The gastraea theory has now convinced us that all the Metazoa or
multicellular animals can be traced to a common stem-form, the
Gastraea. In accordance with the biogenetic law, we find solid proof
of this in the fact that the two-layered embryos of all the Metazoa
can be reduced to a primitive common type, the gastrula. Just as the
countless species of the Metazoa do actually develop in the individual
from the simple embryonic form of the gastrula, so they have all
descended in past time from the common stem-form of the Gastraea. In
this fact, and the fact we have already established that the Gastraea
has been evolved from the hollow vesicle of the one-layered Blastaea,
and this again from the original unicellular stem-form, we have
obtained a solid basis for our study of evolution. The clear path from
the stem-cell to the gastrula represents the first section of our
human stem-history (Chapters 1.8, 1.9, and 2.19).
The second section, that leads from the Gastraea to the Prochordonia,
is much more difficult and obscure. By the Prochordonia we mean the
ancient and long-extinct animals which the important embryonic form of
the chordula proves to have once existed (cf. Figures 1.83 to 1.86).
The nearest of living animals to this embryonic structure are the
lowest Tunicates, the Copelata (Appendicaria) and the larvae of the
Ascidia. As both the Tunicates and the Vertebrates develop from the
same chordula, we may infer that there was a corresponding common
ancestor of both stems. We may call this the Chordaea, and the
corresponding stem-group the Prochordonia or Prochordata.
From this important stem-group of the unarticulated Prochordonia (or
"primitive chorda-animals") the stems of the Tunicates and Vertebrates
have been divergently evolved. We shall see presently how this
conclusion is justified in the present condition of morphological
science.
We have first to answer the difficult and much-discussed question of
the development of the Chordaea from the Gastraea; in other words,
"How and by what transformations were the characteristic animals,
resembling the embryonic chordula, which we regard as the common
stem-forms of all the Chordonia, both Tunicates and Vertebrates,
evolved from the simplest two-layered Metazoa?"
The descent of the Vertebrates from the Articulates has been
maintained by a number of zoologists during the last thirty years with
more zeal than discernment; and, as a vast amount has been written on
the subject, we must deal with it to some extent. All three classes of
Articulates in succession have been awarded the honour of being
considered the "real ancestors" of the Vertebrates: first, the
Annelids (earth-worms, leeches, and the like), then the Crustacea
(crabs, etc.), and, finally, the Tracheata (spiders, insects, etc.).
The most popular of these hypotheses was the annelid theory, which
derived the Vertebrates from the Worms. It was almost simultaneously
(1875) formulated by Carl Semper, of Wurtzburg, and Anton Dohrn, of
Naples. The latter advanced this theory originally in favour of the
failing degeneration theory, with which I dealt in my work, Aims and
Methods of Modern Embryology.
This interesting degeneration theory--much discussed at that time, but
almost forgotten now--was formed in 1875 with the aim of harmonising
the results of evolution and ever-advancing Darwinism with religious
belief. The spirited struggle that Darwin had occasioned by the
reformation of the theory of descent in 1859, and that lasted for a
decade with varying fortunes in every branch of biology, was drawing
to a close in 1870-1872, and soon ended in the complete victory of
transformism. To most of the disputants the chief point was not the
general question of evolution, but the particular one of "man's place
in nature"--"the question of questions," as Huxley rightly called it.
It was soon evident to every clear-headed thinker that this question
could only be answered in the sense of our anthropogeny, by admitting
that man had descended from a long series of Vertebrates by gradual
modification and improvement.
In this way the real affinity of man and the Vertebrates came to be
admitted on all hands. Comparative anatomy and ontogeny spoke too
clearly for their testimony to be ignored any longer. But in order
still to save man's unique position, and especially the dogma of
personal immortality, a number of natural philosophers and theologians
discovered an admirable way of escape in the "theory of degeneration."
Granting the affinity, they turned the whole evolutionary theory
upside down, and boldly contended that "man is not the most highly
developed animal, but the animals are degenerate men." It is true that
man is closely related to the ape, and belongs to the vertebrate stem;
but the chain of his ancestry goes upward instead of downward. In the
beginning "God created man in his own image," as the prototype of the
perfect vertebrate; but, in consequence of original sin, the human
race sank so low that the apes branched off from it, and afterwards
the lower Vertebrates. When this theory of degeneration was
consistently developed, its supporters were bound to hold that the
entire animal kingdom was descended from the debased children of men.
