Under the guidance of the biogenetic law, and on the basis of the
evidence we have obtained, we now turn to the interesting task of
determining the series of man's animal ancestors. Phylogeny us a whole
is an inductive science. From the totality of the biological processes
in the life of plants, animals, and man we have gathered a confident
inductive idea that the whole organic population of our planet has
been moulded on a harmonious law of evolution. All the interesting
phenomena that we meet in ontogeny and paleontology, comparative
anatomy and dysteleology, the distribution and habits of
organisms--all the important general laws that we abstract from the
phenomena of these sciences, and combine in harmonious unity--are the
broad bases of our great biological induction.
But when we come to the application of this law, and seek to determine
with its aid the origin of the various species of organisms, we are
compelled to frame hypotheses that have essentially a DEDUCTIVE
character, and are inferences from the general law to particular
cases. But these special deductions are just as much justified and
necessitated by the rigorous laws of logic as the inductive
conclusions on which the whole theory of evolution is built. The
doctrine of the animal ancestry of the human race is a special
deduction of this kind, and follows with logical necessity from the
general inductive law of evolution.
I must point out at once, however, that the certainty of these
evolutionary hypotheses, which rest on clear special deductions, is
not always equally strong. Some of these inferences are now beyond
question; in the case of others it depends on the knowledge and the
competence of the inquirer what degree of certainty he attributes to
them. In any case, we must distinguish between the ABSOLUTE certainty
of the general (inductive) theory of descent and the RELATIVE
certainty of special (deductive) evolutionary hypotheses. We can never
determine the whole ancestral series of an organism with the same
confidence with which we hold the general theory of evolution as the
sole scientific explanation of organic modifications. The special
indication of stem-forms in detail will always be more or less
incomplete and hypothetical. This is quite natural. The evidence on
which we build is imperfect, and always will be imperfect; just as in
comparative philology.
The first of our documents, paleontology, is exceedingly incomplete.
We know that all the fossils yet discovered are only an insignificant
fraction of the plants and animals that have lived on our planet. For
every single species that has been preserved for us in the rocks there
are probably hundreds, perhaps thousands, of extinct species that have
left no trace behind them. This extreme and very unfortunate
incompleteness of the paleontological evidence, which cannot be
pointed out too often, is easily explained. It is absolutely
inevitable in the circumstances of the fossilisation of organisms. It
is also due in part to the incompleteness of our knowledge in this
branch. It must be borne in mind that the great majority of the
stratified rocks that compose the crust of the earth have not yet been
opened. We have only a few specimens of the innumerable fossils that
are buried in the vast mountain ranges of Asia and Africa. Only a part
of Europe and North America has been investigated carefully. The whole
of the fossils known to us certainly do not amount to a hundredth part
of the remains that are really buried in the crust of the earth. We
may, therefore, look forward to a rich harvest in the future as
regards this science. However, our paleontological evidence will (for
reasons that I have fully explained in the sixteenth chapter of the
History of Creation) always be defective.
The second chief source of evidence, ontogeny, is not less incomplete.
It is the most important source of all for special phylogeny; but it
has great defects, and often fails us. We must, above all, clearly
distinguish between palingenetic and cenogenetic phenomena. We must
never forget that the laws of curtailed and disturbed heredity often
make the original course of development almost unrecognisable. The
recapitulation of phylogeny by ontogeny is only fairly complete in a
few cases, and is never wholly complete. As a rule, it is precisely
the earliest and most important embryonic stages that suffer most from
alteration and condensation. The earlier embryonic forms have had to
adapt themselves to new circumstances, and so have been modified. The
struggle for existence has had just as profound an influence on the
freely moving and still immature young forms as on the adult forms.
Hence in the embryology of the higher animals, especially,
palingenesis is much restricted by cenogenesis; it is to-day, as a
rule, only a faded and much altered picture of the original evolution
of the animal's ancestors. We can only draw conclusions from the
embryonic forms to the stem-history with the greatest caution and
discrimination. Moreover, the embryonic development itself has only
been fully studied in a few species.
