food-
supply.

The animals which now find their home in the open waters of
the ocean are, almost without exception, the descendants of forms
which live upon or near the bottom or along the sea-shore or upon
the land, and the exceptions are all simple animals of minute size.
The metazoa which are primitively pelagic, that is, those which
have been pelagic throughout their whole history and do not owe
their structure to competition with improved forms from the bottom
or the shore, are astonishingly few, and these few are among the
smallest and simplest of the metazoa.

It is only necessary to review the chief groups of metazoa in
order to perceive that most of their pelagic representatives exhibit
the clearest evidence of descent from forms which lived upon or
near the bottom or the shore. Many indeed have no pelagic mem-
bers, but are restricted to the bottom.

The sponges are obviously a bottom group; most of them are
fixed, all are sedentary, and their whole organization is an adapta-
tion for life in the bottom.

The coral polyps, actinias and alcyonarias, are among the most
characteristic bottom forms, and the abundance of the fossil remains
of polyp skeletons proves that these animals became established on
the bottom very early, and that the whole history of their evolution
has taken place at the bottom. The acraspedote meduse are uni-
versally and justly regarded as the descendants of fixed polyp-like
ancestors, and we may state with confidence that they are not primi-
tively pelagic, but that a fixed period in their history has come
between the modern swimming: jelly-fish and its remote and un-
known primitive pelagic ancestor.

The veiled medusz are usually held to have hada similar history,
but I shall soon give my reasons for holding that some of these at
least are primitively pelagic. There can, however, be no doubt
that the evolution of hydroid cormi has taken place at the bottom.
The siphonophores are descended from ancestors like the antho-
medusz, and the various families and genera and species of siph-
onophores have most certainly been produced by divergent special-

SALPA IN RELATION TO EVOLUTION OF LIFE. 141

ization among pelagic forms, and the greater part of their history,
if not the whole of it, is therefore pelagic.

The echinoderms are most characteristic inhabitants of the bottom,
as they have been from palzeozoic times, and while synapta is some-
times found at the surface of the open ocean, this is exceptional, and
we may state without hesitation that the evolution of the echinoderms
has taken place at the bottom. This is equally true of the brachi-
opods and of most of the animals classed as vermes, the gephyreans,
bryozoa, nemertians, and so forth. The pelagic annelids, such as
Tomopteris, are secondary modifications of bottom forms, and while
some of the more primitive annelids may possibly be originally
pelagic, the group as a whole is as characteristic of the bottom as
the echinoderms.

Many groups of crustacea have pelagic representatives, and the
pelagic crustacean fauna is rich and varied, but in most cases the
pelagic forms show unmistakable evidence of secondary change of
habit, and all the higher crustacea have been evolved at the bottom
in adaptation to a bottom life.

I shall soon give my reasons for believing that there is one
important exception to this rule, however, and I shall try to show
that there is good ground for holding that the copepods are primi-
tively pelagic, and that while the greater part of the history of the
crustacea is bottom history, the characteristics of the crustacean
type were outlined in pelagic animals at a very early period in the
history of the metazoa.

The heavy calcareous shells of the molluscs could not have been
acquired at the surface, and that most characteristic molluscan organ,
the lingual ribbon, is adapted for attacking more solid bodies than
the delicate primitive pelagic animals. The classes and orders of
molluscs must have been evolved at the bottom, and there is ample
evidence that the swimming shelless gasteropods and cephalopods
have, like those great pelagic groups the pteropods and heteropods,
been secondarily adapted for a pelagic life.

Many of the marine fishes are strictly pelagic, and the structure
and habits of fishes are in all respects so well fitted for a wandering
life in the open water that the pelagic habit of fishes seems at first
sight to be their most distinctive peculiarity, although a little

142 W. K. BROOKS.

examination will show that there is ample evidence that it is see-
ondary, and not primitive.

The perfection of their adjustment to a free life in the open sea
is no evidence that this life is primitive, for the highest marine
animals and those whose adaptation to a pelagic life is most com-,
plete, the sea-birds and cetacea and marine reptiles, are air-breathing
terrestrial animals which have gone back into the ocean.

The most primitive groups of living fishes are the cyclostomes,
elasmobranchs and ganoids. The cyclostomes are too small a group,
and the living forms are too aberrant in habit, to contribute much
information regarding the nature of the primitive vertebrates, but
they exhibit no evidence of adaptation to a pelagic life, and our
scanty knowledge of them is quite in harmony with the view that
their remote ancestors were bottom animals.

The case is very different as regards the great groups of modern
fishes for which the term palzichthyes is often used ; the sharks,
rays and ganoids.

The living representatives of these great and ancient groups are
of peculiar interest to naturalists on account of their close affinity
to the oldest vertebrate fossils which have been discovered. These
points of resemblance to the more modern, but still ancient, amphibia
and teleosts show that the modern paleeichthyes have preserved their
ancient structure with very slight modification, and that we have
in them one of the most remarkable stem forms in the whole animal
kingdom. This is shown still more conclusively by the fact that
some of the paleeozoic families of elasmobranchs have lived through
period after period of geological history and have held their ground
up to our own times.

The abundance and variety of the remains of elasmobranchs in
the palzozoic rocks prove the great development of the group at
this remote and early period, and the silurian sharks probably dif-
fered but little from those of the present day, although we are forced
to see in them the ancestors of the ganoids and of all the divergent
groups of extinct and living vertebrates.

Of the three groups of modern elasmobranchs, two, the chimeeras
and the rays, are bottom-feeders. The whole organization of the
ray is as obviously adapted for life upon or near the bottom as that
of a bird is for life in the air, and the flat pavement teeth are adapted

SALPA IN RELATION TO EVOLUTION OF LIFE. 143

for crushing and grinding the hard-shelled molluscs and crustacea
and echinoderms of the bottom.

It is true that the sawfish is not confined to the bottom, and the
devil-fishes often capture their prey at the surface. In the West
Indies they are often found very far from land, but these cases are
exceptional, and the true rays rarely leave the bottom, nor are they
adapted for rapid movement through the water.

The rays are undoubtedly much more modern than the true
sharks, but there is ample evidence that they have retained habits
of life which are common to all the primitive elasmobranchs.

Many of the modern sharks live on or near the bottom, where
they are found in immense numbers and at considerable depths.
In 1888 I was invited by Marshall McDonald, the Superintendent
of the U. S. Fish Commission, to make use of the opportunity for
surface collecting which was afforded by an expedition which was
sent out to fish with hook and line on the bottom and along the
edge of the Gulf Stream. The fishing commenced at the 500
fathom line, and every time the line was taken in we found
numbers of dogfish (Scyllium) on the hooks, even when the water
was considerably more than half a mile deep.

Many genera of sharks, such as the houndfish (Mustalus) and
the dogfish (Scyllium), are known to feed upon the molluscs and
crustacea and worms of the bottom, and the flat pavement-teeth of
other genera whose habits are less known show that their mode of
life is the same. Some of the bottom-feeding sharks (Cestracion
for example) are the oldest of living vertebrates.

The mailed ganoids were undoubtedly derived from a shark-like
ancestor, and the structure of the oldest ones, such as perichthys,
coccosteus, and cephalaspis, shows that they were not very rapid
swimmers. They were, undoubtedly, bottom-feeders like the
modern sturgeon, and like many large and important families
of modern teleosts, such as the cod, the siluroids and the
pleuronectidee.

So far as we know the paleozoic waters from fossils, there were
no active locomotor animals of large size to furnish prey for
raptorial fishes and the existence at the present day of so many
species and genera and families of bottom-feeders, and the fact
that the most archaic forms have this habit, are all grounds for

144 W. K. BROOKS.

believing that the fishes are secondarily adapted to a pelagic life,
like the sea-birds and the cetacea. .

So far as amphioxus furnishes evidence, this bears in the same
direction, for its home is in the sand of the bottom. In fact it
may almost be called a subterranean animal, for when it is placed
in an aquarium it sinks into the sand at the bottom and disappears
at once, and it makes its way through the sand with great ease
and rapidity.

All the evidence shows that the primitive vertebrates lived upon
or near the bottom, and that the early steps in the evolution of the
classes of vertebrated animals were made at the bottom.

As the result of this review we see that the evidence from paleon-
tology, from embryology, and from the structure and habits of
living animals all bears in the same direction, and shows that there
are no large or highly organized animals which have been pelagic
through all the stages of their evolution, and that, in this particu-
lar, the life-history of Salpa is not exceptional, but typical.

In its descent from an inhabitant of the bottom and in its
secondary adaptation to a pelagic life, its history resembles that of
all the highly organized pelagic animals.

Embryology also gives us good ground for believing that Salpa
follows the analogy of all the metazoa in its still more remote
descent from a small and simple pelagic ancestor, and there is
good ground for believing that the earliest metazoa were all
pelagic, and that they were represented at a very early period in
the history of life by floating or swimming animals of minute size
and simple structure. We may see in the free larval forms of
many marine metazoa, such as the tornaria of balanoglossus, the
swimming echinoderm larva, the ascidian tadpole, the floating
ciliated larvee of annelids, brachiopods and molluses, in the ecelen-
terate planula, and, as I believe, in the crustacean nauplius, traces
of this primitive mode of life; often obscured or complicated
by more recent adaptation and sometimes almost obliterated by
secondary changes.

When this fact is seen in all its bearings and its full signficance
is grasped, it is certainly one of the most noteworthy and instruc-
tive features of the history of evolution.

SALPA IN RELATION TO EVOLUTION OF LIFE. 145

The food-supply of the ocean consists of a few species of uni-
cellular microscopic plants, and of a few simple protozoa which
feed upon them. ‘This supply is inexhaustible, and it is the only
source of food for all the inhabitants of the ocean, except a few
which live upon floating sargassum and the littoral alge, and the
drainage from the land.

Many marine animals are adapted for direct subsistence upon
these organisms, and some of them, like Salpa, are universally
distributed and are found in enormous numbers in all parts of
the ocean.

The food-supply is not only inexhaustible, it is also primeval,
and all the life of the ocean has gradually taken shape in direct
dependence upon it during the history of its evolution.

In view of all these facts we cannot but be profoundly impressed
by the thought that all the highly organized marine animals are
products of the bottom, or of the shore, or of the land, and that
while the largest animals on earth are pelagic, the few which are
primitively pelagic are very small and very simple.

The reason is obvious. The conditions of pelagic life are so easy
that there is no fierce competition, and the inorganic environment
is so simple that there is little chance for diversity of habits.

The growth of terrestrial plants is limited by the scarcity of
food, but there is no such limit to the growth of pelagic plants or
the animals which feed upon them, and while the balance of life
is undoubtedly adjusted, competition for food is never very fierce
even at the present day, when the ocean swarms with highly
organized animals which have become secondarily adapted for a
pelagic life. Even now the destruction or escape of a microscopic
pelagic organism depends upon the accidental proximity or remote-
ness of an enemy rather than upon defense or protection, and sur-
vival is determined by space relations rather than by a struggle
for existence.

The abundance of food is shown by the ease with which
wanderers from the land, like birds, find places for themselves
in the ocean, and the rapidity with which they spread over its
whole extent.

As a marine animal, the insect, halobates, must be very modern
as compared with most pelagic forms, yet it has spread over all

2

146 W. K. BROOKS.

tropical and subtropical seas, and it may always be found skim-
ming over the surface of the water as much at home as a gerris in
a pond. I never found it absent in the Gulf Stream when con-
ditions were favorable for collecting.

The easy character of pelagic life is also shown by the fact that
the larve of innumerable animals from the bottom and the shore
have retained their pelagic habit, and I shall soon refer to facts
which prove that the larva of a shore animal is safer at sea than it
is near the shore. The absence of fierce competition in the open
ocean is well shown by the simultaneous existence in the modern
ocean of graded stages in the evolution of a type, such as the series
of Pelagide ; and also by the persistency of a stem form like the
elasmobranch, side by side with, and often in competition with,
various improved lines of divergent descendants.

In the primitive pelagic fauna and flora there was little oppor-
tunity for an organism to gain superiority by seizing upon an
advantageous site or by acquiring peculiar habits, for one place was
like another, and peculiar habits could count for little in com-
parison with accidental space relations.

After the pelagic fauna had been enriched by the addition of all
the marine animals which are secondarily pelagic, competition with
these improved forms from the bottom brought about improve-
ments in those which were strictly pelagic in their origin, and
through this competition, complicated animals of considerable size,
like the siphonophores, have been evolved at the surface, but while
their whole history has thus been pelagic they are not primitively
pelagic; that is, they are not the outcome of purely pelagic in-
fluences. The wanderers from the bottom have introduced another
factor in the evolution of pelagic life, for their bodies have been
utilized for purposes of protection or concealment or on account of
other advantages, and we now have fishes which shelter themselves
in the poisoned curtain of physalia; crustacea which live in the
pharynx of salpa; barnacles and sucking fishes fastened to whales
and turtles, besides a host of external and internal parasites. The
primitive ocean furnished no such opportunity, and the conditions
of pelagic life must, at first, have been extremely simple.

Among the higher metazoa and the higher plants size is, in
itself, an important factor in evolution. Variations in the con-

SALPA IN RELATION TO EVOLUTION OF LIFE, 147

stituent cells of a large organism are continually being seized upon
and fixed by natural selection, on account of their value in the
functions of relation to other parts. Primitive pelagic organisms
are all minute, and it is easy to understand why. To plants which
are bathed on all sides by food, like the pelagic protophytes, small
size is advantageous, since a small body has a larger surface in
proportion to its bulk than a large one; and the pelagic plants
are, as I have shown, most favorably placed for rapid growth when
new cells separate as soon as they are formed, and thus expose all
their surface.

The same ratio between bulk and nutritive surface tends to limit
in the same way, if not to the same degree, the growth of the pelagic
animals which live in the midst of an abundant supply of vegeta-
ble food.

Competition was not entirely absent among the primitive pelagic
organisms, for the conditions of life are never absolutely uniform,
although the possibilities of evolution must have been extremely
limited and the progress of divergent modification very slow, so
long as life was restricted to the waters of the ocean.

There can be no doubt that pelagic life was abundant for a long
period during which the bottom was uninhabited. The history of
the slow process of geological change by which the earth gradually
assumed its present character, presents a boundless field for specu-
lation, but there can be no doubt that the surface of the primeval
ocean became fit for life long before the deeper waters or the sea-
floor.

The early steps in the evolution of plants must have been taken
in the transparent surface water under the influence of sunlight,
and as both animals and plants are dependent upon oxygen, the
primal flora and fauna must have lived in aerated water. The
oxygen which is diffused through the ocean from the surface,
where it is absorbed from the air, is gradually exhausted by
oxidizable substances, both inorganic and organic, and it dimin-
ishes with the distance from the source of supply at the surface.
The oceanic circulation tends to equalize its distribution, and no
part of the ocean now seems to be totally without oxygen. Oxygen
has been shown to be reduced to a minimum at the bottom of some
of the great depressions of the sea-floor, and it is clear that a slight

148 W. K. BROOKS.

change in the conditions which influence it might render the sea-
bottom unfit for life.

In early paleozoic times the sea-floor was perhaps more level
than it is now, and there may have been no deep hollows like those
in which the oxygen is now found to be deficient, but the average
depth must have been considerably greater, when all the water
which is now locked up in the sedimentary rocks of the bottom and
of the shores was still free in the ocean. The circulation may also
have been less active when geographical conditions were more simple,
and the air was undoubtedly less rich in oxygen in early paleeozoic
times than it is at present.

It is therefore easy to understand that long after the crust of the
earth had acquired essentially its present character, there may have
been a period when the supply of oxygen was so scanty that the
activities of pelagic organisms and the products of their decompo-
sition used it up in the surface water, so that life on the bottom
was impossible at a time when the superficial water supported a
luxuriant fauna and flora.

During this period the proper conditions for the production of
large and complicated organisms did not exist, and while the total
volume of life was probably very great, it consisted of the organisms
of minute size and simple structure which I have termed the primi-
tive pelagic fauna and flora.

THE PRIMITIVE PELAGIC FAUNA.

In using this term I do not, of course, intend to imply that these
organisms are the beginning of life, or to express any opinion as to
the way in which life first came into existence. I use it merely as
a convenient designation for the total sum of the organisms which
have been evolved by purely pelagic influences from a starting-point
which is absolutely unknown at present.

The attempt to reconstruct in imagination the primitive pelagic
fauna and flora is most fascinating, but all the available evidence
is indirect, and as we can have little hope of finding any record of
it in the rocks, we must trust to deduction rather than observation.

The modern pelagic protophytes have probably retained nearly
their ancient form, but the modern radiolarians and pelagic fora-

SALPA IN RELATION TO EVOLUTION OF LIFE. 149

minifera exhibit indications of secondary adaptation, and they have
undoubtedly been modified by competition with improved organisms
from the bottom.