This theory was most strenuously defended by the Catholic priest and
natural philosopher, Michelis, in his Haeckelogony: An Academic
Protest against Haeckel's Anthropogeny (1875). In still more
"academic" and somewhat mystic form the theory was advanced by a
natural philosopher of the older Jena school--the mathematician and
physicist, Carl Snell. But it received its chief support on the
zoological side from Anton Dohrn, who maintained the anthropocentric
ideas of Snell with particular ability. The Amphioxus, which modern
science now almost unanimously regards as the real Primitive
Vertebrate, the ancient model of the original vertebrate structure,
is, according to Dohrn, a late, degenerate descendant of the stem, the
"prodigal son" of the vertebrate family. It has descended from the
Cyclostoma by a profound degeneration, and these in turn from the
fishes; even the Ascidia and the whole of the Tunicates are merely
degenerate fishes! Following out this curious theory, Dohrn came to
contest the general belief that the Coelenterata and Worms are "lower
animals"; he even declared that the unicellular Protozoa were
degenerate Coelenterata. In his opinion "degeneration is the great
principle that explains the existence of all the lower forms."
If this Michelis-Dohrn theory were true, and all animals were really
degenerate descendants of an originally perfect humanity, man would
assuredly be the true centre and goal of all terrestrial life; his
anthropocentric position and his immortality would be saved.
Unfortunately, this trustful theory is in such flagrant contradiction
to all the known facts of paleontology and embryology that it is no
longer worth serious scientific consideration.
But the case is no better for the much-discussed descent of the
Vertebrates from the Annelids, which Dohrn afterwards maintained with
great zeal. Of late years this hypothesis, which raised so much dust
and controversy, has been entirely abandoned by most competent
zoologists, even those who once supported it. Its chief supporter,
Dohrn, admitted in 1890 that it is "dead and buried," and made a
blushing retraction at the end of his Studies of the Early History of
the Vertebrate.
Now that the annelid-hypothesis is "dead and buried," and other
attempts to derive the Vertebrates from Medusae, Echinoderms, or
Molluscs, have been equally unsuccessful, there is only one hypothesis
left to answer the question of the origin of the Vertebrates--the
hypothesis that I advanced thirty-six years ago and called the
"chordonia-hypothesis." In view of its sound establishment and its
profound significance, it may very well claim to be a THEORY, and so
should be described as the chordonia or chordaea theory.
I first advanced this theory in a series of university lectures in
1867, from which the History of Creation was composed. In the first
edition of this work (1868) I endeavoured to prove, on the strength of
Kowalevsky's epoch-making discoveries, that "of all the animals known
to us the Tunicates are undoubtedly the nearest blood-relatives of the
Vertebrates; they are the most closely related to the Vermalia, from
which the Vertebrates have been evolved. Naturally, I do not mean that
the Vertebrates have descended from the Tunicates, but that the two
groups have sprung from a common root. It is clear that the real
Vertebrates (primarily the Acrania) were evolved in very early times
from a group of Worms, from which the degenerate Tunicates also
descended in another and retrogressive direction." This common extinct
stem-group are the Prochordonia; we still have a silhouette of them in
the chordula-embryo of the Vertebrates and Tunicates; and they still
exist independently, in very modified form, in the class of the
Copelata (Appendicaria, Figure 2.225).
The chordaea-theory received the most valuable and competent support
from Carl Gegenbaur. This able comparative morphologist defended it in
1870, in the second edition of his Elements of Comparative Anatomy; at
the same time he drew attention to the important relations of the
Tunicates to a curious worm, Balanoglossus: he rightly regards this as
the representative of a special class of worms, which he called
"gut-breathers" (Enteropneusta). Gegenbaur referred on many other
occasions to the close blood-relationship of the Tunicates and
Vertebrates, and luminously explained the reasons that justify us in
framing the hypothesis of the descent of the two stems from a common
ancestor, an unsegmented worm-like animal with an axial chorda between
the dorsal nerve-tube and the ventral gut-tube.
The theory afterwards received a good deal of support from the
research made by a number of distinguished zoologists and anatomists,
especially C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and
Julin. Since Hatschek's Studies of the Development of the Amphioxus
gave us full information as to the embryology of this lowest
vertebrate, it has become so important for our purpose that we must
consider it a document of the first rank for answering the question we
are dealing with.