Finally, the third and most valuable source of evidence, comparative
anatomy, is also, unfortunately, very imperfect; for the simple reason
that the whole of the living species of animals are a mere fraction of
the vast population that has dwelt on our planet since the beginning
of life. We may confidently put the total number of these at more than
a million species. The number of animals whose organisation has been
studied up to the present in comparative anatomy is proportionately
very small. Here, again, future research will yield incalculable
treasures. But, for the present, in view of this patent incompleteness
of our chief sources of evidence, we must naturally be careful not to
lay too much stress in human phylogeny on the particular animals we
have studied, or regard all the various stages of development with
equal confidence as stem-forms.
In my first efforts to construct the series of man's ancestors I drew
up a list of, at first ten, afterwards twenty to thirty, forms that
may be regarded more or less certainly as animal ancestors of the
human race, or as stages that in a sense mark off the chief sections
in the long story of evolution from the unicellular organism to man.
Of these twenty to thirty stages, ten to twelve belong to the older
group of the Invertebrates and eighteen to twenty to the younger
division of the Vertebrates.
In approaching, now, the difficult task of establishing the
evolutionary succession of these thirty ancestors of humanity since
the beginning of life, and in venturing to lift the veil that covers
the earliest secrets of the earth's history, we must undoubtedly look
for the first living things among the wonderful organisms that we call
the Monera; they are the simplest organisms known to us--in fact, the
simplest we can conceive. Their whole body consists merely of a simple
particle or globule of structureless plasm or plasson. The discoveries
of the last four decades have led us to believe with increasing
certainty that wherever a natural body exhibits the vital processes of
nutrition, reproduction, voluntary movement, and sensation, we have
the action of a nitrogenous carbon-compound of the chemical group of
the albuminoids; this plasm (or protoplasm) is the material basis of
all vital functions. Whether we regarded the function, in the monistic
sense, as the direct action of the material substratum, or whether we
take matter and force to be distinct things in the dualistic sense, it
is certain that we have not as yet found any living organism in which
the exercise of the vital functions is not inseparably bound up with
plasm.
The soft slimy plasson of the body of the moneron is generally called
"protoplasm," and identified with the cellular matter of the ordinary
plant and animal cells. But we must, to be accurate, distinguish
between the plasson of the cytodes and the protoplasm of the cells.
This distinction is of the utmost importance for the purposes of
evolution. As I have often said, we must recognise two different
stages of development in these "elementary organisms," or plastids
("builders"), that represent the ultimate units of organic
individuality. The earlier and lower stage are the unnucleated
cytodes, the body of which consists of only one kind of albuminous
matter--the homogeneous plasson or "formative matter." The later and
higher stage are the nucleated cells, in which we find a
differentiation of the original plasson into two different formative
substances--the caryoplasm of the nucleus and the cytoplasm of the
body of the cell (cf. Chapter 1.6.)
(FIGURE 2.226. Chroococcus minor (Nageli), magnified 1500 times. A
phytomoneron, the globular plastids of which secrete a gelatinous
structureless membrane. The unnucleated globule of plasm (bluish-green
in colour) increases by simple cleavage (a to d).
The Monera are permanent cytodes. Their whole body consists of soft,
structureless plasson. However carefully we examine it with our finest
chemical reagents and most powerful microscopes, we can find no
definite parts or no anatomic structure in it. Hence, the Monera are
literally organisms without organs; in fact, from the philosophic
point of view they are not organisms at all, since they have no
organs. They can only be called organisms in the sense that they are
capable of the vital functions of nutrition, reproduction, sensation,
and movement. If we were to try to imagine the simplest possible
organism, we should frame something like the moneron.
The Monera that we find to-day in various forms fall into two groups
according to the nature of their nutrition--the Phytomonera and the
Zoomonera; from the physiological point of view, the former are the
simplest specimens of the plant (phyton) kingdom, and the latter of
the animal (zoon) world. The Phytomonera, especially in their simplest
form, the Chromacea (Phycochromacea or Cyanophycea), are the most
primitive and the oldest of living organisms. The typical genus
Chroococcus (Figure 2.226) is represented by several fresh-water
species, and often forms a very delicate bluish-green deposit on
stones and wood in ponds and ditches. It consists of round, light
green particles, from 1/7000 to 1/2500 of an inch in diameter.