All the metazoa have pelagic larve, or else larval or embryonic
stages, which must be regarded as the degenerated vestiges of a pel-
agic habit ; but in most cases these larvee have been so much changed
by the accelerated development of adult features, or by the acqui-
sition of habits or structures to fit them for the conditions of modern
pelagic life, that we can deduce little more from them than the
former existence of pelagic ancestors. When a pelagic larva is still
represented by a modern pelagic adult of minute size and simple
structure, as the tadpole larva of ascidians is represented by A ppen-
dicularia, we may be confident that it is a pelagic production, and
that it existed in the primitive pelagic fauna, although this view is
directly opposed to accepted dogmas regarding the origin of the
Chordata. When all the members of a great group have a definite
pelagic larval stage which adheres to the same plan of structure in
all of them, we may be pretty confident that this larva is the repre-
sentative of a primitive pelagic adult animal, even if this ancestor
has now no unmodified descendants.

To my mind the best example of the retention, by all the mem-
bers of a great group, of a larval stage which represents an extinct
primitively pelagic ancestor is to be found in the crustacean nauplius,
and notwithstanding the popular verdict against it, I do not hesitate
to regard the nauplius as a pure pelagic product, and to include it
in the primitive pelagic fauna, although I shall discuss this question
further on. In cases like that of the echinoderms, where the pel-
agic larvee of the various classes and orders are very different from
each other in the details of their organization, we are hardly safe
in assuming more than the primitive existence of an unknown pel-
agic organism, from which they have been derived. This is true
to even a greater degree of the trochic larve of annelids, molluscs,
ete., but while there is little ground for regarding the forms of these
modern larvee as ancestral, we must regard their pelagic habit as an
inheritance from unknown ancestors in the primitive pelagic fauna,
in which we must therefore include representatives of such larve
as the molluscan veliger, the nemertain pilidium, the actinotrocha
of phoronis, the brachiopod larva, the ccelenterate planula, and so

150 W. K. BROOKS.

forth, although we are quite unable to say how many independent
starting-points these various metozoic lines had in the primitive
pelagic fauna, or what these starting-points were like. Our inability
to describe or picture these ancestral forms is no reason for doubting
their reality, for in biology the weight and certainty of a deduction
are often independent of its definiteness. We may, for example,
feel sure that the cetacea are descended from terrestrial animals and
yet find it impossible to picture their ancestor, or even to decide
whether their ancestral lines converge into one stem before or after
the pelagic habit was acquired.

We may in the same way feel sure, even in the absence of suffi-
cient evidence to trace their direct paths, that all the great groups
of metazoa ran back to minute pelagic ancestors, and we must,
therefore, include in the primitive pelagic fauna a great, but
indefinite, number of distinct and somewhat widely separated
ancestral forms, and together with them, no doubt, an equal or
greater number of somewhat similar forms which have been exter-
minated and have left no descendants. In these extinct forms we
should, if we could study them, find the connecting links between
divergent. groups, and we*would thus be able to complete the
genealogical tree of the metazoa by bringing together the great
divergent branches of the metazoic stem whose primary relation-
ships now seem beyond discovery.

In addition to the primitive pelagic animals which are known
to us only by the traces of their former existence which they have
left in the structure and habits of modern larvee and embryos,
there are a few modern pelagic adult animals which show by their
minute size and simple structure and by their systematic affinities
that they are primitive pelagic animals, owing their structure to
purely pelagic influences.

Appendicularia is a good example of this class, and I believe that
the copepods are the most important group of the primitively pelagic
metazoa.

THE ORIGIN OF THE CRUSTACEA.
The view that the copepods are degenerated descendants from

crustacea like the phyllopods, and that the crustacea were evolved
on the bottom, and that the pelagic habit of the copepods is

SALPA IN RELATION TO EVOLUTION OF LIFE. 151

secondary, is so generally accepted that it is hardly worth while
to advance a different view in this place where there is no room
for its exhaustive treatment.

The consideration which seems to have the greatest weight with
morphologists is the supposed necessity of a phylogenetic explana-
tion of metamerism, but a little reflection will show the persistent
existence of an influence which tends to metamerism at the
present day.

For this influence, which is shown by such phenomena as the
inheritance by the child of polydactylous feet from the polydacty-
lous hands of the parent, or the development of wing-feathers on
the toes of fantail pigeons; the influence which has carried the
feet of the horse family along the same line of evolution with the
hands, I have, in another place, proposed the term ontogenetic
inheritance. Among the arthropods, examples of this sort of
modern metamerization are very common, both as normal features
of their structure, in the movable body-rings of the ocular and
antennary somites of stomatopods, for example, and as monstrosi-
ties, as in the twelve-legged coleoptera.

I believe that a thorough study of this most interesting and
instructive class of facts will convince any one that there is no
philosophical necessity for assuming that the primitive crustacean
had a highly metamerized body like that of a phyllopod, and that
all the common features in the structure of arthropods may have
been derived from a common ancestor as simple as a nauplius.

The analogy between the parapodia of annelids and the limbs
of crustacea has been held to prove that the primitive crustacean
limb was not a rowing organ fitted for a pelagic life, like the limbs
of the nauplius and the copepod, but flat and leaf-like and adapted
for movement over the bottom.

It is hardly possible, however, to believe that the arthropods
have been derived from the higher polychzetous annelids, and as
the simpler and more primitive annelids have no parapodia, the
resemblance, which is not actually very noteworthy, can be noth-
ing more than an analogy.

There are plenty of degenerated copepods, and we have in their
structure abundant proof of the degeneracy, and an adequate
explanation of it in their parasitic habits, but they are degenerated

152 W. K. BROOKS.

descendants of ordinary swimming copepods, and not of phyllo-
pods, and there is no reason for holding that the copepodan type
itself is degenerate, except the supposed exigencies of morpho-
logical philosophy.

The active locomotor habits of the eucopepods of the open
ocean would seem to be conducive to advancement rather than to
degeneration, and the occurrence of phyllopods in the lower Cam-
brian is, of course, no more evidence that they are primitive
crustacea than the occurrence of pteropods and gasteropods is that
they are primitive molluscs.

IT am unable to see any valid objection to the view that the
copepods are primitively pelagic; that they have been evolved at
the surface of the ocean from pelagic nauplii, and that the great
group crustacea has been derived from them.

We have already seen that the eucopepods are the chief inter-
mediary between the micro-organisms of the ocean and the larger
and higher marine animals; that they prey upon the protophytes
and protozoa, and in their turn supply either directly or indirectly
most of the food for the large inhabitants of the water; that most
pelagic larvee feed upon theth; that they are the food of the great
pelagic banks of pteropods and heteropods, of many ccelenterates,
of the young of most fishes, and of some of the most abundant
and important adult fishes, like the herring, and that the sea-birds,
the cetacea, and in fact almost all of the larger pelagic animals,
prey upon animals which in their turn prey upon copepods.

The animals which are most important at one period in the
earth’s history are often replaced by others at another period, and
it is, of course, possible that the modern copepods now fill a place
which was in former times filled by something else; but as their
organization, as compared with that of the other crustacea, exhibits
all the characteristics of a primitive pelagic stem-form, and inas-
much as the remains of animals, like the pteropods, which now
live almost entirely upon copepods, are found in the oldest fossili-
ferous rocks, there is every reason to believe that the group formed
an important constituent of the primitive pelagic fauna.

No one who advocates at one time the morphological heresies
which are involved in the view that appendicularia is a stem-form
which is pelagic in its origin; that the nauplius is a persistent

SALPA IN RELATION TO EVOLUTION OF LIFE. 153

representative of the primitive crustacea, and that the whole
history of the copepods has been pelagic; and that the veiled
medusz have been evolved in direct relation to pelagic influences ;
no one who makes these statements can hope to escape the charge that
his view “ist die unwahrscheinlichste von Unwahrscheinlichkeiten.”

The books all tell us that the free active appendicularia is the
“degenerated” descendant of an ancestor which crept over the
bottom; that the nauplius is a secondary larval form; that the
active free-swimming copepods are degenerated phyllopods ; and
that the locomotor hydro-medusa is, in its origin, a specialized
member of a sessile, polymorphic, hydroid cormus.

The first of these opinions, that appendicularia is a degenerated
form, rests upon a supposed necessity for deriving the body of a .
vertebrate, which consists of a series of segments homologous with
each other, but highly differentiated among themselves, from an
ancestral aggregation of similar, but less differentiated, segments.
The second opinion, that the copepods are degenerated and that
the nauplius is a secondary larval form, is the result of a supposed
necessity for explaining the segmentation of the arthropods in the
same way, while the third view has its origin in the belief that
the polymorphic members of a hydroid cormus must have arisen
through specialization and division of labor from an ancestral
undifferentiated aggregation.

These are a few, from among many, illustrations of the general
acceptance among morphologists of a dogma which, while it is
often refined and qualified until its character is almost lost, may
be broadly stated as a belief that the homology between different
parts of the same organism is always to be explained, like the
homology between corresponding parts of different animals, as the
result of phylogenetic inheritance ; or, to state it ina different way,
that the vegetative duplication of parts in animals has a phylogenetic
significance, and implies descent from a duplicated ancestor.

The dogma is not the dictum of any one teacher, and it has
grown almost imperceptibly from its starting-point in the dis-
covery that the body of a metazoon is an aggregation of cells, each
with an individuality of its own, specialized and differentiated by
polymorphism and division of labor, and each one homologous
with an unicellular organism.

154 W. K. BROOKS.

The dogma has been a most useful and suggestive working
hypothesis when well controlled, but when uncontrolled it has led
to the most fantastic and grotesque unscientific speculation. The
climax of inconsistency into which its blind adherents have been
led was well shown by the simultaneous appearance, in a recent
morphological journal, of two memoirs, one an essay on “ The
Origin of the Vertebrates from the Arachnids,” and the other on
“The Origin of Vertebrates from a Crustacean-like Ancestor.”

After my first examination of the second of these memoirs I
laid it down, much distressed in mind by the thought that this
author had unkindly descended from the sphere of experimental
research in physiology, to expose the unscientific methods of the
morphologists by a severe and well merited, if somewhat ponder-
ous, satire.

In my next chapter on the morphological significance of appendi-
cularia I shall try to show that there is no philosophical necessity
for a phylogenetic explanation of duplicated structures in animals,
whether they are radical, bilateral, metameric or indefinite, and I
must refer the reader to that chapter for my reasons for including
appendicularia, the copepods’ and the veiled medusee among the
primitive pelagic animals.

THE PHYLOGENY OF THE METAZOA.

The primitive pelagic fauna, before the influence of the bottom
and of the shore had been brought to bear upon it, consisted of
small animals of simple structure ; but we are forced, by the facts
of comparative anatomy and embryology, to believe that a number
of distinct types of structure were found among them.

Most of the great metazoic stems show by their embryology that
they run back to simple and minute pelagic ancestors, and that their
common meeting-point must be projected back to a still more re-
mote time, before the differentiation of their pelagic ancestors had
been effected. After we have traced each great line of metazoa as
far back as we can from the study of fossils and by the aid of
comparative morphology, we still find these lines distinctly laid
down. The lower cambrian crustacea, for example, are as distinct
from the lower cambrian echinoderms or pteropods or brachiopods

SALPA IN RELATION TO EVOLUTION OF LIFE. 155

or lamellibranchs, as they are from those of the present day. The
efforts of anatomists and embryologists to reconstruct the primary
phylogeny of the metazoa have so far yielded few trustworthy
results, and the results which are most trustworthy are usually
those which are the most indefinite.

We are therefore forced to believe that the early steps in the
establishment of the various types of metazoa were taken under
conditions which had some essential difference from those which
have prevailed, without any fundamental changes, from the time
of the oldest fossil to the present day ; and we are also forced to
believe that most of the great lines of descent were represented at
some time in the remote past by ancestors which, living a different
sort of life, differed essentially in structure as well as habits, from
the representatives of the same types which are known to us. Fur-
thermore, embryology teaches that each great group still bears in-
ternal evidence of descent from pelagic ancestors, and while the
characteristics of these ancestors are in most cases unknown, a few,
like appendicularia, are still found alive.

Our knowledge of the evolution of the metazoic types has certain
general features which are essentially the same for all, but each
group has also in its history much that is individual, and any
general statement requires so much qualification that the history
of an illustrative group is more instructive than a general summary.

In the echinoderms we have a well-defined type represented by
abundant fossils, very rich in living forms, very diversified in its
modifications, and therefore well fitted for use as an illustration.

This great stem contains many classes and orders, all constructed
on the same plan, which is sharply isolated and quite unlike the
plan of structure in any other group of animals. All through the
series of fossiliferous rocks echinoderms are found, and the plan of
structure is always the same. Paleontology gives us most valua-
ble evidence regarding the course of evolution within the limits of
a class as in the crinoids and in the echinoids ; but we appeal to it
in vain for light upon the organization of the primitive echinoderm,
or for connecting links between the classes. ‘To our questions on
these subjects and on the relation of the echinoderms to other ani-
mals, paleontology is silent, and throws them back upon us as
unsolved riddles.

156 W. K. BROOKS.

The morphologist unhesitatingly projects his imagination, held
in check only by the laws of scientific thought, into the dark period
before the times of the oldest fossils, and feels absolutely certain
of the past existence of a stem-form, from which the classes of
echinoderms have inherited the fundamental plan of their structure,
and he affirms with equal confidence that the structural changes
which have separated this ancient type from the classes which we
know were very much more profound and extensive than all the
changes which each class has undergone from the earliest paleeozoic
times to the present day.

He is also disposed to assume, but, as I shall show, with much
less reason, that the amount of change which structure has under-
gone is an index to the length of time which the change has re-
quired, and that the period which is covered by the fossiliferous
rocks is only an inconsiderable part of that which has been con-
sumed in the evolution of the echinoderms.

The morphologist does not check the flight of his scientific
imagination here, however, for he trusts implicitly to the embryo-
logical evidence which teaches him that, still further back in the
past, all the echinoderms Were represented by a minute pelagic
animal which was not an echinoderm at all in any sense except
the ancestral one, although it was distinguished by features which
natural selection has converted, under the influence of more modern
conditions, into the structure of echinoderms. He finds, in the
embryology of modern echinoderms, phenomena which can bear
no interpretation but this, and he unhesitatingly assumes that they
are an inheritance which has been handed down from generation to
generation through all the ages from the prehistoric times of zodlogy.

Other groups tell the same story with equal clearness. Who
can look at a living lingula without being overwhelmed by the
effort to grasp its immeasurable antiquity, and by the thought that,
while it has passed through all the chances and changes of geological
history, the structure which fitted it for life on the earliest paleeozoic
bottom is still adapted for a life in the sands of the modern sea-floor?

The everlasting hills are the type of venerable antiquity ; but
lingula has seen the continents grow up, and has maintained its
integrity unmoved by the convulsions of nature which have given
to the crust of the earth its present form.

SALPA IN RELATION TO EVOLUTION OF LIFE. 157

As measured by the time-standards of the morphologist, lingula
itself is modern, for its life-history still holds, locked up within it,
the record of a structure and of a habit of life which were lost in
the unknown past at the time of the lower cambrian, and it tells
us, vaguely but unmistakably, of a life at the surface of the primi-
tive ocean at a time when the brachiopod stem was represented by
minute and simple pelagic animals.

Broadly stated, the history of each great line of metazoa has been
like that of the echinoderms or brachiopods, for while the brachio-
pods are certainly much more closely related to the polyzoa or the
gephyreans than to the echinoderms, and while these latter are
nearer to the chordata than to the brachiopods, yet each great line
stands sharply by itself.

The oldest pteropod or lamellibranch or crustacean or echinoderm
or vertebrate which we know from fossils exhibits its own type of
structure with perfect distinctness, and later influences have done
no more than to expand and diversify the type, while anatomy fails
to guide us back to the point where these various lines met each
other in a common source, although it forces us to believe that this
common source once had an individual existence.

Embryology teaches that each line once had its own pelagic
representatives, and that the early stages in the evolution of each
type have passed away and left no record.

The palzontological side of the subject has recently been ably
summed up by Walcott in an interesting memoir on the oldest
fauna which is known to us from fossils (The Fauna of the Lower
Cambrian or Olenellus Zone, by Charles Doolittle Walcott, U.S.
Geological Survey, 10th Annual Report, Washington, 1890).

The fossils of the lower cambrian are not absolutely the oldest
known, but it is the oldest fauna which is represented with sufficient
completeness for a general view, and is, therefore, interesting to
biologists.

Walcott says that no plants are known in the rocks of the lower
cambrian, and that he has satisfied himself, after a study of all the
reputed species of alge, that they are not plants, but the trails of
worms or molluscs.

The number of species is small, but their diversity is most note-
worthy and remarkable.

158 W. K. BROOKS.

Walcott’s collection of 141 American species from the lower cam-
brian is distributed over most of the marine groups of the animal
kingdom, and, except for the absence of the remains of vertebrates,
the whole province of animal life is almost as completely covered
by these 141 species as it could be by a collection from the bottom
of the modern ocean.

Four of the American species are sponges, two are hydrozoa,
nine are actinozoa, one an echinoderm, twenty-nine are brachiopods,
three are lamellibranchs, thirteen are gasteropods, fifteen are ptero-
pods, eight are crustacea, fifty-one are trilobites, and the trails and
burrows show the existence of at least six species of bottom forms,
probably worms or crustacea.