The ontogenetic facts that we gather from this sole survivor of the
Acrania are the more valuable for phylogenetic purposes, as
paleontology, unfortunately, throws no light whatever on the origin of
the Vertebrates. Their invertebrate ancestors were soft organisms
without skeleton, and thus incapable of fossilisation, as is still the
case with the lowest vertebrates--the Acrania and Cyclostoma. The same
applies to the greater part of the Vermalia or worm-like animals, the
various classes and orders of which differ so much in structure. The
isolated groups of this rich stem are living branches of a huge tree,
the greater part of which has long been dead, and we have no fossil
evidence as to its earlier form. Nevertheless, some of the surviving
groups are very instructive, and give us clear indications of the way
in which the Chordonia were developed from the Vermalia, and these
from the Coelenteria.
While we seek the most important of these palingenetic forms among the
groups of Coelenteria and Vermalia, it is understood that not a single
one of them must be regarded as an unchanged, or even little changed,
copy of the extinct stem-form. One group has retained one feature,
another a different feature, of the original organisation, and other
organs have been further developed and characteristically modified.
Hence here, more than in any other part of our genealogical tree, we
have to keep before our mind the FULL PICTURE of development, and
separate the unessential secondary phenomena from the essential and
primary. It will be useful first to point out the chief advances in
organisation by which the simple Gastraea gradually became the more
developed Chordaea.
We find our first solid datum in the gastrula of the Amphioxus (Figure
1.38). Its bilateral and tri-axial type indicates that the
Gastraeads--the common ancestors of all the Metazoa--divided at an
early stage into two divergent groups. The uni-axial Gastraea became
sessile, and gave rise to two stems, the Sponges and the Cnidaria (the
latter all reducible to simple polyps like the hydra). But the
tri-axial Gastraea assumed a certain pose or direction of the body on
account of its swimming or creeping movement, and in order to sustain
this it was a great advantage to share the burden equally between the
two halves of the body (right and left). Thus arose the typical
bilateral form, which has three axes. The same bilateral type is found
in all our artificial means of locomotion--carts, ships, etc.; it is
by far the best for the movement of the body in a certain direction
and steady position. Hence natural selection early developed this
bilateral type in a section of the Gastraeads, and thus produced the
stem-forms of all the bilateral animals.
The Gastraea bilateralis, of which we may conceive the bilateral
gastrula of the amphioxus to be a palingenetic reproduction,
represented the two-sided organism of the earliest Metazoa in its
simplest form. The vegetal entoderm that lined their simple gut-cavity
served for nutrition; the ciliated ectoderm that formed the external
skin attended to locomotion and sensation; finally, the two primitive
mesodermic cells, that lay to the right and left at the ventral border
of the primitive mouth, were sexual cells, and effected reproduction.
In order to understand the further development of the gastraea, we
must pay particular attention to: (1) the careful study of the
embryonic stages of the amphioxus that lie between the gastrula and
the chordula; (2) the morphological study of the simplest Platodes
(Platodaria and Turbellaria) and several groups of unarticulated
Vermalia (Gastrotricha, Nemertina, Enteropneusta).
We have to consider the Platodes first, because they are on the border
between the two principal groups of the Metazoa, the Coelenteria and
the Coelomaria. With the former they share the lack of body-cavity,
anus, and vascular system; with the latter they have in common the
bilateral type, the possession of a pair of nephridia or renal canals,
and the formation of a vertical brain or cerebral ganglion. It is now
usual to distinguish four classes of Platodes: the two free-living
classes of the primitive worms (Platodaria) and the coiled-worms
(Turbellaria), and the two parasitic classes of the suctorial worms
(Trematoda) and the tape-worms (Cestoda). We have only to consider the
first two of these classes; the other two are parasites, and have
descended from the former by adaptation to parasitic habits and
consequent degeneration.
(FIGURE 2.239. Aphanostomum Langii (Haeckel), a primitive worm of the
platodaria class, of the order of Cryptocoela or Acoela. This new
species of the genus Aphanostomum, named after Professor Arnold Lang
of Zurich, was found in September, 1899, at Ajaccio in Corsica
(creeping between fucoidea). It is one-twelfth of an inch long,
one-twenty-fifth of an inch broad, and violet in colour. a mouth, g
auditory vesicle, e ectoderm, i entoderm, o ovaries, a spermaries, f
female aperture, m male aperture.)