(FIGURE 2.227. Aphanocapsa primordialis (Nageli), magnified 1000
times. A phytomoneron, the round plastids of which (bluish-green in
colour) secrete a shapeless gelatinous mass; in this the unnucleated
cytodes increase continually by simple cleavage.)
The whole life of these homogeneous globules of plasm consists of
simple growth and reproduction by cleavage. When the tiny particle has
reached a certain size by the continuous assimilation of inorganic
matter, it divides into two equal halves, by a constriction in the
middle. The two daughter-monera that are thus formed immediately begin
a similar vital process. It is the same with the brown Procytella
primordialis (formerly called the Protococcus marinus); it forms large
masses of floating matter in the arctic seas. The tiny plasma-globules
of this species are of a greenish-brown colour, and have a diameter of
1/10,000 to 1/5000 of an inch. There is no membrane discoverable in
the simplest Chroococcacea, but we find one in other members of the
same family; in Aphanocapsa (Figure 2.227) the enveloping membranes of
the social plastids combine; in Gloecapsa they are retained through
several generations, so that the little plasma-globules are enfolded
in many layers of membrane.
Next to the Chromacea come the Bacteria, which have been evolved from
them by the remarkable change in nutrition which gives us the simple
explanation of the differentiation of plant and animal in the protist
kingdom. The Chromacea build up their plasm directly from inorganic
matter; the Bacteria feed on organic matter. Hence, if we logically
divide the protist kingdom into plasma-forming Protophyta and
plasma-consuming Protozoa, we must class the Bacteria with the latter;
it is quite illogical to describe them--as is still often done--as
Schizomycetes, and class them with the true fungi. The Bacteria, like
the Chromacea, have no nucleus. As is well-known, they play an
important part in modern biology as the causes of fermentation and
putrefaction, and of tuberculosis, typhus, cholera, and other
infectious diseases, and as parasites, etc. But we cannot linger now
to deal with these very interesting features; the Bacteria have no
relation to man's genealogical tree.
We may now turn to consider the remarkable Protamoeba, or unnucleated
Amoeba. I have, in the first volume, pointed out the great importance
of the ordinary Amoeba in connection with several weighty questions of
general biology. The tiny Protamoebae, which are found both in fresh
and salt water, have the same unshapely form and irregular movements
of their simple naked body as the real Amoebae; but they differ from
them very materially in having no nucleus in their cell-body. The
short, blunt, finger-like processes that are thrust out at the surface
of the creeping Protamoeba serve for getting food as well as for
locomotion. They multiply by simple cleavage (Figure 2.228).
(FIGURE 2.228. A moneron (Protamoeba) in the act of reproduction. A
The whole moneron, moving like an ordinary amoeba by thrusting out
changeable processes. B It divides into two halves by a constriction
in the middle. C The two halves separate, and each becomes an
independent individual. (Highly magnified.))
The next stage to the simple cytode-forms of the Monera in the
genealogy of mankind (and all other animals) is the simple cell, or
the most rudimentary form of the cell which we find living
independently to-day as the Amoeba. The earliest process of inorganic
differentiation in the structureless body of the Monera led to its
division into two different substances--the caryoplasm and the
cytoplasm. The caryoplasm is the inner and firmer part of the cell,
the substance of the nucleus. The cytoplasm is the outer and softer
part, the substance of the body of the cell. By this important
differentiation of the plasson into nucleus and cell-body, the
organised cell was evolved from the structureless cytode, the
nucleated from the unnucleated plastid. That the first cells to appear
on the earth were formed from the Monera by such a differentiation
seems to us the only possible view in the present condition of
science. We have a direct instance of this earliest process of
differentiation to-day in the ontogeny of many of the lower Protists
(such as the Gregarinae).