The most noteworthy characteristic is the completeness with which
these new species outline the whole fauna of the modern sea-floor.

Nothing brings home more vividly to the zoologist a picture of
the diversity of the lower cambrian fauna and of its intimate re-
lation to the bottom fauna of to-day than the thought that he would
have found, on the old cambrian shore, about the same opportunity
to study the embryology and anatomy of pteropods, gasteropods
and lamellibranchs and crustacea and medusze that he now has at
a marine laboratory, and that his studies in phylogeny would have
had about the same form then that they have now.

Biological evidence based on embryology and anatomy and on
the habits and affinities of animals is justly regarded, by zoologists
at least, as a more perfect record of the early history of life than
paleontology, and we accept, without question, proofs of phylogeny
which refer to a time very much more remote than the age of the
oldest fossils.

We must not forget, however, that our generalizations in primi-
tive phylogeny rest for the most part on the study of swimming
or floating larvee of minute size and simple structure, which we
can have little hope of finding as fossils.

In the formations which follow the lower cambrian, species
gradually become more numerous, but this is due to divergent
specialization, and Walcott says that if a comparison be made
between the olenellus zone (lower cambrian) and the silurian
fauna, the superiority of the latter in number of species, genera
and families is at once apparent.

SALPA IN RELATION TO EVOLUTION OF LIFE. 159

“Tf the comparison be extended to class characters, the disparity
between the two is very much reduced, and it is made evident that
the evolution of life between the epoch of the Olenellus fauna and
the epoch of the Ordvician fauna has been, with one or two excep-
tions, in the direction of differentiating the class types that existed
in the earlier fauna.”

The ground which we have covered in our review of these vari-
ous broad aspects of the animal kingdom brings us, then, to the
following point of view :

There are no highly organized animals which have been pelagic
through all the stages of their evolution. The metazoa, which
have been pelagic through their whole history, are either small
and simply organized, as compared with the higher representa-
tives of the group to which they belong, like appendicularia, or
else, like the siphonophores, they have been perfected through
competition with higher types.

Marine life is older than terrestrial life, and as all marine life
has shaped itself in relation to the pelagic food-supply, this itself
is the only form of life which is independent, and it must therefore
be the oldest. There must have been a long period in primeval
times during which there was a pelagic flora and fauna, rich
beyond limit in individuals, but made up of only a few small
simple types. During this time the pelagic ancestors of all the
great groups of metazoa were slowly evolved, as well as others
which have no living descendants. So long as life was restricted
to the surface, no great or rapid advancement through the influ-
ences which now modify species was possible, and we know of no
other influence which might have replaced these. We are, there-
fore, forced to believe that the differentiation and improvement of
the primitive flora and fauna was slow, and that for a vast period
of time life consisted of an innumerable multitude of pelagic organ-
isms made up of a few forms. During the time which it took to
form the thick beds of older sedimentary rocks the physical condi-
tions of the ocean gradually took their present form, and during a
part, at least, of this period, the total amount of life in the ocean
may have been about as great as it is now without leaving any
permanent record of its existence, for no rapid advancement took
place until the advantages of a life on the bottom were discovered.

160 W. K. BROOKS.

THe DiscovERY OF THE BoTToM, AND ITS
EFFrect oN Evouurtion.

We must not think of the populating of the bottom as a physi-
cal problem, but as colonization, very much like the colonization
of oceanic islands. Physical conditions for a long time made it
impossible, but its initiation was the result of biological influences,
and there is no reason why the starting-point should be the point
where the physical obstacles were first removed. It is useless to
speculate upon the character of the physical obstacles; there is
reason to believe that one of them, probably a very important one,
was the deficiency of oxygen in deep water.

Whatever their character may have been they were all, no doubt,
of such a nature that they first disappeared in the most shallow
water around the coast, but it is not probable that bottom life was
first established in shallow water, or before the physical conditions
had become favorable at considerable depths.

The sediment near the shore is destructive to most pelagic ani-
mals, and recent explorations have shown that a stratum of water
of very great thickness is necessary for the complete development
of the pelagic flora and fauna. It is a mistake to picture pelagic
life as confined to a thin surface stratum. Pelagic plants probably
flourish as far down as the light penetrates, and pelagic animals
are abundant at very great depths. As the earliest bottom animals
must have depended directly upon the floating organisms for food,
it is not probable that they first established themselves in shallow
water, where the food-supply is not only scanty in amount but
also mixed with sediment; nor is it probably that their .estab-
lishment on the bottom was delayed until the great depths had
become favorable to life.

The belts around elevated areas which are far enough from shore
to be free from sediment and to have above them a sufficient depth
of water to permit the pelagic fauna to reach its full development,
are the most favorable spots, and I shall soon show that there is
palzeontological evidence which indicates that they were seized
upon very early in the history of bottom life. It is very probable
that colony after colony was established on the bottom, and after-

SALPA IN RELATION TO EVOLUTION OF LIFE. 161

wards swept away, like clouds before the wind, by geological changes,
and that the bottom fauna which we know was not the first.

Colonies which started in shallow water were exposed to accidents
from which those in great depths were free, and in view of our
present knowledge of the permanency of the sea-floor and of the
broad outlines of the continents, it is not impossible that the first
fauna which settled in the deep zone around the continents may
have persisted and given rise to our modern life. However this
may be, we must regard this deep zone as the birthplace of the
fauna which has survived; as the ancestral home of all the im-
proved metazoa.

The effect of life upon the bottom is more interesting than the
place where it began, and we have now to consider its influence in
the evolution of animals.

The effect of the secondary acquisition of a sedentary life by
modern animals has been fully discussed by many writers, but no
one, so far as I am aware, has ever considered the effect of the first
settlement of the bottom by pelagic animals, all whose competitors
and enemies had previously been pelagic.

It is doubtful whether the animals which first settled on the
bottom secured any more food than the floating ones, but they un-
doubtedly obtained it with less effort, and were able to devote their
superfluous energy to growth and to multiplication, and thus to
become larger and to increase in numbers faster than pelagic animals.

Their sedentary life must have been favorable to both sexual and
asexual multiplication, and the tendency to multiply by budding
must have been quickly rendered more active. It is sometimes
stated that the capacity for budding has been acquired among the
metazoa as the result of a sedentary life, but this view hardly seems
to be the true one. Capacity for asexual multiplication is very old,
older in all probability than sexual reproduction, and there is no
reason to believe that it has ever been lost even by the highest
animals, for it must be regarded as nothing more, in ultimate
analysis, than discontinuous growth. The tissues of all animals
have vegetative power, and external influences determine whether
this shall result in continuous or discontinuous growth, and a trace
of the power to multiply asexually is retained even among the em-
bryos of mammals. It is therefore wrong to speak of the acquisition

3

162 W. K. BROOKS.

of a capacity for budding, and it is not at all improbable that
the primitive pelagic metazoa multiplied by buds; although the
tendency to form connected cormi, and to retain the connection
between the parent and the bud until the latter was able to obtain
its own food and to care for itself, was a result, and probably one
of the first results, of life on the bottom.

The animals which first acquired the habit of resting upon the
bottom therefore soon began to multiply faster, both sexually and
asexually, than their swimming allies ; and their asexual progeny
remaining for a longer time attached to and nourished by the parent
stock, were much more favorably placed for rapid growth. As
bottom animals live on a surface, or at least a thin stratum, while
swimming animals are distributed through solid space, the rapid
multiplication of bottom animals must soon have led to crowding
and to competition, and it soon became harder and harder for new
forms from the open water to force themselves in among the old ones,
and colonization soon came to an end.

The great antiquity of all the types of structure which are repre-
sented among the modern metazoa is therefore what we should
expect, for after the foundation for the fauna of the bottom was
laid it became, and ever afterwards remained, difficult for new
forms to establish themselves.

Our knowledge of the sea-bottom is for the most part from three
sources: from dredgings and other methods of exploration ; from
rocks which were originally laid down beyond the immediate in-
fluence of the continents, and from the patches of the bottom fauna
which have been gradually brought near its surface by the growth
of coral reefs; and from all these sources we find testimony to the
density of the crowd of animals on favorable spots.

Deep-sea exploration can give only the most scanty and frag-
mentary basis for a picture of the sea-bottom, but it shows that its
animal life may thrive with the dense luxuriance of tropical vege-
tation, and Sir William Thomson says that he once brought up at
one time on a “ tangle,” which was fastened to a dredge, over 20,000
specimens of a single species of sea-urchin.

While cruising on the U. S. Fish Commission schooner Grampus,
I was interested to find that when a ground-line with baited fish-
hooks had been sunk to the bottom in nearly a mile of water, several

‘SALPA IN RELATION TO EVOLUTION OF LIFE. 1638

of the hooks dropped into the mouths of large sea anemones, so
that they were brought up uninjured, and were carried more than
three hundred miles to the laboratory, where they lived for some
time in an aquarium.

The number of remains of paleozoic crinoids and brachiopods
and trilobites which are crowded into a slab of fine-grained lime-
stone is most astounding, and it testifies most vividly and forcibly
to the wealth of life on the old sea-floor.

No description can convey an adequate conception of the bound-
less luxuriance of a coral island, but nothing else affords such a
vivid picture of the capacity of the sea-floor for supporting life.

The marine plants are not abundant on coral islands, and the
animals depend either directly or indirectly upon the pelagic food-
supply, so that in this respect their life is like that of animals in
the deep sea far from land.

The abundant life is not restricted to the growing edge of the
reef, and the inner lagoons are often like crowded aquaria. At
Nassau, my party of eight persons found so much to study in a
little reef in a lagoon, close to our laboratory, that for four months
and more we found new things every day, and our explorations
seldom carried us beyond this little tract of bottom. Every inch
of the surface was carpeted with living animals, while others were
darting about among the corals and gorgonias in all directions ;
but this was not all, for the solid coral was honeycombed every-
where by tubes and burrows; and, when broken to pieces with a
hammer, each mass of coral gave us specimens of nearly every
great group in the animal kingdom. Fishes, crustacea, annelids,
molluses, echinoderms, hydroids and sponges could be picked out
of every fragment, and the abundance of life inside the solid rock
was most wonderful.

The absence of pelagic life in the landlocked waters of coral islands
is as impressive and noteworthy as the luxuriance of life upon and
near the bottom.

On my first visit to the Bahama Islands I was sadly disappointed
by the absence of pelagic animals where all the conditions seemed
to be peculiarly favorable.

The deep ocean is so near that, as one cruises through the inner
sounds past the openings between the islets which form the outer

164 W. K. BROOKS.

barrier, the deep-blue water of mid-ocean is seen to meet the white
sand of the beach, and soundings show that the outer edge is a
precipice as high as the side of Chimborazo and much steeper.
Nowhere else in the world is the pure water of the deep sea found
nearer land or more free from sediment, and on the days when the
weather was favorable for towing outside, we found siphonophores
and pteropods and pelagic molluscs, crustacea, salpze, and all sorts
of pelagic larve in great abundance in the open sea just outside
the inlets.

Inside the barrier the water was always calm, and day after day
it was as smooth as the surface of an inland lake. When I first
entered one of these beautiful sounds where the calm, transparent
water stretches as far as the eye can reach, and new beauties of islets
and winding channels open before one, as those which are passed
fade away on the horizon, I felt sure that I had at last found a
place where the pelagic fauna of mid-ocean could be taken home
alive and studied on shore.

The water proved to be not only as pure as air, but also as
empty. At high water we sometimes captured a few pelagic
animals near the inlets, bat we dragged our surface-nets through
the sounds day after day only to find them as clean as if they had
been hung out in the wind todry. The water in which we washed
them usually remained as pure and empty as if it had been filtered,
and we often returned from our towing expeditions in the sounds
without even a copepod or a zoea or a pluteus.

The absence of floating larve is most remarkable, for the sounds
swarm with bottom animals which give birth every day to millions
of swimming larvee. The mangrove swamps and the rocky shores
are fairly alive with crabs carrying eggs at all stages of develop-
ment, and the boat passes over great black patches of sea-urchins
crowded together by thousands, and the number of animals which
are engaged in laying their eggs or in hatching their young is
infinite, yet we rarely captured any larve in the tow-net, and most
of those which we did find were old and nearly through their
Jarval life.

It is often said that the water of the coral sounds is too full of
lime to be inhabited by the animals of the open ocean, but this is
a mistake, for the water is perfectly fitted for supporting the most

SALPA IN RELATION TO EVOLUTION OF LIFE. 165

delicate and sensitive animals, and we had no difficulty in keeping
alive, in water taken from the sounds, the surface animals which
we caught outside. Even trachomeduse and doliolums, which are
extremely sensitive to impurities in the water, could be kept alive
in the house very much better than in any other place where I
have ever tried to keep them, and instead of being injurious, the
pure water of the sounds is peculiarly favorable for use in aquaria
for surface animals.

The scarcity of floating organisms can have only one explana-
tion. They are eaten up, and competition for food is so fierce that
nearly every organism which is swept in by the tide, and nearly
every larva which is born in the sounds, is snatched by the tenta-
cles around some hungry mouth.

Nothing could illustrate the fierceness of the struggle for food
among the animals on a crowded sea-bottom more vividly than
the emptiness of the water in coral sounds. The only larve
which have much chance of establishing themselves for life are
those which are so fortunate as to be swept out into the open
ocean, where they can complete their larval life under the milder
competition of the pelagic fauna, and while it is usually stated
that the pelagic habit has been retained by the larve of bottom ani-
mals for the purpose of distributing the species, it is more probable
that it has been retained on account of its comparative safety.

There can be no doubt, in view of these facts, that competition
came swiftly after the establishment of the first bottom fauna, and
that it soon became very rigorous and led to rapid evolution ; and
we must also remember that life on the bottom introduced many
new opportunities for divergent modification and for the perfect-
ing of animals.

The increase in size, which came with the economy of energy,
increased the possibilities of variation, and led to the natural selec-
tion of those peculiarities which improved the efficiency of various
parts of the body in their functions of relations to each other, and
this has certainly been an important factor in the evolution ot
complicated organisms.

The new mode of life also permitted the acquisition of protec-
tive shells, hard supporting skeletons, and other imperishable
structures, and it is therefore probable that the history of evolu-

166 W. K. BROOKS.

tion in later times gives us no index as to the time which was
required to evolve, from pelagic ancestors, the oldest animals
which were likely to be preserved as fossils.

Life on the bottom also introduced another most important
influence in evolution—competition between blood relations. In
the animals which we know most intimately, divergent modifica-
tion, with the extinction of connecting forms, results from the fact
that the fiercest competitors of each animal are its closest allies,
which, having the same habits, living upon the same food, and
avoiding enemies in the same way, are constantly striving to hold
exclusive possession of all the essentials to their life. When a
stock gives rise to two divergent branches, each of them escapes
competition with the other, so far as they differ in structure and
habits, while the parent stock, competing with both at a disadvan-
tage, is exterminated.

Among the animals which we know best, evolution leads to a
branching tree-like phylogeny with the topmost twigs represented
by living animals, while the rest of the tree is buried in the dead
past. The connecting form between two species must, therefore,
be constructed in imagination or sought in the records of the past.

Even at the present day things are somewhat different in the
open ocean, and they must have been very different in the primi-
tive ocean, for a pelagic animal has no fixed home, one locality is
like another, and the competitors and enemies of each individual
are determined, in great part, by accidents. We accordingly
find, even now, that the evolution of pelagic animals is ‘often
linear instead of divergent, and the early steps in the series often
live on side by side with the later and more evolved forms. The
radiolarians and the meduse and the siphonophores furnish many
well-known illustrations of this feature of pelagic life.

No one is much surprised to find in the South Pacific or in the
Indian Ocean a salpa, or a pelagic crustacean, or a surface fish
which has previously been known only in the North Atlantic, and
the list of species of marine animals which are found in all seas is
a very long one. The fact that pelagic animals are so independent
of those laws of geographical distribution which limit land animals
is additional evidence of the easy character of the conditions of
pelagic life. We have seen that one of the first results of life

SALPA IN RELATION TO EVOLUTION OF LIFE. 167

upon the bottom was to increase asexual multiplication and to
lengthen the time during which buds remained united to and
nourished by their parents. One result of this is the crowding
together of individuals of the same species, and competition
between relations. We have in this and in other obvious
peculiarities of life on the bottom a sufficient explanation of the
fact that, since the first establishment of the bottom fauna, evolu-
tion has resulted in the elaboration and divergent specialization of
the types of structure which were already established, rather than
in the production of new types.

Another result of the struggle for existence on the bottom was
the escape of varieties from competition with their allies by flight
from the crowded spots and a return to the open water above ;
just as in later times the cetacea and sea-birds have gone back
from the land to the ocean. These emigrants, like the civilized
men who invade the homes of peaceful islanders, brought with
them the improvements which had come from fierce competition,
and they carried everything before them and produced a great and
rapid change in the character of the pelagic fauna.