The primitive worms (Platodaria) are very small flat worms of simple
construction, but of great morphological and phylogenetic interest.
They have been hitherto, as a rule, regarded as a special order of the
Turbellaria, and associated with the Rhabdocoela; but they differ
considerably from these and all the other Platodes (flat worms) in the
absence of renal canals and a special central nervous system; the
structure of their tissue is also simpler than in the other Platodes.
Most of the Platodes of this group (Aphanostomum, Amphichoerus,
Convoluta, Schizoprora, etc.) are very soft and delicate animals,
swimming about in the sea by means of a ciliary coat, and very small
(1/10 to 1/20 inch long). Their oval body, without appendages, is
sometimes spindle-shaped or cylindrical, sometimes flat and
leaf-shaped. Their skin is merely a layer of ciliated ectodermic
cells. Under this is a soft medullary substance, which consists of
entodermic cells with vacuoles. The food passes through the mouth
directly into this digestive medullary substance, in which we do not
generally see any permanent gut-cavity (it may have entirely
collapsed); hence these primitive Platodes have been called Acoela
(without gut-cavity or coelom), or, more correctly, Cryptocoela, or
Pseudocoela. The sexual organs of these hermaphroditic Platodaria are
very simple--two pairs of strings of cells, the inner of which (the
ovaries, Figure 2.239 o) produce ova, and the outer (the spermaria, s)
sperm-cells. These gonads are not yet independent sexual glands, but
sexually differentiated cell-groups in the medullary substance, or, in
other words, parts of the gut-wall. Their products, the sex-cells, are
conveyed out behind by two pairs of short canals; the male opening (m)
lies just behind the female (f). Most of the Platodaria have not the
muscular pharynx, which is very advanced in the Turbellaria and
Trematoda. On the other hand, they have, as a rule, before or behind
the mouth, a bulbous sense-organ (auditory vesicle or organ of
equilibrium, g), and many of them have also a couple of simple optic
spots. The cell-pit of the ectoderm that lies underneath is rather
thick, and represents the first rudiment of a neural ganglion
(vertical brain or acroganglion).
The Turbellaria, with which the similar Platodaria were formerly
classed, differ materially from them in the more advanced structure of
their organs, and especially in having a central nervous system
(vertical brain) and excretory renal canals (nephridia); both
originate from the ectoderm. But between the two germinal layers a
mesoderm is developed, a soft mass of connective tissue, in which the
organs are embedded. The Turbellaria are still represented by a number
of different forms, in both fresh and sea-water. The oldest of these
are the very rudimentary and tiny forms that are known as Rhabdocoela
on account of the simple construction of their gut; they are, as a
rule, less than a quarter of an inch long and of a simple oval or
lancet shape (Figure 2.240). The surface is covered with ciliated
epithelium, a stratum of ectodermic cells. The digestive gut is still
the simple primitive gut of the gastraea (d), with a single aperture
that is both mouth and anus (m). There is, however, an invagination of
the ectoderm at the mouth, which has given rise to a muscular pharynx
(sd). It is noteworthy that the mouth of the Turbellaria (like the
primitive mouth of the Gastraea) may, in this class, change its
position considerably in the middle line of the ventral surface;
sometimes it lies behind (Opisthostomum), sometimes in the middle
(Mesostomum), sometimes in front (Prosostomum). This displacement of
the mouth from front to rear is very interesting, because it
corresponds to a phylogenetic displacement of the mouth. This probably
occurred in the Platode ancestors of most (or all?) of the Coelomaria;
in these the permanent mouth (metastoma) lies at the fore end (oral
pole), whereas the primitive mouth (prostoma) lay at the hind end of
the bilateral body.
In most of the Turbellaria there is a narrow cavity, containing a
number of secondary organs, between the two primary germinal layers,
the outer or animal layer of which forms the epidermis and the inner
vegetal layer the visceral epithelium. The earliest of these organs
are the sexual organs; they are very variously constructed in the
Platode-class; in the simplest case there are merely two pairs of
gonads or sexual glands--a pair of testicles (Figure 2.241 h) and a
pair of ovaries (e). They open externally, sometimes by a common
aperture (Monogonopora), sometimes by separate ones, the female behind
the male (Digonopora, Figure 2.241). The sexual glands develop
originally from the two promesoblasts or primitive mesodermic cells
(Figure 1.83 p). As these earliest mesodermic structures extended, and
became spacious sexual pouches in the later descendants of the
Platodes, probably the two coelom-pouches were formed from them, the
first trace of the real body-cavity of the higher Metazoa
(Enterocoela).