The unicellular form that we have in the ovum has already been
described as the reproduction of a corresponding unicellular
stem-form, and to this we have ascribed the organisation of an Amoeba
(cf. Chapter 1.6). The irregular-shaped Amoeba, which we find living
independently to-day in our fresh and salt water, is the least
definite and the most primitive of all the unicellular Protozoa
(Figure 1.16). As the unripe ova (the protova that we find in the
ovaries of animals) cannot be distinguished from the common Amoebae,
we must regard the Amoeba as the primitive form that is reproduced in
the embryonic stage of the amoeboid ovum to-day, in accordance with
the biogenetic law. I have already pointed out, in proof of the
striking resemblance of the two cells, that the ova of many of the
sponges were formerly regarded as parasitic Amoebae (Figure 1.18).
Large unicellular organisms like the Amoebae were found creeping about
inside the body of the sponge, and were thought to be parasites. It
was afterwards discovered that they were really the ova of the sponge
from which the embryos were developed. As a matter of fact, these
sponge-ova are so much like many of the Amoebae in size, shape, the
character of their nucleus, and movement of the pseudopodia, that it
is impossible to distinguish them without knowing their subsequent
development.
Our phylogenetic interpretation of the ovum, and the reduction of it
to some ancient amoeboid ancestral form, supply the answer to the old
problem: "Which was first, the egg or the chick?" We can now give a
very plain answer to this riddle, with which our opponents have often
tried to drive us into a corner. The egg came a long time before the
chick. We do not mean, of course, that the egg existed from the first
as a bird's egg, but as an indifferent amoeboid cell of the simplest
character. The egg lived for thousands of years as an independent
unicellular organism, the Amoeba. The egg, in the modern physiological
sense of the word, did not make its appearance until the descendants
of the unicellular Protozoon had developed into multicellular animals,
and these had undergone sexual differentiation. Even then the egg was
first a gastraea-egg, then a platode-egg, then a vermalia-egg, and
chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg,
reptile-egg, and finally bird's egg. The bird's egg we have experience
of daily is a highly complicated historical product, the result of
countless hereditary processes that have taken place in the course of
millions of years.
The earliest ancestors of our race were simple Protophyta, and from
these our protozoic ancestors were developed afterwards. From the
morphological point of view both the vegetal and the animal Protists
were simple organisms, individualities of the first order, or
plastids. All our later ancestors are complex organisms, or
individualities of a higher order--social aggregations of a plurality
of cells. The earliest of these, the Moraeada, which represent the
third stage in our genealogy, are very simple associations of
homogeneous, indifferent cells--undifferentiated colonies of social
Amoebae or Infusoria. To understand the nature and origin of these
protozoa-colonies we need only follow step by step the first embryonic
products of the stem-cell. In all the Metazoa the first embryonic
process is the repeated cleavage of the stem-cell, or first
segmentation-cell (Figure 2.229). We have already fully considered
this process, and found that all the different forms of it may be
reduced to one type, the original equal or primordial segmentation
(cf. Chapter 1.8). In the genealogical tree of the Vertebrates this
palingenetic form of segmentation has been preserved in the Amphioxus
alone, all the other Vertebrates having cenogenetically modified forms
of cleavage. In any case, the latter were developed from the former,
and so the segmentation of the ovum in the Amphioxus has a great
interest for us (cf. Figure 1.38). The outcome of this repeated
cleavage is the formation of a round cluster of cells, composed of
homogeneous, indifferent cells of the simplest character (Figure
2.230). This is called the morula (= mulberry-embryo) on account of
its resemblance to a mulberry or blackberry.
(FIGURE 2.229. Original or primordial ovum-cleavage. The stem-cell or
cytula, formed by fecundation of the ovum, divides by repeated regular
cleavage first into two (A), then four (B), then eight (C), and
finally a large number of segmentation-cells (D).
FIGURE 2.230. Morula, or mulberry-shaped embryo.)
It is clear that this morula reproduces for us to-day the simple
structure of the multicellular animal that succeeded the unicellular
amoeboid form in the early Laurentian period. In accordance with the
biogenetic law, the morula recalls the ancestral form of the Moraea,
or simple colony of Protozoa. The first cell-communities to be formed,
which laid the early foundation of the higher multicellular body, must
have consisted of homogeneous and simple amoeboid cells. The oldest
Amoebae lived isolated lives, and even the amoeboid cells that were
formed by the segmentation of these unicellular organisms must have
continued to live independently for a long time. But gradually small
communities of Amoebae arose by the side of these eremitical Protozoa,
the sister-cells produced by cleavage remaining joined together. The
advantages in the struggle for life which these communities had over
the isolated cells favoured their formation and their further
development. We find plenty of these cell-colonies or communities
to-day in both fresh and salt water. They belong to various groups
both of the Protophyta and Protozoa.