The rapid intellectual improvement which has taken place among
the mammalia since the middle tertiaries, and the rapid structural
development which took place in animals and plants when the
land fauna and flora were first established, are well known; but
the fact that the discovery of the bottom initiated a much earlier,
and probably much more important era of rapid development in
the forms of animal life has never received the attention which it
so well merits.

If the views which I have advanced are correct, the primitive
bottom fauna must have had the following characteristics :

1. It was entirely animal, without plants, and it at first depended
directly upon the pelagic food-supply.

2. It was established around elevated areas in water deep enough —
to be beyond the influence of the shore.

3. The great groups of metazoa were rapidly established from
pelagic ancestors.

4, There was a rapid increase in the size of the bottom animals
and hard parts were quickly acquired.

168 W. K. BROOKS.

5. The bottom fauna soon produced progressive development
among pelagic animals.

6. After the establishment of the bottom fauna, elaboration and
differentiation among the representatives of each primitive type
soon set in and led to the extinction of the connecting forms.

There is no reason to suppose that the first animals which were
adapted for preservation as fossils have been discovered, and many
of the oldest fossils, like the pteropods, are most certainly the
modified descendants of simpler ancestors with hard parts, but it
is interesting to note that the oldest fossil fauna which is known to
us is an unmistakable approximation to the primitive bottom fauna
as I have outlined it.

Walcott has given the following sketch of the broad general
characteristics of the lower cambrian fauna :

The lower cambrian fossils are distributed through strata which
in Washington and Rensselaer counties in New York, are nearly
two miles thick, and some of them, at least, were deposited in
water of considerable depth. This is shown by the fineness of the
sediment and by the perfect preservation of tracks and burrows in
soft mud and of soft animals Jike jelly-fishes. These show that the
sediment was laid down slowly and gently, in water so deep as to
be free from disturbance, and under conditions so favorable that
it contains the remains of some animals which are not found again
until we reach a very much more modern period. The fossil
medusee of the lower cambrian are so perfect that their identity is
unquestionable, yet it is not until the Solenhofen lithographic slate
of the Jura is reached in ascending the geological scale, that
medusz are again met with; and corals and lamellibranchs are
found in the lower cambrian, although as they are not found again
until the silurian rocks are reached, we have no record of their
existence through the long period covered by the middle and upper
cambrian.

The fauna of the lower cambrian, while it undoubtedly lived in
water of very considerable depth, was not oceanic but continental,
and Walcott says that “one of the most important conclusions is,
that the fauna of the lower Cambrian lived on the eastern and
western shores of a continent that in its general configuration out-
lines the American continent of to-day. Strictly speaking, the

SALPA IN RELATION TO EVOLUTION OF LIFE. 169

fauna did not live upon the outer shore, facing the ocean, but on
the shores of interior seas, straits, or lagoons that occupied the
intervals between the several ridges that ran from the central plat-
form east and west of the main continental land-surface of the
time.”

The lower cambrian fauna was rich and varied, but it was not
self-supporting, for no fossil plants are found, and the primary
food-supply was pelagic. Animals adapted for a rapacious life at
the surface, such as the pteropods, were abundant, and they prove
the existence of a rich supply of pelagic animals. All the forms
are either carnivorous animals, such as meduse, corals, crustacea
and trilobites, or they are adapted, like the sponges, brachiopods
and lamellibranchs, for straining minute organisms out of the
water, or for gathering up those which rained down from above,
and the conditions under which they lived were obviously very
similar to those on the bottom at the present day.

Walcott’s studies show that the earliest known fauna had the
following characteristics :

1. So far as the record goes it consisted of animals alone, and
these animals were dependent upon the pelagic food-supply for
support.

2. While small in comparison with many modern animals, they
were gigantic in size as compared with primitive pelagic animals.

3. The species were few, but they represented a very wide range
of types.

4, All the types have modern representatives, and most of the
modern types are represented in the lower cambrian.

5. The habitat was not the bottom of the deep ocean, but the
submerged surface of a sinking continent, under water of con-
siderable depth.

Remains of bottom animals are found in rocks below the cam-
brian, and Walcott believes that while the olenellan fauna adds
a little more to our knowledge of the rate of convergence back-
wards in geological time of the lines representing the evolution of
animal life, it also proves, at the same time, that an immense
interval has elapsed between the beginning of life and the epoch
represented by the olenellan fauna. He says: “That the life in
the pre-olenellus seas was large and varied, there can be little, it

170 W. K. BROOKS.

any, doubt. The few traces known of it prove little of its charac-
ter, but they prove that life existed in a period far preceding lower
Cambrian times, and they foster the hope that it is only a question
of search and favorable conditions to discover it.”

No one can question the validity of the basis for Walcott’s hope,
for pelagic animals have undoubtedly established themselves on the
shores of elevated tracts again and again, during the oscillations of
the sea-bottom, and we have every reason to expect and look for
their remains.

If, however, it is true that the primitive stem-forms were pelagic
and minute, there is little hope of finding their delicate microscopic
remains in the sedimentary rocks of the shore.

The cambrian fauna is usually regarded as a half-way station in
a series of organisms which reaches back into the past for an im-
measurable period, and it is even stated that the history of life
before the cambrian is longer, by many fold, than its history since.

So far as this opinion rests on the diversity of types in cambrian
and silurian times it has no good basis, for if the view which I have
advocated is correct, the evolution of the ancestral stem-forms took
place at the surface, and all the necessary conditions for the rapid
production of types were present when the bottom fauna first became
established.

As we pass backwards towards the lower cambrian we find closer
and closer agreement with the biological conception of the primi-
tive life at the bottom.

We cannot regard the olenellan fauna as the first bottom fauna,
for it contains forms which have been secondarily adapted for a
pelagic life, such as the pteropods.

We may, however, feel confident that the first bottom fauna re-
sembled that of the lower cambrian in its physical conditions, and
in its most distinctive peculiarity, the abundance of types and the
slight amount of differentiation among the representatives of these
types.

Far from seeing in the lower cambrian fauna a half-way station
in a long series of bottom animals, the biologist must regard it as
an unmistakable and decided approximation to the primitive fauna
of the bottom, beyond which life was represented only by simple
and minute pelagic organisms.

SALPA IN RELATION TO EVOLUTION OF LIFE. 171

THE ORIGIN OF THE CHORDATA, CONSIDERED IN ITS RELATION
TO PELAGIC INFLUENCES.

Section 1.—The Aneestral Chordata.

I shall now attempt to study the origin and significance of the
structure of appendicularia in accordance with those conditions which
must, as Dohrn has pointed out (Studien, ete., VIII, p. 79), direct
all inquiry into the genealogy of animals.

All biologists will agree with Dohrn that no amount of morpho-
logical information, or of exhaustive microscopical study of the
structure and development of animals, can suffice, in the absence
of comprehensive knowledge of their mode of life and of the con-
ditions of their existence, for the institution of inquiries into their
phylogenetic relationship.

Unquestionably the first condition for genealogical inquiry is, as
Dohrn says, the establishment of a direct connection between our
morphological studies and the facts of physiology and biology.

“The homologies which are established by comparative anatomy,
and the primitive identities which are established by comparative
embryology, are only the means for this end. They are in them-
selves valuable in phylogenetic inquiry only so far as they furnish
us the opportunity to pass from the consideration of the structure
of organs as they now exist, and of the functions of these organs at
the present time, to the consideration of conditions which have
passed away ; to the study of the history of the modifications which
have come between these structures and functions and those which
we must attribute to the same organs at an earlier genealogical
stage.”

Keeping these conditions of genealogical inquiry in view, let us
try to study the structure of appendicularia in relation to the con-
ditions of its life, so far as these are known to us, and let us see
what functions we must, according to the principle of change of
function, attribute to the organs of the remote ancestors of the tuni-
cates, and what are the paths these organs have traversed in reach-
ing their modern structure.

If the reader of the following pages should think that I wander
too far from the beaten paths of observation, I must plead as my

172 W. K. BROOKS.

excuse that the study of phylogeny is impossible without the use
of the imagination, and that the field is already occupied by a
phylogeny of the tunicata which cannot be set aside until a more
satisfactory one has been found.

Appendicularia is a very simple organism, and while much
ingenuity has been expended in the negative task of accounting
for the absence of all the structures which it lacks, I hope that the
more positive attempt to account for its actual structure will not
lead us into any great difficulties.

In the belief that the sequel will justify the assumption, I shall,
as my starting-point, picture the ancestor of appendicularia as a
simple, minute, unsegmented, chordate animal, leading a free,
locomotor, pelagic life and subsisting upon the micro-organisms
of the ocean. I shall also assume that this ancestor had an
elongated, unsegmented body stiffened by an axial, unpaired,
unsegmented notochord, like that of amphioxus, appendicularia,
and the ascidian larva; that it had a simple, elongated, dorsal,
nervous system, and an elongated, ventral, digestive tube, without
pharyngeal clefts ; that this tube was nearly straight ; that it had a
capacious lumen, and thatg as in amphioxus and the tunicates,
this was permanently distended and ciliated, and that the water,
with the micro-organisms that float in it, was swept through it by
endodermal] cilia and not by muscular contractions.

In order to entangle the floating particles of food and to hold
them while the water swept on through the intestine and out of
the anus, gland-cells for the excretion of slime were scattered
among the ordinary ciliated endoderm cells of the digestive tract.
In origin, these slime-cells may have been modified or specialized
digestive gland-cells.

As particles which are entangled and held captive near the oral
end of the gut are more perfectly exposed to its digestive action
than those which continue to float with the stream, the most
anterior slime-cells are most efficient and valuable, and as each
variation in this direction gave its possessor an advantage, the
slime-cells gradually, through the action of natural selection,
became localized in the pharyngeal region, and this region
gradually became enlarged and was thus set apart, at a very early
period, as a specialized tract of the gut.

SALPA IN RELATION TO EVOLUTION OF LIFE. 173

It is also probable that, at a very early stage in the phylogeny
of these primitive chordata, a blind pouch was developed, behind
the pharynx, to catch the food-particles as they were hurried past
with the stream of water and to retain them long enough for per-
fect digestion, and that the rudiment of the organ which has in
the higher vertebrates become the liver was thus established.

In these primitive animals the current of water through the
digestive organs was most useful as the vehicle for floating food,
but while necessary, it was a necessary evil, for the large dis-
tended Jumen which furnished it a channel also permitted undi-
gested food to be swept away and lost.

The immovable, permanently distended, ciliated digestive tract
of a modern lamellibranch is very similar to that of these primi-
tive chordata, but the lamellibranchs have acquired an apparatus
for straining off the water from the captured food, so that the
digestive tract is relieved from this disadvantageous current.

If, after the pharynx had been established, a secondary opening
from it to the exterior were to be formed, this opening would per-
mit the water to escape without passing through the intestine, and
as the advantage of this new arrangement is obvious, there can be
no doubt that after an opening of this sort was once formed, it
would be preserved and perfected by natural selection, as a channel
for the escape of the water after the food has been strained out and
entangled by the excretion of the pharyngeal slime-glands.

I shall show, further on, that if an useful opening of this sort
were to be fixed and preserved by natural selection on one side of
the body, the laws of growth would soon cause it to be duplicated
on the other side. These two openings are the so-called gill-slits
of appendicularia, although they are beyond question much older
than the modern appendicularia, dating back to a time before this
animal had acquired the features which distinguish it from its more
primitive chordata ancestors.

{tam not able to suggest what led to the first establishment of a
secondary opening into the pharynx ; but, once formed, its preser-
vation and gradual improvement, by natural selection, as a channel
for the escape of superfluous water, and its duplication on opposite
sides of the body, are easily intelligible.

174 W. K. BROOKS.

If we accept the view that the chordata type was evolved under
purely pelagic influences, we are forced to believe that the first
chordata were minute, and that their small bodies were soft, and
unprotected by a hard covering. If we also admit that their di-
gestive tract was a channel for a current of water, we can hardly
believe that they needed respiratory organs, or, for that matter,
excretory organs, for all the tissues of a minute soft animal, bathed
within and without by pure water, must have been sufficiently
aerated and purified without any organs for this purpose.

It is not at all probable, then, that the pharyngeal clefts were origi-
nally either gills or renal organs, and we have seen that the conditions
of pelagic life furnish a much more simple explanation of their advan-
tage, and I believe that the view that they were originally concerned
in nutrition rather than in respiration will commend itself to all
who approach the subject without any philosophical preconception.

After they were once established they gradually effected a rear-
rangement of the slime-cells and ciliated cells of the pharynx, for
as it now became important that all the food particles should be
entangled by the product of the slime-cells before it reached the
pharyngeal clefts, the slimé-cells were gradually restricted to the
anterior part of the pharynx, while the ciliated cells gradually be-
came specialized to carry the entangled food past the openings and
to convey it safely into the cesophagus.

All the parts of the pharynx of appendicularia are beautifully
constructed for this purpose. The pharyngeal clefts are situated
far back in the pharynx, and are separated by nearly its whole
width from the cesophagus. They are fringed by large cilia to
expel the water, and they are separated from each other by a
vertical shelf or velum on the ventral floor of the pharynx, so
placed as to prevent cross-currents.

In front of this shelf the slime-cells are brought together in two
rows, near the middle line, just inside the mouth, to form the hypo-
pharyngeal band or endostyle. Between these two rows of slime-
cells there is a median row of large ciliated cells, so placed that they
drive the slime forwards to the point where a ciliated peripharyngeal
band receives it and carries it up each side of the pharynx just
behind the mouth, into the most favorable place for entangling the
food, as this enters with the current of fresh water.

SALPA IN RELATION TO EVOLUTION OF LIFE. 175

On the dorsal middle line the threads of slime are gathered up
and guided along the epipharyngeal band or dorsal lamella, beyond
the influence of the current of water which sets backwards, on each
side of the ventral velum, to the pharyngeal clefts, and the food is
thus safely conducted into the cesophagus while the water escapes.

Up to this point I believe that the ancestral history of the tuni-
cates was identical with that of the vertebrates, for the hepatic
cecum, the dilated pharynx, the pharyngeal clefts, the hypo-
pharyngeal gland and the peripharyngeal bands have been in-
herited by all the chordata, and have impressed themselves so
firmly in their organization that even the highest vertebrates still
retain them, either as vestiges, or as organs which have been fitted
to new functions.

I believe, however, that while they were acquired before the
tunicates diverged from the chordata stem, they were acquired by
an organism whose environment and habits of life were essentially
like those of the modern appendicularia.

All the parts of the pharynx of appendicularia are so beautifully
co-ordinated for effecting a purpose so useful and so well adapted
to the conditions of its simple pelagic life, that we find it difficult
to resist the belief that its ancestors had essentially the same habits,
and that they lived under essentially the same conditions, and that
this simple organization was directly acquired in adaptation to
these conditions.

If this view involved any great or unusual difficulties we might
well distrust it, notwithstanding its simplicity ; but I shall try to
show that it does not. In the preceding chapter I have shown
that it accords with our knowledge of the fundamental principles
of the general biology of the ocean, and further on I shall try to
show that it is equally in accord with the principles of morphology.

At present we must devote our attention to the history of the
evolution of the tunicates from this primitive chordata stem.

Section 2.—The Origin of the Tunicates.

Like most recent students of the tunicates, I believe that we
have in appendicularia a persistent representative of the primitive
tunicata; but, unlike many of them, I fail to find in its structure

176 W. K. BROOKS.

any evidence of degeneracy, or in its habits any basis for the
assumption that it is degenerated. In most respects its structure
is like that of the hypothetical ancestor whose evolution we have
traced. It has an unsegmented notochord, and a capacious lumen
throughout the whole course of the digestive tract from mouth to
anus. This lumen is permanently distended and food is carried
through it by cilia. It has a blind diverticulum from the stomach,
and the greatly expanded pharynx opens laterally through two
ciliated pharyngeal clefts, through which the water escapes while
the food passes into the cesophagus. There is a ventral slime-gland
just inside the mouth, and its excretion is conveyed upwards around
the pharynx by the cilia of the peripharyngeal bands, and is then
swept into the cesophagus with the entangled food.

This increasing complexity and perfection of the pharynx is
accompanied by an increase in its size, so that in the primitive tuni-
cates it soon comes to be the most important and dominant organ
of the body, and brings about adaptive changes in other parts. One
of these is the differentiation of a stomach for the retention and
digestion of the food, in the direct course of the gut. As long as
the food was mixed with great quantities of water, digestion and
assimilation probably went on simultaneously in all parts of the
post-pharyngeal gut, but as the water found another exit and the
food thus became more compact and solid, the stomach of appen-
dicularia became established and thus divided the gut into an
cesophageal, a gastric, and an intestinal region.

Our knowledge of the primitive vertebrates seems to me to be
too scanty to show whether this differentation occurred before or
after the tunicates diverged from the ancestors of the vertebrates.
We are now concerned with the history of the tunicata line alone,
and the fact that the differentiation now exists in all tunicates shows
that it was brought about very early in their history.