The gonads are among the oldest organs, the few other organs that we
find in the Platodes between the gut-wall and body-wall being later
evolutionary products. One of the oldest and most important of these
are the kidneys or nephridia, which remove unusable matter from the
body (Figure 2.240 nc). These urinary or excretory organs were
originally enlarged skin-glands--a couple of canals that run the
length of the body, and have a separate or common external aperture
(nm). They often have a number of branches. These special excretory
organs are not found in the other Coelenteria (Gastraeads, Sponges,
Cnidaria) or the Cryptocoela. They are first met in the Turbellaria,
and have been transmitted direct from these to the Vermalia, and from
these to the higher stems.
Finally, there is a very important new organ in the Turbellaria, which
we do not find in the Cryptocoela (Figure 2.239) and their gastraead
ancestors--the rudimentary nervous system. It consists of a couple of
simple cerebral ganglia (Figure 2.241 g) and fine nervous fibres that
radiate from them; these are partly voluntary nerves (or motor fibres)
that go to the thin muscular layer developing under the skin; and
partly sensory nerves that proceed to the sense-cells of the ciliated
epiderm (f). Many of the Turbellaria have also special sense-organs; a
couple of ciliated smell pits (na), rudimentary eyes (au), and, less
frequently, auditory vesicles.
On these principles I assume that the oldest and simplest Turbellaria
arose from Platodaria, and these directly from bilateral Gastraeads.
The chief advances were the formation of gonads and nephridia, and of
the rudimentary brain. On this hypothesis, which I advanced in 1872 in
the first sketch of the gastraea-theory (Monograph on the Sponges),
there is no direct affinity between the Platodes and the Cnidaria.
(FIGURE 2.240. A simple turbellarian (Rhabdocoelum). m mouth, sd
gullet epithelium, sm gullet muscles, d gastric gut, nc renal canals,
nm renal aperture, au eye, na olfactory pit. (Diagram.)
FIGURE 2.241. The same, showing the other organs. g brain, au eye, na
olfactory pit, n nerves, h testicles, male symbol male aperture,
female symbol female aperture, e ovary, f ciliated epiderm. (Diagram.)
(FIGURES 242 AND 243. Chaetonotus, a rudimentary vermalian, of the
group of Gastrotricha. m mouth, s gullet, d gut, a anus, g brain, n
nerves, ss sensory hairs, au eye, ms muscular cells, h skin, f
ciliated bands of the ventral surface, nc nephridia, nm their
aperture, e ovaries.))
Next to the ancient stem-group of the Turbellaria come a number of
more recent chordonia ancestors, which we class with the Vermalia or
Helminthes, the unarticulated worms. These true worms (Vermes, lately
also called Scolecida) are the difficulty or the lumber-room of the
zoological classifier, because the various classes have very
complicated relations to the lower Platodes on the one hand and the
more advanced animals on the other. But if we exclude the Platodes and
the Annelids from this stem, we find a fairly satisfactory unity of
organisation in the remaining classes. Among these worms we find some
important forms that show considerable advance in organisation from
the platode to the chordonia stage. Three of these phenomena are
particularly instructive: (1) The formation of a true (secondary)
body-cavity (coeloma); (2) the formation of a second aperture of the
gut, the anus; and (3) the formation of a vascular system. The great
majority of the Vermalia have these three features, and they are all
wanting in the Platodes; in the rest of the worms at least one or two
of them are developed.
Next and very close to the Platodes we have the Ichthydina
(Gastrotricha), little marine and fresh-water worms, about 1/250 to
1/1000 inch long. Zoologists differ as to their position in
classification. In my opinion, they approach very close to the
Rhabdocoela (Figures 2.240 and 2.241), and differ from them chiefly in
the possession of an anus at the posterior end (Figure 2.242 a).
Further, the cilia that cover the whole surface of the Turbellaria are
confined in the Gastrotricha to two ciliated bands (f) on the ventral
surface of the oval body, the dorsal surface having bristles.
Otherwise the organisation of the two classes is the same. In both the
gut consists of a muscular gullet (s) and a glandular primitive gut
(d). Over the gullet is a double brain (acroganglion, g). At the side
of the gut are two serpentine prorenal canals (water-vessels or
pronephridia, nc), which open on the ventral side (nm). Behind are a
pair of simple sexual glands or gonads (Figure 2.243 e).