To have some idea of those ancestors of our race that succeeded
phylogenetically to the Moraeada, we have only to follow the further
embryonic development of the morula. We then see that the social cells
of the round cluster secrete a sort of jelly or a watery fluid inside
their globular body, and they themselves rise to the surface of it
(Figure 1.29 F, G). In this way the solid mulberry-embryo becomes a
hollow sphere, the wall of which is composed of a single layer of
cells. We call this layer the blastoderm, and the sphere itself the
blastula, or embryonic vesicle.
This interesting blastula is very important. The conversion of the
morula into a hollow ball proceeds on the same lines originally in the
most diverse stems--as, for instance, in many of the zoophytes and
worms, the ascidia, many of the echinoderms and molluscs, and in the
amphioxus. Moreover, in the animals in which we do not find a real
palingenetic blastula the defect is clearly due to cenogenetic causes,
such as the formation of food-yelk and other embryonic adaptations. We
may, therefore, conclude that the ontogenetic blastula is the
reproduction of a very early phylogenetic ancestral form, and that all
the Metazoa are descended from a common stem-form, which was in the
main constructed like the blastula. In many of the lower animals the
blastula is not developed within the foetal membranes, but in the open
water. In those cases each blastodermic cell begins at an early stage
to thrust out one or more mobile hair-like processes; the body swims
about by the vibratory movement of these lashes or whips (Figure 1.29
F).
We still find, both in the sea and in fresh water, various kinds of
primitive multicellular organisms that substantially resemble the
blastula in structure, and may be regarded in a sense as permanent
blastula-forms--hollow vesicles or gelatinous balls, with a wall
composed of a single layer of ciliated homogeneous cells. There are
"blastaeads" of this kind even among the Protophyta--the familiar
Volvocina, formerly classed with the infusoria. The common Volvox
globator is found in the ponds in the spring--a small, green,
gelatinous globule, swimming about by means of the stroke of its
lashes, which rise in pairs from the cells on its surface. In the
similar Halosphaera viridis also, which we find in the marine plancton
(floating matter), a number of green cells form a simple layer at the
surface of the gelatinous ball; but in this case there are no cilia.
Some of the infusoria of the flagellata-class (Signura, Magosphaera,
etc.) are similar in structure to these vegetal clusters, but differ
in their animal nutrition; they form the special group of the
Catallacta. In September, 1869, I studied the development of one of
these graceful animals on the island of Gis-Oe, off the coast of
Norway (Magosphaera planula), Figures 2.231 and 2.232). The
fully-formed body is a gelatinous ball, with its wall composed of
thirty-two to sixty-four ciliated cells; it swims about freely in the
sea. After reaching maturity the community is dissolved. Each cell
then lives independently for some time, grows, and changes into a
creeping amoeba. This afterwards contracts, and clothes itself with a
structureless membrane. The cell then looks just like an ordinary
animal ovum. When it has been in this condition for some time the cell
divides into two, four, eight, sixteen, thirty-two, and sixty-four
cells. These arrange themselves in a round vesicle, thrust out
vibratory lashes, burst the capsule, and swim about in the same
magosphaera-form with which we started. This completes the life-circle
of the remarkable and instructive animal.
If we compare these permanent blastulae with the free-swimming
ciliated larvae or blastulae, with similar construction, of many of
the lower animals, we can confidently deduce from them that there was
a very early and long-extinct common stem-form of substantially the
same structure as the blastula. We may call it the Blastaea. Its body
consisted, when fully formed, of a simple hollow ball, filled with
fluid or structureless jelly, with a wall composed of a single stratum
of ciliated cells. There were probably many genera and species of
these blastaeads in the Laurentian period, forming a special class of
marine protists.