Another most important change in the relations of the gut also
took place very early in their history. The intestinal portion became
bent upon the enlarged pharynx so as to form a C with the intesti-
nal bar of the C ventral to the pharyngeal portion, and with the
anus on the ventral middle line under the pharynx. Herdman
represents the primitive condition of the digestive tract of tunicates
as a C4, with the intestine and anus dorsal instead of ventral (page

SALPA IN RELATION TO EVOLUTION OF LIFE. 177

128); but I shall show further on that the relations exhibited by
appendicularia are the primitive ones, from which we must derive
those which are exhibited by other tunicates.

By this change the tail was freed from the gut and was made
much more efficient as an organ of locomotion, while the feeces were
discharged from the anus into the current of water which set out
through the pharyngeal clefts. This latter feature may not have
been of any value so long as habits of active locomotion were re-
tained, but, as we shall see, it became very important at a later stage.

The embryology of the ascidians shows that this arrangement of
the digestive tract was secondary ; that at one time it was straight,
extending into that region of the body which is now specialized in
appendicularia as a tail. The advantage to an active pelagic animal
of this change is obvious, since it permits the tail to become purely
locomotor. As each slight variation in this direction must have
given a slight increase in the freedom of movement, the shape of
the body of appendicularia is easily intelligible as the result of
natural selection, and while the change is complete in this, the most
primitive tunicate which we know, so that we can only conjecture
the transitional stages, the change itself is not a complicated one.
It presents little difficulty, although the resulting differentiation of
appendicularia into two regions or “segments,” a body and a tail,
has been made the basis of much speculation.

The great development of the pharynx and the reduction of the
tail to an organ of locomotion soon resulted in a pronounced change,
of the sort for which Dana long ago proposed the term cephalization.

As the functions of the pharynx, and of its oral end in particular,
became more and more complicated and more and more exactly co-
ordinated, while those of the tail became simplified, the elongated
nervous system became differentiated in a corresponding way, and
its caudal portion became reduced to a caudal nerve, while its oral
extremity became evolved into a cerebral vesicle with sense-organs
and nerves in relation with the co-ordinated structures of the
pharynx.

All the characteristics of appendicularia, except the structure of
the heart and the structure and position of the reproductive organ,
are thus seen to be intelligible as direct adaptations to a pelagic
life ; for its distinctive features, as compared with other primitive

4

178 W. K. BROOKS.

chordata, are the U-shaped folds of the digestive cavity, the sharp
separation of the tail from the body, and the differentiation of the
nervous system into a caudal nerve and anterior vesicle.

We have little basis for speculation as to the path by which the
reproductive organ acquired its present position, and it is by no
means certain whether the tunicate heart is homologous with that
of the other chordata.

The conditions of pelagic life are so permanent that we may
safely make use of the structure and habits of the modern pelagic
forms to reconstruct this part of the ancestral history of the tuni-
cates, for time writes no wrinkles on the azure brow of the ocean.

As regards the later history the case is different. Between ap-
pendicularia and the ascidians there is a great gap which we can
bridge only in imagination. The transitional animals are totally
unknown, and the conditions of life on the bottom of the modern
ocean may, possibly, be very different from those which prevailed
when the fixed ascidians were first evolved.

It is easy to imagine changes which might have gradually con-
verted an ancestor like appendicularia into a descendant like the
fixed ascidians, through successive adaptations to a sedentary life,
but in the absence of all evidence we cannot feel implicit confidence
that the imaginary picture bears any minute and detailed resemblance
to the actual history.

It seems probable that after the bottom of the ocean became fit
for life, some of the descendants of the primitive pelagic tunicates
gradually acquired the habit of sometimes swimming upon or near
it in an inclined position with the mouth downwards to suck up the
organic sediment, and that they also acquired the habit of resting
upon the bottom in this position.

We may well doubt whether these animals obtained any more
food than their pelagic ancestors, but it is well known that it is not
the amount of food, but the ratio between the supply and the amount
of expended energy which affects size. As this new habit econo-
mized energy both during rest and during activity, it permitted an
increase in size, and it is interesting in this connection to note that
Chun has found at great depths appendicularias which may well be
called gigantic as compared with all which are known to exist at
the surface.

SALPA IN RELATION TO EVOLUTION OF LIFE. 179

With each increase in size, the habit of visiting the bottom must
itself have become more and more fixed, until the life upon the
bottom, which may have been at first only intermittent and more
or less accidental, at last became established in the ancestors of the
ascidians as a constant characteristic peculiarity.

As this new mode of life was gradually acquired, some method
of aerating the fluids of the body must also have been gradually
evolved ; for without it, a minute animal adapted for a free active
life in the highly aerated surface-water, could not, at the same time
that it grew larger, acquire a less active habit of life in the bottom
strata where the water is less perfectly aerated, the products of de-
composition of organisms more concentrated, and the capacity for
passing from exhausted and impure water to a fresh environment,
restricted both by the more stationary habit and by the fact that
life in space has been exchanged for a home which is limited by a
surface.

Undoubtedly the change of habit was accompanied by the gradual
perfection of the system of blood-spaces around the pharynx, which,
at first indefinite and irregular, became constant on the margins of
the pharyngeal clefts, which thus gradually acquired a new function
and became gill-slits, and also became duplicated as the animals
grew larger and the need for more perfect respiration itfcreased
with their change of habits.

I hope that no one will interpret the last sentence as an expression
of the belief that the need for respiration caused the gill-slits to
multiply. I believe, and shall try to show further on, that the
tendency to duplicate a structure, either radially, bilaterally or
serially, is a result of the method of growth by cell multiplication,
and that in the case in question the serial reduplication has been
fixed and preserved by natural selection on account of its value in
respiration.

The context shows that I also regard the gill-slits of vertebrates
and those of tunicates as homologous structures inherited from a
common source, the primary pharyngeal clefts; but that I regard
the increase in their number as a secondary change which has
occurred in both lines after their genealogical paths had diverged.

It does not seem necessary to defend the thesis that the number
of gill-slits in the ascidians is the result of secondary multiplication,

180 W. K. BROOKS.

since, as I shall show further on, it is accepted by Dohrn (Studien,
etc., LX, 417), who has proved himself a most rigorous critic of the
logic of morphology.

There is reason to believe that the multiplication of gill-slits in
the tunicates has not only taken place independently, but that it
has taken place in a peculiar way. Anatomy and embryology give
evidence that while the perforations of the tunicate pharynx multi-
plied, the perforations of the outer wall of the body did not; and
that the external portions of the two primary clefts became distended
into a pair of spacious perithoracic chambers, each with numerous
ciliated openings into the pharynx, and a single opening to the
exterior which perhaps became enlarged as the gill-slits multiplied.

So long as the primary function of the first pair of pharyngeal
clefts, the discharge of the superfluous water, was the only one,
they probably remained circular like those of appendicularia ; but
as they became concerned in respiration and increased in number,
and were furnished with definite, blood-vessels, they became elongated
vertically and, forming a series side by side over a considerable area
on each side of the pharynx, they thus became much more efficient
organs for the aeration of the blood.

In this simple way metamerism, that fetish of the morphologists,
was established among the tunicates, and there is no evidence that
it has ever involved any of their organs except the gill-slits and the
pharyngeal blood-vessels.

A vertical series of slits, elongated longitudinally, would un-
doubtedly have permitted the water to escape just as well as a
longitudinal series elongated vertically, but it is possible that, during
the gradual establishment of the respiratory circulation, those of the
irregular and variable blood-spaces which were most nearly trans-
verse to the current of water from the mouth to the primary clefts,
were the ones which were first made definite by natural selection,
and that the arrangement of the gill-slits was thus determined.

We can only conjecture how this unknown ancestral swimming
organism first became fixed, but the discovery of its descendants
on the modern sea-floor is among the possibilities of future expla-
nation.

The sedentary habit undoubtedly came gradually, and at first it
may have been temporary, confined perhaps to the breeding season,

SALPA IN RELATION TO EVOLUTION OF LIFE. 181

when, loaded down with eggs, the animal may have learned to rest
upon the fragments of crinoids, or the shells of trilobites or brachio-
pods or molluscs, to avoid clogging its delicate ciliated and vascular
pharynx with sediment. At the point where the heavy anterior
end of the tadpole-shaped body rested, the ectoderm cells, just be-
low the mouth, probably became modified for the excretion of an
adhesive cement.

The sedentary habit, which must have resulted in a still greater
economy of energy and a corresponding increase of size, undoubtedly
became more and more firmly established, and the changes which
followed and resulted in the evolution of the ascidian type are easily
intelligible as adaptations to a fixed home, although we have little
to show the sequence of their acquisition.

So long as the animal led a free life the fate of the deoxidized
water after it left the gill-slits had no meaning, but with the fixed
habit came the need for driving it away as far as possible, and the
external apertures of the perithoracic chambers became small,
moved towards each other, and finally united to give to the
exhaled current the strength of concentration. The attitude of
the animal upon the bottom undoubtedly determined the dorsal
instead of ventral location of the common aperture and of the
median atrium or cloaca. As each step in this process of concen-
tration must have been advantageous, its evolution by natural
selection is easily intelligible. The accumulation of feces from
the intestine, around a fixed animal, is so unsanitary that the anus
has disappeared in many sedentary metazoa, while in others, such
as the crinoids and the lamellibranchs for example, secondary
adaptations for sweeping away the refuse matter have been ac-
quired.

The folding of the originally straight digestive tract of the
primitive chordata into a U with the anus and intestine ventral to
the pharnyx, took place in the ancestral tunicates as an adaptation
to locomotion, but, as appendicularia shows, it incidentally brought
the anus into the region of the pharyngeal clefts. As the seden-
tary habit became slowly established the anus became shifted from
the middle line into the exhaled current from the left perithoracic
chamber, and finally into the margin of its aperture, so that, during
the migration of the exhalent openings, the U of the digestive tract

182 W. K. BROOKS.

became twisted into an 8 in such a way that, as Plate VIII, Fig.
2 shows, the intestine p passed on the left side of the cesophagus,
g, to open dorsally into the atrium, near the middle line, but a
little to the left.

This arrangement of the digestive organs is very characteristic
of the tunicates, and the few exceptions are clearly due to later
changes. Thus in doliolum the atrium has moved backwards as
an adaptation to locomotion, and the anus has followed it until the
gut has become nearly straight. The intestine and anus of the
adult ageregated Salpa pinnata, Plate I, Fig. 1, are ventral; but
I have shown that in the young the intestine crosses to the left of
the cesophagus to open dorsally, as it does in the adults of all
ordinary salpee. In the Polyclinide the loop of the intestine has
been elongated, with the elongation of the body, until the bend of
the 8 has been obliterated, and the presence of the characteristic 8
in more primitive ascidians such as clavelina shows that the Poly-
clinidee have been more recently modified.

All sedentary animals which take their food by means of cilia
have their apertures raised in some way above the reach of sedi-
ment. In the crinoids this end is reached by a stalk; in the
lamellibranchs it is attained either by siphons, or by the vertical
elongation of the shell as in the oyster; and the shifting of the area
of attachment of the ascidians from the oral end to the aboral end,
the elongation and approximation of the mouth and the atrial
aperture, the acquisition of oral and atrial sphincter muscles, the
degeneration and disappearance of the locomotor tail, and the sim-
plification of the nervous system, are such obvious adaptations to a
sedentary life that it is not necessary to discuss them.

Section 3.—The Annelidian Hypothesis.

I believe that the structure of the tunicates has been acquired as
an adaptation to the biological conditions which prevailed at the
surface of the primitive ocean, and that it has been evolved by the
gradual addition of successive complications on to the body of a
still more primitive and simple ancestor. This involves the total
rejection of the dogma that the vertebrates are modified annelids,
and that the tunicates are degenerated vertebrates.

SALPA IN RELATION TO EVOLUTION OF LIFE. 1838

While it is not my purpose to discuss the ancestral history of the
vertebrates, the remote phylogeny of the tunicates is unquestionably
identical with that of the other chordata, and I cannot ignore the
general acceptance of an opinion which is absolutely irreconcilable
with the one which I have presented.

This prevailing opinion has interwoven itself with the literature
in such a complicated way that one may well shrink from the in-
terminable labor which the critical revision of the whole of it would
involve. I myself decline to undertake what I regard as an un-
profitable and useless task ; unprofitable, as the literature rests on
an untenable and false basis, and useless, since I do not hope to
induce those who have stored their minds with the endless details
of morphology docketed and pigeon-holed according to a false
system, to unload all this rubbish and to build again on a new
foundation.

I shall therefore restrict myself to a discussion of the origin of
the two most characteristic systems of tunicata organs, the gill-slits,
and the pharyngeal ciliated cells and gland cells; and I shall here
confine myself to the observations and reflections of a single writer,
Dr. Dohrn.

I make this selection the more willingly, as Dohrn’s name is
most intimately associated with the annelidian hypothesis, and
because his writings are not only the ones which have been most
influential, but also the ones which are most comprehensive and
most attractive to the reader.

The “ Ursprung der Wirbelthiere” is a most fascinating book.
Soon after it appeared I placed it in the list of works which my
students are advised to read, and for many years an acquaintance
with it has been expected of all who have been examined for the
degree of Ph. D. in the Johns Hopkins University.

My students have even prepared for their own use an English
translation of it, and I have read it with them several times with
interest and pleasure. At the first reading my pleasure was almost
that of conviction, but as the ingenious details became familiar,
and the essay was more sharply focused in its completeness, and
was held, as it were, at arm’s length, so that the whole picture
could be seen at one view, I have read it, as I have read Gulliver’s
Travels, with admiration for the skill which has elaborated it in

184 W. K. BROOKS.

such logical minuteness from a fundamental assumption which is
purely imaginary.

The story, as told by Dohrn in the “ Ursprung,” is so consistent
and logical that I see no reason why animals like the tunicates
might not have been evolved in the way which he pictures so
vividly, although I believe that the actual tunicates have been pro-
duced in a very different way.

I shall therefore examine the account of the origin of the gill-
slits which Dohrn gives in the “ Ursprung,” and the view of the
ciliated and glandular structures of the pharynx which is developed
in his “Studien,” especially in Parts VII, VIII and IX, in order
to determine how far the origin of these structures is accounted for
by the annelidian hypothesis, and what superiority, if any, this has
over the much simpler hypothesis which is here advanced.

Dohrn says (Ursprung, p. 10) that the branchial apparatus of
the tunicates and that of balanoglossus are so much more complicated
than that of the selachians, and their origin is so much more
difficult to understand, that they are of no help to us in our attempt
to trace the origin of gill-slits.

I am quite at a loss fér the meaning of this passage, for no
secondary perforation of the pharynx could possibly be less com-
plicated than the gill-slits of appendicularia, nor could it be de-
veloped in a simpler way than by the involution of a pit on the side
of the body.

It is quite true that we do not know how the gill-slits of appen-
dicularia first came into existence, or what influence led to their
formation, but their usefulness as channels for the escape of the
water which, before they were formed, must have passed through
the intestine, is clear, and we can understand why they have been
preserved, by natural selection, on account of this advantage.

We are forced to believe that the pharynx did, in some way,
acquire a secondary communication with the exterior, although we
are not able to say how it was acquired.

Dohrn’s view of the origin of gill-slits is based upon the need
for an explanation of the original formation of the perforation. He
says (Ursprung, p. 10): “What is a gill-slit? Perforations of the
body-wall do not take place directly, and still less do they form
connections with corresponding perforations of the digestive tract,”

SALPA IN RELATION TO EVOLUTION OF LIFE. 185

and he therefore undertakes to study the origin and primitive
function of gill-slits by the aid of the law of the change of function,
and to find in a more primitive function an explanation of their
present function as channels for water.

As his point of departure is the need for an explanation of the
origin of the perforation, we feel a natural hope that we are to be
led to this explanation, but this hope ends in disappointment.

He regards the gill-slits as modified segmental organs, but he
tells us explicitly, on page 10, that “ we are not able to assign any
reason why segmental organs should unite with the gut,” and his
explanation of the origin of the perforations is no explanation at
all, since it simply assumes, but does not account for, the very
phenomenon which it is supposed to make clear.

His inability to understand the direct origin of the secondary
perforations of the gut has one most remarkable result, for the
view that the gill-slits are segmental organs involves the view
(Ursprung, p. 57) that the anus of the tunicates is not a primary
anus nor a secondary one, but a tertiary one, and that the ancestors
of the tunicates have not only acquired two new secondary anal
apertures, but that they have lost one mouth and acquired a second,
and that they have lost this and acquired a third. As these mouths
are supposed to be modified segmental organs, we are, according to
the acknowledgment on page 10, “unable to assign any reason why
they should have united with the gut.”

The original mouth of the ancestors of the chordata was, accord-
ing to Dohrn (page 3), on what is now the dorsal surface, and the
primitive cesophagus passed through what is now known as the
fossa rhomboidea of the brain.