While the Ichthydina are thus closely related to the Platodes, we have
to go farther away for the two classes of Vermalia which we unite in
the group of the "snout-worms" (Frontonia). These are the Nemertina
and the Enteropneusta. Both classes have a complete ciliary coat on
the epidermis, a heritage from the Turbellaria and the Gastraeads;
also, both have two openings of the gut, the mouth and anus, like the
Gastrotricha. But we find also an important organ that is wanting in
the preceding forms--the vascular system. In their more advanced
mesoderm we find a few contractile longitudinal canals which force the
blood through the body by their contractions; these are the first
blood-vessels.
(FIGURE 2.244. A simple Nemertine. m mouth, d gut, a anus, g brain, n
nerves, h ciliary coat, ss sensory pits (head-clefts), au eyes, r
dorsal vessel, l lateral vessels. (Diagram.)
FIGURE 2.245. A young Enteropneust (Balanaglossus). (From Alexander
Agassiz.) r acorn-shaped snout, h neck, k gill-clefts and gill-arches
of the fore-gut, in long rows on each side, d digestive hind-gut,
filling the greater part of the body-cavity, v intestinal vein or
ventral vessel, lying between the parallel folds of the skin, a anus.
Figure 2.246. Transverse section of the branchial gut. A of
Balanoglossus, B of Ascidia. r branchial gut, n pharyngeal groove,
asterisk ventral folds between the two. Diagrammatic illustration from
Gegenbaur, to show the relation of the dorsal branchial-gut cavity (r)
to the pharyngeal or hypobranchial groove (n).)
The Nemertina were formerly classed with the much less advanced
Turbellaria. But they differ essentially from them in having an anus
and blood-vessels, and several other marks of higher organisation.
They have generally long and narrow bodies, like a more or less
flattened cord; there are, besides several small species, giant-forms
with a width of 1/5 to 2/5 inch and a length of several yards (even
ten to fifteen). Most of them live in the sea, but some in fresh water
and moist earth. In their internal structure they approach the
Turbellaria on the one hand and the higher Vermalia (especially the
Enteropneusta) on the other. They have a good deal of interest as the
lowest and oldest of all animals with blood. In them we find
blood-vessels for the first time, distributing real blood through the
body. The blood is red, and the red colouring-matter is haemoglobin,
connected with elliptic discoid blood-cells, as in the Vertebrates.
Most of them have two or three parallel blood-canals, which run the
whole length of the body, and are connected in front and behind by
loops, and often by a number of ring-shaped pieces. The chief of these
primitive blood-vessels is the one that lies above the gut in the
middle line of the back (Figure 2.244 r); it may be compared to either
the dorsal vessel of the Articulates or the aorta of the Vertebrates.
To the right and left are the two serpentine lateral vessels (Figure
2.244 l).
After the Nemertina, I take (as distant relatives) the Enteropneusta;
they may be classed together with them as Frontonia or Rhyncocoela
(snout-worms). There is now only one genus of this class, with several
species (Balanoglossus); but it is very remarkable, and may be
regarded as the last survivor of an ancient and long-extinct class of
Vermalia. They are related, on the one hand, to the Nemertina and
their immediate ancestors, the Platodes, and to the lowest and oldest
forms of the Chordonia on the other.
The Enteropneusta (Figure 2.245) live in the sea sand, and are long
worms of very simple shape, like the Nemertina. From the latter they
have inherited: (1) The bilateral type, with incomplete segmentation;
(2) the ciliary coat of the soft epidermis; (3) the double rows of
gastric pouches, alternating with a single or double row of gonads;
(4) separation of the sexes (the Platode ancestors were
hermaphroditic); (5) the ventral mouth, underneath a protruding snout;
(6) the anus terminating the simple gut-tube; and (7) several parallel
blood-canals, running the length of the body, a dorsal and a ventral
principal stem.