It is an interesting fact that in the plant kingdom also the simple
hollow sphere is found to be an elementary form of the multicellular
organism. At the surface and below the surface (down to a depth of
2000 yards) of the sea there are green globules swimming about, with a
wall composed of a single layer of chlorophyll-bearing cells. The
botanist Schmitz gave them the name of Halosphaera viridis in 1879.
The next stage to the Blastaea, and the sixth in our genealogical
tree, is the Gastraea that is developed from it. As we have already
seen, this ancestral form is particularly important. That it once
existed is proved with certainty by the gastrula, which we find
temporarily in the ontogenesis of all the Metazoa (Figure 1.29 J, K).
As we saw, the original, palingenetic form of the gastrula is a round
or oval uni-axial body, the simple cavity of which (the primitive gut)
has an aperture at one pole of its axis (the primitive mouth). The
wall of the gut consists of two strata of cells, and these are the
primary germinal layers, the animal skin-layer (ectoderm) and vegetal
gut-layer (entoderm).
The actual ontogenetic development of the gastrula from the blastula
furnishes sound evidence as to the phylogenetic origin of the Gastraea
from the Blastaea. A pit-shaped depression appears at one side of the
spherical blastula (Figure 1.29 H). In the end this invagination goes
so far that the outer or invaginated part of the blastoderm lies close
on the inner or non-invaginated part (Figure 1.29 J). In explaining
the phylogenetic origin of the gastraea in the light of this
ontogenetic process, we may assume that the one-layered cell-community
of the blastaea began to take in food more largely at one particular
part of its surface. Natural selection would gradually lead to the
formation of a depression or pit at this alimentary spot on the
surface of the ball. The depression would grow deeper and deeper. In
time the vegetal function of taking in and digesting food would be
confined to the cells that lined this hole; the other cells would see
to the animal functions of locomotion, sensation, and protection. This
was the first division of labour among the originally homogeneous
cells of the blastaea.
(FIGURE 2.231. The Norwegian Magosphaera planula, swimming about by
means of the lashes or cilia at its surface.
FIGURE 2.232. Section of Magosphaera planula, showing how the
pear-shaped cells in the centre of the gelatinous ball are connected
by a fibrous process. Each cell has a contractile vacuole as well as a
nucleus.)
The effect, then, of this earliest histological differentiation was to
produce two different kinds of cells--nutritive cells in the
depression and locomotive cells on the surface outside. But this
involved the severance of the two primary germinal layers--a most
important process. When we remember that even man's body, with all its
various parts, and the body of all the other higher animals, are built
up originally out of these two simple layers, we cannot lay too much
stress on the phylogenetic significance of this gastrulation. In the
simple primitive gut or gastric cavity of the gastrula and its
rudimentary mouth we have the first real organ of the animal frame in
the morphological sense; all the other organs were developed
afterwards from these. In reality, the whole body of the gastrula is
merely a "primitive gut." I have shown already (Chapters 1.8 and 1.9)
that the two-layered embryos of all the Metazoa can be reduced to this
typical gastrula. This important fact justifies us in concluding, in
accordance with the biogenetic law, that their ancestors also were
phylogenetically developed from a similar stem-form. This ancient
stem-form is the gastraea.
The gastraea probably lived in the sea during the Laurentian period,
swimming about in the water by means of its ciliary coat much as free
ciliated gastrulae do to-day. Probably it differed from the existing
gastrula only in one essential point, though extinct millions of years
ago. We have reason, from comparative anatomy and ontogeny, to believe
that it multiplied by sexual generation, not merely asexually (by
cleavage, gemmation, and spores), as was no doubt the case with the
earlier ancestors. Some of the cells of the primary germ-layers
probably became ova and others fertilising sperm. We base these
hypotheses on the fact that we do to-day find the simplest form of
sexual reproduction in some of the living gastraeads and other lower
animals, especially the sponges.
The fact that there are still in existence various kinds of
gastraeads, or lower Metazoa with an organisation little higher than
that of the hypothetical gastraea, is a strong point in favour of our
theory. There are not very many species of these living gastraeads;
but their morphological and phylogenetic interest is so great, and
their intermediate position between the Protozoa and Metazoa so
instructive, that I proposed long ago (1876) to make a special class
of them. I distinguished three orders in this class--the Gastremaria,
Physemaria, and Cyemaria (or Dicyemida). But we might also regard
these three orders as so many independent classes in a primitive
gastraead stem.