This ancestral mouth degenerated and disappeared as it was
gradually superseded in the remote progenitors of the vertebrates
by a second mouth (page 5), which is the mouth of the vertebrates
of the present day, and of the ancestors of the tunicates (page 57)
as well, although it was gradually converted first into a sucker, and
finally into an organ for fastening the tunicata to foreign bodies,
while these animals gradually acquired a tertiary mouth (page 58)
by the formation of a secondary communication between the nasal
chamber and the gut.

186 W. K. BROOKS.

Dohrn says (page 60) that these assumptions “set the relation
between the fishes and the ascidians in the right light,” although
the perforation of the gut, which the hypothesis is to explain, is
not only left unaccounted for, but is multiplied so many times
that, like the man with an unclean spirit, its last state is worse
than the first.

Dohrn says that the secondary nature of the mouth of the verte-
brates is proved by its very late appearance in the young vertebrate
after its embryonic body and its great systems of organs are fully
formed, and by the fact that, when it does make its appearance, it
does not lie at the anterior end of the body, in the place which it
finally occupies in the great majority of vertebrates, but at a spot
some distance behind this place.

It is not possible to attach much weight to either of these argu-
ments, for slight changes in the position of organs are not unusual,
and it is well known that the ontogenetic acceleration or retarda-
tion in the relative time of appearance of structures is by no means
exceptional, and it would be as safe to assume that the change in
the pitch of the voice of man is phylogenetically older than the
sexual maturity of the ancestors of man, as it is to assume, from
the same sort of evidence, that the aortic system of vertebrates is
older than the mouth.

The vertebrate mouth unquestionably bears a great morphological
resemblance to a pair of gill-slits. As Dohrn points out, it is bor-
dered, like the gill-slits, by a pair of visceral arches, it lies in front
of the first pair of true gill-slits, it arises at the same time with
them in the embryo, and like them it opens into a section of the gut.

A ventral view of a shark shows the resemblance between the
mouth and the true gill-slits in the most impressive way, and if
any pair of them were to be united with each other at their ventral
ends, they would become perfectly equivalent to the mouth. The
armature of the mouth is repeated on the gills, and there is reason
to believe that the jaw-arches have at one time carried gills like the
gill-arches.

This resemblance is not imaginary. Beyond all question it is
real, and it is certainly most remarkable and suggestive, but does
it prove that the vertebrate mouth is phylogenetically a pair of
gill-slits ?

SALPA IN RELATION TO EVOLUTION OF LIFE. 187

When, in my student days, my instructor held before me the
skull of a turtle and called upon me to observe the centrum, the
transverse processes and the neural arch of the occipital vertebra, I
was, for the time, convinced that the occipital bone had arisen by
the differentiation and specialization of a bony vertebra, like those
in the neck of a turtle, and that its history had been identical with
that of the thoracic vertebrae, which have been differentiated and
specialized in the same way into constituent parts of the bony box
which covers the body of the turtle, as the skull covers the brain.

In all these cases the morphological resemblance is undeniable,
but our opinion of its phylogenetic significance depends upon our
view of the nature and origin of the metamerism of vertebrates, a
question which will soon be discussed.

At present we must confine ourselves to a narrower point of view,
and learn where we are led by Dohrn’s opinion that the vertebrate
mouth is actually a pair of gill-slits.

If the present mouth of the vertebrates was once a pair of gill-
slits, the ancestors of the vertebrates must have had at that time
another mouth, and during the long series of stages of development,
while the gill-slits were gradually assuming the function of a mouth,
food must have been taken in through both openings ; for the new
function of the gill-slits must have been acquired slowly alongside
their old function, until the new mouth finally became so perfectly
adapted for its new function that it supplanted and replaced the
old one.

According to Dohrn, these considerations force us to believe that
the primitive mouth of the ancestors of the vertebrates and of the
tunicates was situated in the fossa rhomboidea, where an cesophagus
pushed inwards to join the mid-gut, in the same way that it is
joined in insect embryos by the fore-gut. This primitive mouth
and its oesophagus were homologous with the corresponding organs
of modern arthropods and annelids. The mouth of the modern
vertebrates is then to be regarded as a secondary mouth, which has
gradually supplanted and replaced the old one on account of its
greater efficiency.

It follows from this, according to Dohrn (p. 56), that the
“so-called larva” of the ascidians is a degenerated fish, and that
all the features which show the derivation of the cyclostomes from

188 W. K. BROOKS.

the fishes show also that the process of degeneration has reached its
extreme in the tunicates. The cyclostomes are held to owe their
degeneracy to parasitism, and the most important element in the
more advanced process of degeneration is that the ascidians no
longer fasten themselves to fishes nor make use of their bodies as
food, but that they fasten themselves to stones, to ships, or to the
bodies of other animals which do not serve as food, such as the
shells of crabs or the tubes of annelids.

The mouth (p. 57) which in the cyclostomes serves both as an
organ for attachment to the skin of fishes, and also as a sucker for
extracting their blood, has become converted in the ascidians into
an organ for attachment; and these animals have thus lost their
old mouth, which was homologous with that of the true vertebrates,
and have acquired a new one which is homologous with the verte-
brate nasal chamber.

The process, Dohrn says, must be represented as follows: The
fishes take in the water for respiration through the mouth, but as
this is used by the parasitic cyclostomes as a sucker, they have
acquired another arrangement, and the water is not only discharged
through the gill-slits, but i$ also inhaled through them, and, in
the myxenoids, through the nasal passage also, which has in the
tunicates become the functional mouth. The vertebrate mouth has
lost its old function in the cyclostome-like ancestors of the tunicates,
as these have gradually lost their parasitic habit, and have estab-
lished themselves on lifeless bodies; but the original lips have
remained, and they are to be recognized in the so-called sucking
knobs of the ascidian larva, while the teeth of the cyclostomes are
supposed to be represented by “ bristle-carrying end knobs”? upon
the suckers.

The “so-called larva” of the ascidians is represented in almost
every feature of its organization by the adult, sexually mature,
appendicularia. No better example of the correspondence between
an adult animal and an ontogenetic stage in the history of another
can be desired, and we may feel confident that, whatever the
phylogenetic history of appendicularia has been, that of the ascidian
larva has been the same.

Nearly all of the students who have devoted themselves to the
study of the tunicates agree in regarding appendicularia as a per-

SALPA IN RELATION TO EVOLUTION OF LIFE. 189

sistent representative of their primitive condition ; but appendi-
cularia is an active swimming organism, and I have shown that its
simple structure is so well adapted to the needs of its pelagic life,
that there can be no inherent improbability in the view that it owes
its origin to simple pelagic influences.

Nothing whatever in its habit of life or in its structure lends the
least support to the view that it is a degenerated animal, and if we
accept it as evidence, we are forced to believe that, far from being
the fixed and degenerated descendants of parasitic vertebrates, the
tunicates are descended from free, active, pelagic animals of very
simple structure and minute size.

Even Dohrn seems to admit that the ancestors of the tunicates
were swimming animals, for he tells us in support of his view of
the homology of the endostyle (Studien, etc., VIII, p. 62) that the
ancestors of the tunicates were “obviously” free swimming ani-
mals, and therefore in the position to seize their food by hunting.
“ Offenbar waren sie frei schwimmende Geschépfe und damit in der
Lage, ihre Nahrung durch Jagd selbst zu packen.”

If the tunicates are, as their embryology and comparative anatomy
indicate, the descendants of an ancestor which was obviously a free
swimming animal, it is surely simpler, in view of all the facts, to
regard the gill-slits as perforations which were originally retained
and fixed by natural selection as channels for the exit of the water
which was taken into the mouth with the food, than to refer them
back to imaginary segmental organs which have left no other trace
of their existence in the body of any known tunicata.

Minute pelagic animals, with soft bodies bathed on all sides by
pure water, do not need special organs of excretions or respiration,
and it is not at all probable that the pharyngeal clefts were
originally respiratory ; but it is easy to understand how the chan-
nels through which the water flowed became converted into gill-
slits, in accordance with the law of change of function, as the
descendants of the primitive tunicates grew larger and became
sedentary, and thus came to need respiratory organs.

It may be argued that the thing to be explained is not the exist-
ence of gill-slits, but their serial reduplication or metamerism. It
may be held that the metameric repetition of the gill-slits of
ascidians forces us to regard the ascidian pharynx as the primitive

190 W. K. BROOKS.

form, from which that of appendicularia has been produced by
“degeneration.” As we are told, however, by no less an authority
than Dohrn (Studien, IX, p. 417, and VIII, p. 61) that the great
number of gill-slits in the ascidians is due to secondary multiplica-
tion, “nachtrigliche Vermehrung,” this consideration need not
detain us. .

If the logical conditions of sound morphological philosophy
admit the possibility of “ nachtrigliche Vermehrung,” and permit
us to believe that the twenty or thirty pairs of gill-slits which are
found in ascidians are to be traced back to the eight pairs which the
primitive fishes are said to have possessed, the same logic will surely
permit us to believe, on sufficient evidence, that they have arisen
not from eight but from a single pair like those of appendicularia.

All the vertebrates have a peculiar organ known as the thyroid
gland, and while it holds no prominent place in our general con-
ception of a vertebrate, this gland is actually one of their most
constant and characteristic organs.

In all the jawed vertebrates, from the sharks up to man, its
typical structure is adhered to so closely as to prove that the gland
as it exists in man is an “organ of vast antiquity. In all these
animals it is a ductless gland, situated far back in the throat, behind
the hyoid skeleton; but at an early stage in its ontogeny it is a
part of the endodermal epithelium of the pharynx, and it arises on
the middle line just within the mouth.

Its function in the jawed vertebrates is problematical, but these
two features in its ontogeny seem to show that far back in the
remote past, before it had assumed its characteristic form, it had
another function which stood in some direct relation to the mouth.

The tunicate endostyle is a conspicuous organ which attracts the
eye of all observers, but its true structure was first demonstrated by
Fol, who proved that it is a pharyngeal gland with excretory cells
to produce slime, and with ciliated cells to drive the slime out
through the long, narrow, slit-like duct into the pharynx. Fol
also showed its true relation to the ciliated peripharyngeal bands
- and dorsal lamella, and proved by simple but conclusive experi-
ments that these organs are co-ordinated parts of a single system,
which has for its function the capture of the microscopic floating
food which enters the mouth with the water.

°

SALPA IN RELATION TO EVOLUTION OF LIFE. 191

W. Miller was the first to point out the homology of the tunicate
endostyle with the vertebrate thyroid gland, and this homology has
been established beyond the possibility of doubt by Schneider’s
discovery that the thyroid body of ammoccetes is a slime gland
with an opening into the pharynx near the mouth, and that on
each side of this opening a ciliated furrow or peripharyngeal band
runs upwards on the, inner wall of the pharynx, just in front of
the first gill-slit, to its dorsal middle line, where the two unite to
form an epipharyngeal band or dorsal lamella which runs back-
wards to the esophagus. Even more conclusive proof of this
homology is afforded by Dohrn’s account (Studien, VIIT) of the
histological structure of the pharyngeal gland of ammoceetes, for
his studies show on the one hand a most complete fundamental
identity with the very peculiar and characteristic histological struc-
ture of the tunicate endostyle, and they also, on the other hand,
prove its identity with the vertebrate thyroid gland, by showing
that, as development progresses, it is cut up by ingrowths of con-
nective tissue into the isolated follicles which are so characteristic
of the thyroid gland. Still further confirmation is furnished by
Dohrn’s discovery in the torpedo embryo (Studien, VIII, p. 60) of
two endodermal grooves which run from the ventral margins of
the spiracles to the ventral middle line of the pharynx, to end at
the median unpaired thyroid invagination in such a way as to prove
that they are rudimentary peripharyngeal grooves.

This most remarkable homology can no longer be questioned.
The simplest explanation, and the one which first presents itself, is
the one which Miiller advances, that the common ancestor of the
tunicates and of the other chordata, possessed this system of organs
in the form in which we now find it in the tunicates, and that while
all the jawed vertebrates have inherited the ventral pharyngeal
gland, it has been turned in them to some new use, as yet undis-
covered by the physiologists, and has lost its primitive connection
with the pharynx and its functional relation to the mouth, and has
become a ductless aggregation of follicles far back in the throat.

I have tried to show that the structure and anatomical relations
of this system of organs in the tunicates are quite consistent with
the view that it was originally acquired for the purpose which it |
now serves, the capture of food. :

192 W. K. BROOKS.

The simplest explanation of its origin is that which attributes it
to the preservation by natural selection of a long series of slight
changes, each of which improved the adaptation to the simple con-
ditions of primitive pelagic life.

Dohrn disputes this position, and says that ‘ many persons would
have great difficulty in believing that this simple mechanism is
primitive” (Studien, VIII, p. 62). The future must show how
many of these persons there are, but I shall now lay before
them Dohrn’s own explanation, that they make comparisons for
themselves.

“We ask,” he says (p. 62), “how the ancestors of the tunicates
obtained their food before the endostyle was formed. Obviously
they were free swimming animals, and therefore in the position
to seize their food by hunting. It is as certain that they needed
other contrivances than the ciliated furrows and the slime-gland, as
it is that the ancestors of the cirripeds sought food in some other
way than by the formation of little vortices to sweep into their
mouths everything within their influence. The limbs of the swim-
ming forefathers of the cirripeds were certainly different from
the cirri of modern barnacles ; even so were the ancestors of the
tunicates differently constructed from the modern ones, and before
the slime-gland and the ciliated grooves became the exclusive
means of nutrition, they must have been the accessory aids to some
more primitive mode of capturing food” . . .

“ Ammoccetes lives in the sand, into which even the youngest
larvee bore. Although direct observations fail, it must be assumed
that the excretion of slime and the ciliation have some advantage
in the nutritive or respiratory functions of organisms which live in
the mud. May we not believe that, in spite of all the sifting
through the oral tentacles and the velum, the hard particles of sand
would be injurious to the delicate epithelium of the gut, if this
were not protected by a thick coating of slime; that the ciliated
furrows are adapted for conveying this slime to the most exposed
parts, and that, in this function, they have their starting-point?
Once brought into existence, it is not remarkable to see these useful
structures further evolved until the whole mass of food is invested
with a slimy admixture to facilitate its passage through the gut.
It is not impossible that besides acting mechanically as an invest-

SALPA IN RELATION TO EVOLUTION OF LIFE. 1938

ment, the slime also acts chemically as an aid to digestion. If this
is the case, it is easy to understand how a peculiarity so useful to
sedentary animals like the ascidians, or to floating ones like the
salpe, gradually assumed the whole function of nutrition. Thus
the problem of the change of function is solved.”

Although it seems as if the delicate walls of the gut of a bur-
rowing animal would be more effectively protected if slime were
directly excreted “upon the most exposed spots,” than by this
highly specialized system of organs, we might yet believe that the
system “ has its starting-point ” in the habits of ammoccetes, if we
did not find in the structure and embryology of every chordata
animal which is known to exist evidence of descent from an ances-
tor in which it had attained, not a starting-point merely, but its full
development.

The ontogenetic evidence that the vertebrate thyroid body was at
one time a pharyngeal gland opening just within the mouth, and
the discovery by Dohrn of rudimentary peripharyngeal grooves in
the torpedo embryo (Studien, ete., VIII, Plate H, Figs. 7f, 7g, 7h
and 7i), seem to me to be convincing proofs that the organs did not
have their starting-point in the habits of ammoccetes nor in any
degenerated fish, but that they arose in a lineal ancestor of the
selachians and of the higher vertebrates, which was also an ancestor
of the tunicates and cyclostomes.

Passing now from the biological relations of the system of the
endostyle to its homologies, we are told by Dohrn that it is equiva-
lent to two pairs of gill-slits; that these gill-slits were present and
functional in the fish-like ancestors of the cyclostomes and tunicates,
and that two of them, the mandibular clefts, moved downwards
and met on the ventral middle line to form the thyroid gland
or endostyle, while the endodermal portions of the others, the
spiracular clefts, lost their connection with the exterior and
became converted into the peripharyngeal grooves (Studien, etc.,
VII and VIII).

Homologies are expressions of genetic relationship, and Dohrn
tells us (p. 79) that they are valuable in phylogeny only as they
furnish us with the opportunity to pass from the consideration of
the structure of organs as they now exist, and of the functions of
these organs at the present time, to the consideration of conditions

5

194 W. Kk. BROOKS.

which have passed away ; to the study of the history of the modifi-
cations which have come between these structures and functions,
and those which we must attribute to the same organs at an earlier
genealogical stage.

I regard the structures which we find in the tunicates and in
ammocecetes as primitive, and as homologous with those which we
find in the jawed vertebrates ; and I have tried to trace the history
of the modifications which have come between these structures of
modern vertebrates and those which we must attribute to the same
organs at an earlier genealogical stage in the primitive history of
the ancestral pelagic chordata. The reader must judge of my success.

Let us now see what light Dohrn’s homology throws on the
history of these primitive modifications. He tells us (Studien, etc.,
VII, p. 47) that he will point out, further on, the significance of
the changes which have led to the fusion, on the middle line, of
structures which were originally paired; but I have been able to
find nothing more upon this point except the acknowledgment, on
page 63, that “TI frankly admit that I have at present no available
argument to bring the peculiar organization (of the ciliated grooves)
of ammoccetes from a.pair of imperforated (spiracular) gill-slits,
into accordance with the concept of change of function; and that
the origin of the slime-gland of ammoccetes from two ventrally
fused (mandibular) gill-slits must for the present remain an un-
solved problem.”