On the other hand, the Enteropneusta differ from their Nemertine
ancestors in several features, some of which are important, that we
may attribute to adaptation. The chief of these is the branchial gut
(Figure 2.245 k). The anterior section of the gut is converted into a
respiratory organ, and pierced by two rows of gill-clefts; between
these there is a branchial (gill) skeleton, formed of rods and plates
of chitine. The water that enters at the mouth makes its exit by these
clefts. They lie in the dorsal half of the fore-gut, and this is
completely separated from the ventral half by two longitudinal folds
(Figure 2.246 A*). This ventral half, the glandular walls of which are
clothed with ciliary epithelium and secrete mucus, corresponds to the
pharyngeal or hypo-branchial groove of the Chordonia (Bn), the
important organ from which the later thyroid gland is developed in the
Craniota (cf. Chapter 2.16). The agreement in the structure of the
branchial gut of the Enteropneusts, Tunicates, and Vertebrates was
first recognised by Gegenbaur (1878); it is the more significant as at
first we find only a couple of gill-clefts in the young animals of all
three groups; the number gradually increases. We can infer from this
the common descent of the three groups with all the more confidence
when we find the Balanoglossus approaching the Chordonia in other
respects. Thus, for instance, the chief part of the central nervous
system is a long dorsal neural string that runs above the gut and
corresponds to the medullary tube of the Chordonia. Bateson believes
he has detected a rudimentary chorda between the two.
Of all extant invertebrate animals the Enteropneusts come nearest to
the Chordonia in virtue of these peculiar characters; hence we may
regard them as the survivors of the ancient gut-breathing Vermalia
from which the Chordonia also have descended. Again, of all the
chorda-animals the Copelata (Figure 2.225) and the tailed larvae of
the ascidia approach nearest to the young Balanoglossus. Both are, on
the other hand, very closely related to the Amphioxus, the Primitive
Vertebrate of which we have considered the importance (Chapters 2.16
and 2.17). As we saw there, the unarticulated Tunicates and the
articulated Vertebrates must be regarded as two independent stems,
that have developed in divergent directions. But the common root of
the two stems, the extinct group of the Prochordonia, must be sought
in the vermalia stem; and of all the living Vermalia those we have
considered give us the safest clue to their origin. It is true that
the actual representatives of the important groups of the Copelata,
Balanoglossi, Nemertina, Icthydina, etc., have more or less departed
from the primitive model owing to adaptation to special environment.
But we may just as confidently affirm that the main features of their
organisation have been preserved by heredity.
We must grant, however, that in the whole stem-history of the
Vertebrates the long stretch from the Gastraeads and Platodes up to
the oldest Chordonia remains by far the most obscure section. We might
frame another hypothesis to raise the difficulty--namely, that there
was a long series of very different and totally extinct forms between
the Gastraea and the Chordaea. Even in this modified chordaea-theory
the six fundamental organs of the chordula would retain their great
value. The medullary tube would be originally a chemical sensory
organ, a dorsal olfactory tube, taking in respiratory-water and food
by the neuroporus in front and conveying them by the neurenteric canal
into the primitive gut. This olfactory tube would afterwards become
the nervous centre, while the expanding gonads (lying to right and
left of the primitive mouth) would form the coeloma. The chorda may
have been originally a digestive glandular groove in the dorsal middle
line of the primitive gut. The two secondary gut-openings, mouth and
anus, may have arisen in various ways by change of functions. In any
case, we should ascribe the same high value to the chordula as we did
before to the gastrula.
In order to explain more fully the chief stages in the advance of our
race, I add the hypothetical sketch of man's ancestry that I published
in my Last Link [a translation by Dr. Gadow of the paper read at the
International Zoological Congress at Cambridge in 1898]:--
A. MAN'S GENEALOGICAL TREE, FIRST HALF: EARLIER SERIES OF ANCESTORS,
WITHOUT FOSSIL EVIDENCE.
COLUMN 1 : CHIEF STAGES.
COLUMN 2 : ANCESTRAL STEM-GROUPS.
COLUMN 3 : LIVING RELATIVES OF ANCESTORS.
STAGES 1 TO 5. PROTIST ANCESTORS. UNICELLULAR ORGANISMS.
1 to 2. Protophytes. : 1. Monera. Without nucleus. : Chromacea.
(Chroococcus.) Phycochromacea.
1 to 2. Protophytes. : 2. Algaria. Unicellular algae. : 2. Paulotomea.
Palmellacea. Eremosphaera.
3 to 5. Protozoa. : 3. Lobosa. Unicellular (amoebina) rhizopods. : 3.
Amoebina. Amoeba Leucocyta.
3 to 5. Protozoa. : 4. Infusoria. Unicellular. : 4. Flagellata.
Euflagellata. Zoomonades.