The Gastremaria and Cyemaria, the chief of these living gastraeads,
are small Metazoa that live parasitically inside other Metazoa, and
are, as a rule, 1/50 to 1/25 of an inch long, often much less (Figure
2.233, 1 to 15). Their soft body, devoid of skeleton, consists of two
simple strata of cells, the primary germinal layers; the outer of
these is thickly clothed with long hair-like lashes, by which the
parasites swim about in the various cavities of their host. The inner
germinal layer furnishes the sexual products. The pure type of the
original gastrula (or archigastrula, Figure 1.29 I) is seen in the
Pemmatodiscus gastrulaceus, which Monticelli discovered in the
umbrella of a large medusa (Pilema pulmo) in 1895; the convex surface
of this gelatinous umbrella was covered with numbers of clear
vesicles, of 1/25 to 1/8 inch in diameter, in the fluid contents of
which the little parasites were swimming. The cup-shaped body of the
Pemmatodiscus (Figure 2.233, 1) is sometimes rather flat, and shaped
like a hat or cone, at other times almost curved into a semi-circle.
The simple hollow of the cup, the primitive gut (g), has a narrow
opening (o). The skin layer (e) consists of long slender cylindrical
cells, which bear long vibratory hairs; it is separated by a thin
structureless, gelatinous plate (f) from the visceral or gut layer
(i), the prismatic cells of which are much smaller and have no cilia.
Pemmatodiscus propagates asexually, by simple longitudinal cleavage;
on this account it has recently been regarded as the representative of
a special order of gastraeads (Mesogastria).
Probably a near relative of the Pemmatodiscus is the Kunstleria
Gruveli (Figure 2.233, 2). It lives in the body-cavity of Vermalia
(Sipunculida), and differs from the former in having no lashes either
on the large ectodermic cells (e) or the small entodermic (i); the
germinal layers are separated by a thick, cup-shaped, gelatinous mass,
which has been called the "clear vesicle" (f). The primitive mouth is
surrounded by a dark ring that bears very strong and long vibratory
lashes, and effects the swimming movements.
Pemmatodiscus and Kunstleria may be included in the family of the
Gastremaria. To these gastraeads with open gut are closely related the
Orthonectida (Rhopalura, Figure 2.233, 3 to 5). They live
parasitically in the body-cavity of echinoderms (Ophiura) and
vermalia; they are distinguished by the fact that their primitive
gut-cavity is not empty, but filled with entodermic cells, from which
the sexual cells are developed. These gastraeads are of both sexes,
the male (Figure 1.3) being smaller and of a somewhat different shape
from the oval female (Figure 1.4).
The somewhat similar Dicyemida (Figure 1.6) are distinguished from the
preceding by the fact that their primitive gut-cavity is occupied by a
single large entodermic cell instead of a crowded group of sexual
cells. This cell does not yield sexual products, but afterwards
divides into a number of cells (spores), each of which, without being
impregnated, grows into a small embryo. The Dicyemida live
parasitically in the body-cavity, especially the renal cavities, of
the cuttle-fishes. They fall in several genera, some of which are
characterised by the possession of special polar cells; the body is
sometimes roundish, oval, or club-shaped, at other times long and
cylindrical. The genus Conocyema (Figures 1.7 to 1.15) differs from
the ordinary Dicyema in having four polar pimples in the form of a
cross, which may be incipient tentacles.
The classification of the Cyemaria is much disputed; sometimes they
are held to be parasitic infusoria (like the Opalina), sometimes
platodes or vermalia, related to the suctorial worms or rotifers, but
having degenerated through parasitism. I adhere to the
phylogenetically important theory that I advanced in 1876, that we
have here real gastraeads, primitive survivors of the common
stem-group of all the Metazoa. In the struggle for life they have
found shelter in the body-cavity of other animals.
(FIGURE 2.233. Modern gastraeads.
Figure 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal
section.