Whatever may be thought of my own view, it must be admitted
that Dohrn’s homology of the endostylic system with two pairs of
gill-slits has very little phylogenetic value, even when measured by
his own test: the opportunity it furnishes for passing from the
structure and functions of modern organs to the history of earlier
genealogical stages.

Dohrn’s memoirs upon the thyroid body are full of interesting
anatomical details, such as the similarity between the thyroid body
of the shark embryo and the true gill-slits, in their relations to the
cartilages, to the muscles and to the blood-supply (VII, p. 44) ;
and the resemblance between the peripharyngeal grooves of am-
mocoetes and the spiracular gills of selachians (VIII, p. 55); but
as he admits that the annelidian hypothesis leaves the origin of the
endostylic structures of tunicates an unsolved problem, our subject,

SALPA IN RELATION TO EVOLUTION OF LIFE. 195

the history of the tunicates, does not require us to enter into the
discussion of these complicated details of vertebrate morphology.

The considerations which I have presented will undoubtedly be
met by the assertion that while the simple and direct origin of the
tunicates seems plausible so long as we confine ourselves to these
animals alone, such a restricted view is unscientific. I shall no
doubt be told that we are forced by more fundamental evidence
to believe that the body cavity of the chordata is, in ultimate
analysis, a segmented enteroccel formed from a series of pairs of
gut-pouches, and that the simplicity of appendicularia cannot be
primitive, inasmuch as the ancestors of the tunicates once possessed
these complicated structures.

The first step to take in discussing this objection is to learn
whether there are any traces of gut-pouches in the tunicates.

Seeliger (p. 9) has given us a very minute and detailed account
of the history of the mesoderm in the clavelina embryo, and has
shown that it arises from two rows of endoderm cells which give
origin, in the tail, to the caudal muscles and, in the body, to free
mesoderm cells which multiply with great rapidity and wander
everywhere through the body cavity, which is bounded on one
side by the endodermal wall of the gut, and on the other by the
ectoderm.

He says emphatically (p. 128) that the mesoderm arises as two
totally unsegmented rows of cells, each forming a single layer ; that
the body cavity is not an enterocel, but a primary body cavity;
and that the ontogeny of the tunicate mesoderm gives no evidence
of derivation from paired pouches comparable to the ccelomic pouches
of amphioxus.

It is a rare thing for students of tunicate morphology to agree,
but in this case the phenomena are simple, and Davidoff (p. 16)
completely confirms Seeliger’s observations, so far as they bear upon
the question, by his own studies of clavelina and distaplia.

His account of the origin of the mesoderm differs from Seeliger’s
in only one minor point, which has no bearing upon the question
under consideration. Like Seeliger, he derives the mesoderm from
two rows of endoderm cells, but he says that these cells remain as
endoderm cells after they have given rise to the mesoderm, while
Seeliger states that they become converted into the mesoderm.

196 W. K. BROOKS.

In all other respects Davidoff’s observations are a complete
confirmation of Seeliger’s, for he says (600) that in distaplia the
mesoderm of the caudal region persists as a solid rudiment and
becomes the muscular layer of the tail, while elsewhere it breaks
up into wandering mesenchyma cells. “It is to be particularly
emphasized that in no part of the mesoderm is any trace of seg-
mentation to be discovered, and that there is not the least indication
of any cavity comparable to a myoceel. The embryonic history
of the mesoderm of distaplia cannot be referred back in any way
whatever to anything comparable to Hertwig’s conception of the
enteroccelomata.”

Of clavelina he says (607): “There is not even a transitory
division of the mesoderm into a somatopleur and a splanchnopleur.
Even where the mesoderm is two-layered, so that a parietal and
a visceral layer may be distinguished, there is no homology
between these layers and the bounding walls of the ccelom of the
enteroccelomata.”’

After reviewing all the literature on the subject, he gives as the
general result of his studies the statement (p. 622) that “the body
cavity of the ascidians lies between the two primary germ layers
and must be regarded as a blastoccel, which would be identical with
the segmentation cavity if this were not temporarily obliterated
during gastrulation by the contact of the ectoderm cells and endo-
derm cells.”

While the salpa-embryo is very complicated and unfavorable
for studying this question, my own observations, which have
already been described, seem to show that the body cavity of
salpa is, like that of clavelina and distaplia, a primary one,
fundamentally identical with the segmentation cavity, and that
the mesoderm arises as free mesenchyma cells derived from the
endodermal blastomeres.

The body cavity of the salpa-embryo is identical with the space
between the somatic and visceral layers of follicle cells, and while
there is a stage in which these two layers are in contact, the
follicular cavity is undoubtedly the same as the cavity shown at
16 in Plate XI, Fig. 3, of my memoir on salpa, and this is the
same as the space which is shown in Plate X, Fig. 3, between the
segmenting egg and the follicle.

SALPA IN RELATION TO EVOLUTION OF LIFE. 197

In the chapter on the significance of the salpa-embryo I have
given my reasons for believing that this space is homologous with
the segmentation cavity of more normal tunicate embryos, and if
this view be correct the body cavity of salpa is not an enteroccel
but a primary body cavity or blastoccel. The mesoderm of salpa
consists of free migrating cells, and the chamber of the heart is part
of the body cavity, so that these cells pass through it ; and while
salpa is a peculiarly unfavorable subject, my observations are in
complete accord with those which Seeliger and Davidoff have made
under simpler and more favorable conditions.

No student of the embryology of tunicates has ever described any
trace of a series of body cavities, and Kowalevsky, the discoverer
of the coelomic pouches of amphioxus, failed to find anything com-
parable to them in the tunicates, although the existence of a single
pair of enteroccels has been claimed by certain observers. Van
Beneden and Julin (Zool. Anzeiger, 4, 1881; Bull. Acad. Belg. (3)
7, 1884; Arch. Biol. 6, 1884) believe that the anterior portion of
the body cavity of ascidians arises as a pair of gut-pouches, and that
its mesoderm consists of a somatopleur and a splanchnopleur, but
Davidoff has shown by careful serial sections that this statement is
probably based upon erroneous observations.

Salensky holds (17, 460) that the mesoderm of the blastoderm of
pyrosoma consists of two symmetrically placed ecelomic pouches,
and that pyrosoma is, therefore, to be placed among the true entero-
celomata. The space between the vertebrate blastoderm and the
yolk is undoubtedly homologous with the enteron, but it is by no
means certain that this is the case in pyrosoma, where the food-yolk
is an independent acquisition ; nor do Salensky’s figures show, as
clearly as we might wish, that the two ccelomic vesicles open into
this space, and even if this is the case, we must remember that the
pyrosoma-embryo is very aberrant, and that the structure of its body
cavity may be a secondary adaptation to the presence of the yolk.
Taken alone it certainly is not enough to prove, without corrobora-
tion from other sources, that the body cavity of the tunicate is an
enteroccel.

The ontogeny and homology of the tunicate mesoderm have been
recently discussed at very great length by Seeliger (11, pp. 85-104
and pp. 126-131), by Davidoff (16, pp. 592-628), and by Salensky

198 W. K. BROOKS.

(17, pp. 456-462 and pp. 468-470 and 36-46), and as those who
wish can find in these papers an extended presentation of the com-
plicated and perplexing theory (?) of the mesoderm, I have attempted
to treat it very briefly.

The literature shows that there is no direct evidence whatever of
the existence, at any time in the history of the tunicates, of a meta-
meric series of coelomic pouches, and the supposed necessity for
believing that such a series existed in the primitive chordata is only
another aspect of the dogma that the metamerism of the vertebrates
must have been inherited from a primitive metameric ancestor.

If, as I believe, the metamerism of vertebrates is secondary, the
metamerism of the mesoderm and body cavity may have resulted
from the duplication of a single pair of coelomic pouches similar to
those of echinoderm larve, and it is quite conceivable that these
may have been acquired by the ancestors of the vertebrates after the
divergence of the tunicates.

If, however, future research should show that there is a pair of
gut-pouches in the embryo of appendicularia, or should prove in
some other way that the structures which Salensky describes are
true enteroccels inherited from an ancestral tunicate, such a dis-
covery, which is certainly among the possibilities, would be no
evidence that the primitive tunicate was the degenerated descendant
of an ancestor with metameric gut-pouches.

At present, however, the evidence all tends to show that the ances-
tors of the tunicates had no such structure, and that the presence
of ccelomic vesicles in pyrosoma is an adaptation to its peculiar mode
of development.

SALPA IN RELATION TO EVOLUTION OF LIFE. 199

As these chapters from my memoir on Salpa are passing through
the press for the second time, I take this opportunity to refer briefly
to a paper which has appeared in the meantime by Willey (Studies
on the Protochordata, Q. J. Mic. Se., Jan. 1893, pp. 317-360).

While the author seems to agree with me in rejecting Dohrn’s
view that the tunicates are degenerated fishes, he holds that the
ascidians exhibit, during their development, certain features of re-
semblance to other primitive chordata which are not exhibited by
appendicularia ; and he believes that these characteristics prove that
the ascidians are more closely related than appendicularia to these
protochordata.

The features upon which he lays most emphasis are these: I.
The endostyle is at first vertical and pre-oral; II. The organ of
fixation is a pre-oral lobe, and its cavity is the pre-oral or anterior
body cavity; and III. The first four primary stigmata of Ciona
intestinalis are developed from one primitive gill-slit ; the first and
fourth representing the two halves of one slit separated by the pre-
cocious development of a tongue-bar, while the third and fourth are
formed by constriction from the first and fourth. The six primary
stigmata are actually equivalent to three gill-slits, and the innumer-
able branchial stigmata of the adult are formed by subdivision of
the primary stigmata and not by new perforations.

T cannot believe that students of the tunicata will regard the
first and second of these arguments as entitled to the least considera-
tion. It has long been known that the endostyle of ascidian larvae
is at first vertical or at right angles to the long axis and it is so
figured and described by Seeliger, but the relative position of organs
is so much influenced by changes in other organs that we cannot
attribute a phylogenetic significance to the position of the endostyle.
The mouth is formed very late in ascidian larvee, and as the oral
region of the pharynx is rudimentary when the endostyle is formed,
this latter organ does not occupy its true position until the floor of
the pharynx is pushed downwards by the development of the oral
region. That this is the true explanation of its change of position
is shown by the fact that when the mouth continues to enlarge, as
it does in the nutritive zooids of doliolum, the endostyle is pushed
past its horizontal position until it finally becomes turned upside
down with its long axis again at right angles to the long axis of the

200 W. K. BROOKS.

body, but with its oral end below, instead of above as it is in as-
cidian larve. See, for example, Doliolum und sein Generations-
wechsel, by Grobbin. Plate IV., Fig. 19.

Seeliger’s carefully drawn figures of the organ of attachment of
the young clavelina (Jenaische Zeitschr., X VIII, 1885, Plate 4,
Figs. 40, 41, 42 and 44) show that it is at first represented by three
distinct and separate tracts of cylindrical ectodermal gland cells,
p- 36, ventral to the place of the future mouth, and it is only
necessary to read his minute account of its ontogeny, p. 36 and p. 52,
to see that its embryonic history is exactly what we should expect
it to be if it has been directly acquired as an organ of fixation.
If the phylogenetic history of the fixed ascidians has been as I
have pictured it, each successive stage in Seeliger’s account of the
ontogeny of the organ of fixation is a recapitulation of a useful
stage in its ancestral history, and to my mind, furnishes conclu-
sive evidence that the ascidians are the descendants of swimming
tunicates.

Willey’s observations add nothing to Seeliger’s excellent account
of the organ of fixation, and he gives no reason for holding that it
is a pre-oral lobe, except tliat “it contains loose mesenchyma cells
derived from the two lateral mesodermic bands.” This is equally
true of other parts of the body cavity, and there is no more evidence
that the organ of fixation is a pre-oral lobe, than there is that it is
homologous with the jaws and teeth of sharks.

If it isa pre-oral lobe it is a ventral one, and it cannot be com-
pared with the dorsal one of such protochordata as balanoglossus
and amphioxus.

Willey’s observation on the origin of the gill-slits of ciona are
most novel and interesting and they show that we may look for
most valuable results from the study of the subject in other tuni-
cates, but they are not sufficient in themselves to prove that what
he has found in one species is typical for all.

It seems to me that the author exaggerates our ignorance of the
subject, for while Seeliger’s descriptive account of their origin in
pyrosoma is very short (Jenaische Zeitschr., XXII, 1888, p. 623),
his figures show that he has traced their history most minutely,
step by step, both in surface views and by sections, and that, in the
buds of pyrosoma, each gill-slit is an independent perforation, and

SALPA IN RELATION TO EVOLUTION OF LIFE. 201

that they are formed in succession from in front backwards, as
transverse slits without any trace of a U-shaped fold. Salensky
has more recently (Zool. Jahrbiicher, IV, 1891) studied their
origin in the primary assidiozooids of pyrosoma, by sections, and
has confirmed Seeliger’s statement that each slit is an independent
perforation. We have pretty satisfactory evidence that this is true
of doliolum also, and Barrois’ observations indicate that it is true
of anchinia.

The supposed resemblance between the first pair of gill-slits of
ciona and a single horseshoe-shaped slit with a tongue bar, com-
parable to the gill-slits of amphioxus is not based upon direct
observation, however, for while Willey tells us in his summary,
p. 353, that “The four primary stigmata of Ciona intestinalis are
developed from one primitive gill-slit,’ we find, on page 322,
that “in the actual ontogeny ” the two primary gill-slits “arise by
independent perforations.”

THE ORIGIN OF THE CRASPEDOTA.

This section was intended, when written, to be part of my memoir
on the genus Salpa, but lack of room prevented its publication.

Another group of animals, which, notwithstanding the text-
books, I regard as a product of pelagic influences, is the craspedota,
or so called hydro-meduse.

The opinion that the hydroid jelly-fish is one of the polymor-
phic members of a hydroid-cormus, is, like the prevailing views
on the origin of the chordata and arthropods, a result from the
dogma that the aggregation of units into a compound whole
must necessarily be earlier in time than the high evolution of
the units.

The blastostyles and machopolyps of hydroids are, unquestion-
ably, modified hydranths, which have arisen, in a cormus, by di-
vision of labor, and our first impulse is to believe that the origin
of the sexual medusze must have been the same.

The persistency of this opinion is natural. There seem to be
many proofs that the remote ancestors of the hydro-medusze were
sessile, but I shall try to show that none of them are valid.

202 W. K. BROOKS.

It may be that the pelagic habit of the acraspeda is secondary,
and while the polyps, the craspedota and the acraspeda have un-
questionably had a common starting point in a free pelagic ancestor,
it is quite possible that the more immediate ancestors of the acras-
peda were fixed polyp-like inhabitants of the bottom.

As our opinion of the origin of the medusz has been evolved
under the erroneous notion that the craspedota and acraspeda are
very closely related and have travelled the same path in company,
it has been assumed that, so far as remote ancestry is concerned,
what is true of some meduse must be true of all.

If the remote ancestors of the acraspeda were inhabitants of the
bottom, and if their pelagic habit is a secondary acquisition, it is
natural to assume that this must be true of the craspedota as well,
although all authorities now agree that the swimming habit has
been independently acquired in the two groups, and that the acras-
peda teach us nothing of the phylogeny of the craspedota.

Of the four grand divisions of the craspedota, two, the narco-
meduse and trachomeduse are inhabitants of the open sea, seldom
found by the shore collector, and they are so active and irritable
and so easily destroyed that general collections of pelagic animals
contain few traces of their existence.

While the geryonids are not uncommon near the shore, it is only
by oceanic collecting and by research at sea that we can form a just
conception of the abundance and diversity and scientific importance
of the trachomeduse and narcomeduse; and thus the impression
has arisen that they are aberrant and exceptional, and that the more
familiar leptomedusse and anthomeduse and hydroids are the
characteristic and typical members of the group.

The hydroid-cormus is a most conspicuous and impressive feature
in the life of all our common hydro-medusz, and in many of them,
as in eudendrium, it is everything, and the medusoid structure is so
degenerated that its existence is a matter of philosophy, rather than
an observed fact.

All these factors have combined, during the historical growth
of zodlogy, to give to the hydroid cormus a fictitious value, and
this result has been promoted by the fact that many specialists have
devoted themselves to the systematic study of the hydra-stage alone,
to the neglect of the medusa-stage.

SALPA IN RELATION TO EVOLUTION OF LIFE. 2038

Still more impressive and significant is the fact that all craspe-
dota have a hydra-stage in their life history. Conclusive evidence
shows that they are all descended from a hydra-like ancestor.

This undoubted truth has been assumed to involve the belief that
this common ancestor was a fixed hydroid-cormus, although this
implication is by no means inevitable.