3 to 5. Protozoa. : 5. Blastaeades. Multicellular hollow spheres. : 5.
Catallacta. Magosphaera, Volvocina, Blastula.
STAGES 6 TO 11. INVERTEBRATE METAZOA ANCESTORS.
6 to 8. Coelenteria, without anus and body-cavity. : 6. Gastraeades.
With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria.
6 to 8. Coelenteria, without anus and body-cavity. : 7. Platodes I.
Platodaria (without nephridia). : 7. Cryptocoela. Convoluta, Proporus.
6 to 8. Coelenteria, without anus and body-cavity. : 8. Platodes II.
Platodinia (with nephridia). : 8. Rhabdocoela. Vortex, Monotus.
9 to 11. Vermalia, with anus and body-cavity. : 9. Provermalia.
(Primitive Worms.) Rotatoria. : 9. Gastrotricha. Trochozoa,
Trochophora.
9 to 11. Vermalia, with anus and body-cavity. : 10. Frontonia.
(Rhynchelminthes.) Snout-worms. : 10. Enteropneusta. Balanoglossus,
Cephalodiscus.
9 to 11. Vermalia, with anus and body-cavity. : 11. Prochordonia.
Chorda-worms. : 11. Copelata. Appendicaria. Chordula-larvae.
STAGES 12 TO 15. MONORHINA ANCESTORS.
Oldest vertebrates without jaws or pairs of limbs, single nose. : 12.
Acrania I. (Prospondylia.) : 12. Amphioxus larva.
Oldest vertebrates without jaws or pairs of limbs, single nose. : 13.
Acrania II. More recent. : 13. Leptocardia. Amphioxus.
Oldest vertebrates without jaws or pairs of limbs, single nose. : 14.
Cyclostoma I. (Archicrania.) : 14. Petromyzonta larvae.
Oldest vertebrates without jaws or pairs of limbs, single nose. : 15.
Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta.
B. MAN'S GENEALOGICAL TREE, SECOND HALF: LATER ANCESTORS, WITH FOSSIL
EVIDENCE.
COLUMN 1 : GEOLOGICAL PERIODS.
COLUMN 2 : ANCESTRAL STEM-GROUPS.
COLUMN 3 : LIVING RELATIVES OF ANCESTORS.
Silurian. : 16. Selachii. Primitive fishes. Proselachii. : 16.
Natidanides. Chlamydoselachius. Heptanchus.
Silurian. 17. Ganoides. Plated-fishes. Proganoides. : 17.
Accipenserides. (Sturgeons.) Polypterus.
Devonian. : 18. Dipneusta. Paladipneusta. : 18. Neodipneusta.
Ceratodus. Protopterus.
Carboniferous. : 19. Amphibia. Stegocephala. : 19. Phanerobranchia.
Salamandrina. (Proteus, triton.)
Permian. : 20. Reptilia. Proreptilia. : 20. Rhynchocephalia. Primitive
lizards. Hatteria.
Triassic. : 21. Monotrema. Promammalia. : 21. Ornithodelphia. Echidna.
Ornithorhyncus.
Jurassic. : 22. Marsupalia. Prodidelphia. : 22. Didelphia. Didelphys.
Perameles.
Cretaceous. : 23. Mallotheria. Prochoriata. : 23. Insectivora.
Erinaceida. (Ictopsida +.)
Older Eocene. : 24. Lemuravida. Older lemurs. Dentition. 3. 1. 4. 3. :
24. Pachylemures. (Hyopsodus +), (Adapis +).
Neo-Eocene. : 25. Lemurogona. Later lemurs. Dentition. 2. 1. 4. 3. :
25. Autolemures. Eulemur. Stenops.
Oligocene. : 26. Dysmopitheca. Western apes. Dentition. 2. 1. 3. 3. :
26. Platyrrhinae. (Anthropops +), (Homunculus +).
Older Miocene. : 27. Cynopitheca. Dog-faced apes (tailed). : 27.
Papiomorpha. Cynocephalus.
Neo-Miocene. : 28. Anthropoides. Man-like apes (tail-less). : 28.
Hylobatida. Hylobates. Satyrus.
Pliocene. : 29. Pithecanthropi. Ape-men (alali, speechless). : 29.
Anthropitheca. Chimpanzee. Gorilla.
Pleistocene. : 30. Homines. Men, with speech. : 30. Weddahs.
Australian negroes.