Figure 2. Kunstleria gruveli (Delage), in longitudinal section. (From
Kunstler and Gruvel.)
Figures 3 to 5. Rhopalura Giardi (Julin): Figure 3 male, Figure 4
female, Figure 5 planula.
Figure 6. Dicyema macrocephala (Van Beneden).
Figures 7 to 15. Conocyema polymorpha (Van Beneden): Figure 7 the
mature gastraead, Figures 8 to 15 its gastrulation. d primitive gut, o
primitive mouth, e ectoderm, i entoderm, f gelatinous plate between e
and i (supporting plate, blastocoel).)
The small Coelenteria attached to the floor of the sea that I have
called the Physemaria (Haliphysema and Gastrophysema) probably form a
third order (or class) of the living gastraeads. The genus Haliphysema
(Figures 2.234 and 2.235) is externally very similar to a large
rhizopod (described by the same name in 1862) of the family of the
Rhabdamminida, which was at first taken for a sponge. In order to
avoid confusion with these, I afterwards gave them the name of
Prophysema. The whole mature body of the Prophysema is a simple
cylindrical or oval tube, with a two-layered wall. The hollow of the
tube is the gastric cavity, and the upper opening of it the mouth
(Figure 2.235 m). The two strata of cells that form the wall of the
tube are the primary germinal layers. These rudimentary zoophytes
differ from the swimming gastraeads chiefly in being attached at one
end (the end opposite to the mouth) to the floor of the sea.
In Prophysema the primitive gut is a simple oval cavity, but in the
closely related Gastrophysema it is divided into two chambers by a
transverse constriction; the hind and smaller chamber above furnishes
the sexual products, the anterior one being for digestion.
The simplest sponges (Olynthus, Figure 2.238) have the same
organisation as the Physemaria. The only material difference between
them is that in the sponge the thin two-layered body-wall is pierced
by numbers of pores. When these are closed they resemble the
Physemaria. Possibly the gastraeads that we call Physemaria are only
olynthi with the pores closed. The Ammoconida, or the simple tubular
sand-sponges of the deep-sea (Ammolynthus, etc.), do not differ from
the gastraeads in any important point when the pores are closed. In my
Monograph on the Sponges (with sixty plates) I endeavoured to prove
analytically that all the species of this class can be traced
phylogenetically to a common stem-form (Calcolynthus).
(FIGURES 2.234 AND 2.235. Prophysema primordiale, a living gastraead.
FIGURE 2.234. The whole of the spindle-shaped animal (attached below
to the floor of the sea).
FIGURE 2.235. The same in longitudinal section. The primitive gut (d)
opens above at the primitive mouth (m). Between the ciliated cells (g)
are the amoeboid ova (e). The skin-layer (h) is encrusted with grains
of sand below and sponge-spicules above.
FIGURES 2.236 TO 2.237. Ascula of gastrophysema, attached to the floor
of the sea. Figure 2.236 external view, 2.237 longitudinal section. g
primitive gut, o primitive mouth, i visceral layer, e cutaneous layer.
(Diagram.)
FIGURE 2.238. Olynthus, a very rudimentary sponge. A piece cut away in
front.)
The lowest form of the Cnidaria is also not far removed from the
gastraeads. In the interesting common fresh-water polyp (Hydra) the
whole body is simply an oval tube with a double wall; only in this
case the mouth has a crown of tentacles. Before these develop the
hydra resembles an ascula (Figures 2.236 and 2.237). Afterwards there
are slight histological differentiations in its ectoderm, though the
entoderm remains a single stratum of cells. We find the first
differentiation of epithelial and stinging cells, or of muscular and
neural cells, in the thick ectoderm of the hydra.
In all these rudimentary living coelenteria the sexual cells of both
kinds--ova and sperm cells--are formed by the same individual; it is
possible that the oldest gastraeads were hermaphroditic. It is clear
from comparative anatomy that hermaphrodism--the combination of both
kinds of sexual cells in one individual--is the earliest form of
sexual differentiation; the separation of the sexes (gonochorism) was
a much later phenomenon. The sexual cells originally proceeded from
the edge of the primitive mouth of the gastraead.