Every one now believes that the hydro-medusa is an expanded
and perfected expression of the hydroid type, and that it has been
evolved from a simple, hydra-like, starting point, but the writers
who have most clearly seen this homology, and who have demon-
strated it most conclusively have gone a step further, and have
assumed that the locomotor medusa-stage has been added on to the
life of hydroids for the purpose of distributing the species, and that
~ it has been evolved, according to the law of the division of labor,
by the gradual specialization of certain ones among the members of
a polymorphic hydroid community.

This view, which seems to have commended itself to all students,
and which seems to derive support from the well-known fact that
the blastostyles are, actually, nutritive hydranths which have been
secondarily differentiated and specialized, by division of labor, in
adaptation to the physiological needs of the cormus as a whole, this
view has become so firmly established that it is now regarded as a
settled and closed question.

I have no hope of effecting any change in views which are so
firmly rooted, but I shall now try to show that this established
opinion will not stand the test of searching examination.

From the earliest times the very ancient ciliated planula stage
has provided for the distribution of coelenterates, and there is no
need of other means of dispersal, nor is there any reason to think
that the distribution of the species is any less important now than
it has been in the past; yet one of the most remarkable and note-
worthy peculiarities of hydroids, is the pronounced tendency of the
medusa to degenerate and to lose its locomotor habit, and to become
a sessil and degraded gonophore. The view that the sessil medusa-
buds of hydroids, like hydractinia, are nascent medusze has, very
properly been abandoned, and all morphologists admit that they are
degenerated and that their sessil condition is secondary.

204 W. K. BROOKS.

I for one cannot believe that hydroid cormi acquired locomotor
medusee to distribute the species, and that then these meduse lost
their locomotor habit, and the medusoid structure which they had
slowly acquired, and that they sank into sessil, degenerated, bud-
like gonophores.

If this degeneration were rare and exceptional, or if it had taken
place only once or twice, or if it were an adaptation to any modern
change in the ocean it would not be so remarkable, but in reality it
it is one of the most noteworthy peculiarities of hydroids, and it has
taken place over and over again. In every group of hydroids, in
- the tubularians, in the campanularians, in the hydrocorals and in the
siphonophores, there are species or genera or larger groups with de-
generated gonophores, and there is ample evidence that this has not
been inherited from a common source, but that it has been acquired
again and again. MHydractinia, with its sessil degenerated gono-
phores is clearly related to podocoryne with its free medusa (dys-
morphosa). Tubularia with medusa buds is closely related to
corymorpha with its free medusa (steenstrupia). Laomed:ea stands
in the same relation to obelia; bougainvillia to heterocordyla and
so on, and by far the best example in the whole animal kingdom
of independent modification along parallel lines, is to be found in
the degeneration of the sexual locomotor meduse of compound
hydroids. Among the most widely distributed genera and species
are some of those in which this degeneracy is most complete, such
as the species of eudendrium, hydractinia, laomedea, cordylophora,
and hydra, and I think we may dismiss the idea that the medusa
was originally acquired for the purpose of distributing the species,
and with it the idea that it has been produced by division of labor,
which is disproved by the following facts.

The four great groups of craspedota, the narcomedusee, the tracho-
meduse, the anthomedusze and the leptomeduse agree with each
other in the possession of a gelatinous bell, a muscular sub-umbrella,
and velum, and in the presence of tentacles and sense-organs on the
bell margin, and we are therefore justified in assuming that these
common structural characteristics were acquired before these groups
diverged from each other; that they are older than the modern
genera and families, and older than the differentiation of the hydroids
into tubularians and campanularians.

SALPA IN RELATION TO EVOLUTION OF LIFE. 205

Among these are some, the narcomeduse and trachomeduse,
in which the hydra larva completes its individual development and
grows up into a sexual medusa, and others, the anthomeduse and
leptomedusze, in which there is polymorphic differentiation into
hydranths, and sexual meduse or gonophores. The presumption
in morphology, whenever there is nothing which demands the
contrary, is that the simple is older than the complex, and we should
therefore be justified in holding that the craspedota without poly-
morphism, where each hydra grows up into a medusa, are more
primitive than those with polymorphism, even if there were no
positive evidence, and I shall now show that this view is supported
by positive proof. Omitting hydra, which is too aberrant and too
much isolated to be available for comparison, the hydroids are,
almost without exception, larval or sexually immature, and the
reproductive persons are all meduse, or else gonophores which
show by their structure that they are degenerated meduse ; even
hydra itself is probably a hydroid with gonophores in an extreme
stage of degradation. The affinities of hydra are doubtful, however,
but in most cases we may state definitely that the hydroid is larval
in its sexual nature ; and all the hydroids which we know must be
traced back to an ancestor with a larval hydra stage and an adult
sexual medusa stage; for we must attribute to inheritance from a
common ancestor all they have in common except what can be shown
to be due to secondary modification.

The question of the origin of the craspedota then narrows itself
down to this; was the sessil mode of life, and the habit of forming
cormi which has resulted from it, acquired before or after the
evolution of the medusa ; was the hydra-like ancestor of the hydro-
medusz, a solitary pelagic animal as a sessil cormus.

In discussing this question we must keep in mind the following
considerations ; first, that the history of the polyps and acraspeda
has no bearing on the subject, since these animals belong to another
line of descent ; secondly, we have the very remarkable fact that
the sex of all the progeny of a hydroid egg is usually fixed from
the start, and is either exclusively male or exclusively female. The
egg of a hydractinia may give rise to hundreds of hydranths and
machopolyps before it produces blastostyles, and these grow up
before they produce medusa buds; the number of medusa buds

206 W. K. BROOKS.

which ultimately spring from the progeny of a single egg is very
great indeed and practically unlimited, and yet when these buds
produce reproductive elements they are of the same sex in all the
members of the colony.

This fact is very remarkable when we bear in mind the length
of the path from the egg to the sexual bud, and the complicated
character of the alternation of generations in hydroids.

One of the most constant results of a sedentary or fixed habit
of life is hermaphroditism ; and its influence in this direction is ‘so
potent that it has, in the barnacles, broken down one of the most
ancient and persistent of all the characteristics of animals, the
separation of the sexes of arthropods.

If the sessil habit of hydroids were primitive we should certainly
expect them to be hermaphrodites and the potential unisexuality
of each hydroid cormus would be unintelligible ; although it is
easily understood if the cormi have arisen by the asexual multipli-
cation of the larvz of unisexual locomotor adults, for in this case
each larva, before it began to form cormi, must have been potentially
either a male or a female, and all its progeny, produced by budding,
would naturally inherit the same sex, so long as sex remained po-
tential and was not called into activity, for there is no reason why
the sedentary habit of larvee should be followed by hermaphro-
ditism in locomotor adults, nor is there any reason why the sex of
larvee should be modified by a sessil life so long as sex remains
latent or undeveloped in the bodies of these larvee during the sessil
stage. It is remarkable that those species in which the gonophores
are most degenerated should adhere to the same law, but this fact
shows the firm hold which the separation of the sexes has taken
upon the organization of hydroids, and is to my mind one of the
most conclusive proofs that the sessil cormus is secondary.

In the third place we must remember that the craspedota with a
sessil hydroid cormus form only a part of the group; that the
trachomeduse in which, so far as we know, the hydra larva is
always a free, floating solitary animal, are numerous and diversified,
and that, while the life history of the various narcomeduse has
been greatly modified by parasitism, they may all be reduced to a
type in which each egg gives rise to a simple floating hydra-like
larva which grows up into an adult like the larva of a trachomedusa.

SALPA IN RELATION TO EVOLUTION OF LIFE. 207

The structure of an ordinary nutritive hydranth from a cormus,
with its funnel-shaped crown of numerous tentacles placed in a
circle around the mouth at the oral end of an elongated body, is
undoubtedly an adaptation to a life on the bottom, but there is
ample evidence that this hydroid type, while the most familiar one,
is not the most primitive, nor the one from which the veiled
meduse have been evolved.

Every one who has reared from the egg many species and indi-
viduals knows that the hydra which is formed from the planula,
shows a marked tendency to pass through a stage with first four
and then eight symmetrical, stiff tentacles, so placed that they do
not form a crown, since four point forwards and the other four
backwards in such a way that they radiate from the body like the
pseudopodia of a pelagic rhizopod.

In the simplest and most primitive of the hydroids, the Codonidee
or tubularias, this larva often has at first, a floating habit, and a
distinctive name, actinula, has been given to it on account of its
star-like shape.

The number of tentacles, like the number of arms in a star-fish,
varies somewhat, but they are radially arranged in two alternating
sets, and in the majority of the individuals there are four primary
radii.

The quadrate structure of young hydroids has been noted by
Haeckel, and he points out (Ueber die Individualitait des Thier-
korpers, Jena. Zeitschr., XII, 1878, p. 16) that while the number
of tentacles may, at first, have been indeterminate and variable, the
early establishment of the number four in the organization of the
ancestors of the acalephs is shown by the fact that in the young
hydra, as well as in many other hydro-polyps, and in the young
actinia and in many other acalephs there are only four primary
tentacles.

Haeckel also says that he regards the four-fold condition as primi-
tive for the medusz, and that all the six rayed and eight rayed
medusee are to be derived from a form with four rays, and that the
oral lobes, the primary radial canals and the primary marginal
tentacles all lie in the four primary radia. He also gives his reasons
for believing that the corals and ctenophores retain traces of this
primitive quadrate organization.

208 W. K. BROOKS.

Now what does this quadrate structure mean? No one who has
watched a pelagic hydro-medusa, as it first swims to the surface,
with contracted tentacles, by rapid vigorous pulsations of its bell,
and then sinks slowly down, permitting its tentacles to be drawn
out by the friction of the water, into long slender threads until they
form a living poisoned net, stretched like a spider’s web, to entangle
the floating organisms of the ocean ; no one who has witnessed this
can doubt the perfect adaptation of its structure to the conditions of
its pelagic life. It does not, like a bilateral organism, pursue its
prey in horizontal lines, but it captures it while sinking, and then
rises to the surface to repeat the process, and its most important
relations to space, are radial to the earth and to gravity while those
of bilateral animals are concentric with the earth’s surface.

We do not know enough about the biological relations of the
medusz to say what physiological superiority definite radiation has
over indefinite radiation, but I think we may feel confident that the
quadrate radiation of the hydro-meduse is an adaptation to a more
varied environment than that of a sessil hydra, and that it has
been acquired by free animals. In fact the quadrate structure of
sessil gonophores is generally accepted as evidence of their descent
from free medusz, and the advocates of the hypothesis of polymor-
phism attribute the acquisition of the quadrate structure to the free
locomotor life of the reproductive persons of the cormus.

If we admit this what shall we say of the quadrate structure of
young hydroids? We might see in the four primary tentacles the
accelerated acquisition of medusoid characters if the larval hydroid
were not in most cases separated from the medusa by the intervention
of numerous generations of hydranths with a great and variable
number of tentacles.

In the actinula of tubularia and in the cunina larva the proboscis
or manubrium is long and the body is short, so that the zone where
the tentacles are inserted is more nearly equatorial than it is in
ordinary hydranths; and the larva, which is ciliated, is thus more
perfectly adapted HE a pelagic life.

I shall now show that the quadrate floating hydra and not an
ordinary hydranth with an oral crown, is the ancestral type from
which hydro-medusz have arisen.

SALPA IN RELATION TO EVOLUTION OF LIFE. 209

The craspedota of modern times fall into two divergent groups,
which must have separated from their common stem very early.
In the one group are the trachomedusze, the anthomedusz and the
leptomedusx ; agreeing in the possession of circular and radial
chymiferous tubes, differentiated out of the simple larval digestive
cavity by the formation of areas of adhesion between its oral and
aboral walls ; and in the other we have the narcomedusz, in which
chymiferous tubes are absent and the digestive cavity is formed out
of that of the hydra-like larva in a much more simple and direct
way, as Wilson’s account (The structure of Cunocantha in the adult
and larval stages, by H. V. Wilson, Studies from the Biol. Lab.,
Johns Hopkins Univ.) which has recently been corroborated by
the researches of Maas (Ueber Bau und Entwicklung der Cuninen-
knospen. Zoologischen Jahrbiichern, V, 272) shows.

While the lines of descent represented by these two types are
quite distinct we cannot doubt their origin in a common ancestor
from which both the hydra-like larva, and the bell, sub-umbrella
velum, and marginal tentacles, sense organs and nervous system of
all the craspedota have been inherited, neither can we doubt that
this primitive veiled medusa was still more primitively derived
from a hydra-like form.

Now the fact that the sessil funnel-shaped hydra is restricted to
a part of the members of one of these primary subdivisions of the
craspedota, while the solitary star-shaped floating hydra-like larva
is found in both of them ; in the larva of cunina, as well as in that
of the geryonids, and in the actinula of tubularia, shows that the
ancestral form which the hydra-larva represents was of the latter
type, and that the more familiar hydranth is a secondary modifica-
tion of the more simple and ancient form.

In a more extensive discussion of this subject (The Life History
of the Hydromeduse, Mem. Boston Soc. Nat. Hist., 1886) I have
referred to the history of parasitism and its effects, in the narco-
medusee, in order to show how easily this primitive larval type
may become converted into a polymorphic cormus with alterna-
tion of generations. Nothing could have been further from my
mind than a belief that the cormi of ordinary hydroids have
been phylogenetically derived from the parasitic cormus of cunina
larvee.

6

210 W. K. BROOKS.

The knowledge of the true structure and mode of origin of the
adult cunina which we owe to Wilson’s researches, already referred
to, proves that the narcomeduse represent an independent and
very primitive branch of the craspedota and while I was ignorant
of this at the time my memoir was written, and while I then, with
all other naturalists, believed the narcomedusae to be much nearer
to the hydroids than they really are, I recognized, and thought that
I had clearly stated my opinion, that the cormus and the alternation
of generations in parasitic cuninas is a secondary and independent
acquisition, although I still think, as I did then, that the analogy
is most suggestive and instructive, inasmuch as it shows how easily
secondary complications, essentially similar to those of hydroid
cormi, may be grafted on to the structure of a simple hydra.

I regret that I am not able to refer to my memoir as I write,
for critics of my views have spoken as if I regarded the parasitic
cunina-cormus, as the ancestor of modern hydroids. Thus, for
example, Maas says, p. 295, that Metschnichoff and Brooks hold
that the narcomeduse are primitive forms, and that the alternation
of generations of the-hydroid polyps is derived from that of cunina.

As far as I myself am concerned, I certainly regard the unmodi-
fied cunina metamorphosis as primitive, but I regard the compli-
cations which have been introduced by parasitism, as analogous to,
hut quite independent of, the peculiarities of hydroids.

A general view of a subject so full of complicated details as the
life history of the craspedota is difficult, as illustrations and minute
accounts of specific instances are so necessary. I hope, however,
that, notwithstanding Lang’s verdict (Ueber die Einfluss der fest-
sitzenden Lebensweise, p. 159) that it is ‘‘the most improbable of
improbabilities” the view that the craspedota owe their origin to
pelagic influences will commend itself to those who are most
familiar with these animals.

It is hardly probable, however, that our modern craspedota are
primitively pelagic, for their size, the complication of their organi-
zation, and especially their great diversity of structure and habits,
would seem to show that while they have been evolved in the open
ocean, they are not the product of primitive and simple conditions,
but that they show the influence of the more intense and compli-
cated struggle for existence which has come from competition with

SALPA IN RELATION TO EVOLUTION OF LIFE. 211

animals which, having been evolved at the bottom, have then become
secondarily pelagic and have taken their improved structure back
into the open ocean.

Haeckel has shown that the highest representatives of the craspe-
dote stem, the Disconectze (velella, porpita, etc.) and the siphono-
phores, are pelagic productions, and that the Disconecte can be.
traced back to an ancestor similar to the Trachomeduse, while the
Siphonanthee or true siphonophores have arisen from simple Antho-
meduse. (Report on the Siphonophora collected by H. M. 8.
Challenger; and System der Siphonophoren, Jenaische Zeitschrift,
Vol. XXII, p. 1. 1888).

Both these groups are therefore pelagic in their history, and they
go back, not to ancestral hydroid cormi, but to ancestral meduse,
but they can hardly be primitively pelagic, and we must regard
them as the product of the more modern conditions of pelagic life.

The craspedota were undoubtedly represented in the primitive
pelagic fauna, by floating hydras with stiff radiating poisoned
pseudopodia-like tentacles, and also by small and simple veiled
meduse, but the higher forms of the group are probably more
modern, although there is paleontological evidence that they are
as old as the lower cambrian.

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TABLE OF CONTENTS.

Chapter VII. Salpa in its Relation to ihe Evolution of Life.
VIII. The Origin of the Chordata, considered in its Relation to Pela
Influences.

Egg of Salpa. ‘Chapter X. Sex in » ae
and Segmentation. Chapter XII. The Foetal Membranes.
The Follicle. Chapter XIV. The Ontogeny of the Organs of Salpa.

A

Part IV.—By M. M. Metcalf. The Eye and Sub-neural Gland of Salpa.

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