In turning from the embryology to the phylogeny of man--from the
development of the individual to that of the species--we must bear in
mind the direct causal connection that exists between these two main
branches of the science of human evolution. This important causal
nexus finds its simplest expression in "the fundamental law of organic
development," the content and purport of which we have fully
considered in the first chapter. According to this biogenetic law,
ontogeny is a brief and condensed recapitulation of phylogeny. If this
compendious reproduction were complete in all cases, it would be very
easy to construct the whole story of evolution on an embryonic basis.
When we wanted to know the ancestors of any higher organism, and,
therefore, of man--to know from what forms the race as a whole has
been evolved we should merely have to follow the series of forms in
the development of the individual from the ovum; we could then regard
each of the successive forms as the representative of an extinct
ancestral form. However, this direct application of ontogenetic facts
to phylogenetic ideas is possible, without limitations, only in a very
small section of the animal kingdom. There are, it is true, still a
number of lower invertebrates (for instance, some of the Zoophyta and
Vermalia) in which we are justified in recognising at once each
embryonic form as the historical reproduction, or silhouette, as it
were, of an extinct ancestor. But in the great majority of the
animals, and in the case of man, this is impossible, because the
embryonic forms themselves have been modified through the change of
the conditions of existence, and have lost their original character to
some extent. During the immeasurable course of organic history, the
many millions of years during which life was developing on our planet,
secondary changes of the embryonic forms have taken place in most
animals. The young of animals (not only detached larvae, but also the
embryos enclosed in the womb) may be modified by the influence of the
environment, just as well as the mature organisms are by adaptation to
the conditions of life; even species are altered during the embryonic
development. Moreover, it is an advantage for all higher organisms
(and the advantage is greater the more advanced they are) to curtail
and simplify the original course of development, and thus to
obliterate the traces of their ancestors. The higher the individual
organism is in the animal kingdom, the less completely does it
reproduce in its embryonic development the series of its ancestors,
for reasons that are as yet only partly known to us. The fact is
easily proved by comparing the different developments of higher and
lower animals in any single stem.
In order to appreciate this important feature, we have distributed the
embryological phenomena in two groups, palingenetic and cenogenetic.
Under palingenesis we count those facts of embryology that we can
directly regard as a faithful synopsis of the corresponding
stem-history. By cenogenesis we understand those embryonic processes
which we cannot directly correlate with corresponding evolutionary
processes, but must regard as modifications or falsifications of them.
With this careful discrimination between palingenetic and cenogenetic
phenomena, our biogenetic law assumes the following more precise
shape:--The rapid and brief development of the individual (ontogeny)
is a condensed synopsis of the long and slow history of the stem
(phylogeny): this synopsis is the more faithful and complete in
proportion as the original features have been preserved by heredity,
and modifications have not been introduced by adaptation.
In order to distinguish correctly between palingenetic and cenogenetic
phenomena in embryology, and deduce sound conclusions in connection
with stem-history, we must especially make a comparative study of the
former. In doing this it is best to employ the methods that have long
been used by geologists for the purpose of establishing the succession
of the sedimentary rocks in the crust of the earth. This solid crust,
which encloses the glowing central mass like a thin shell, is composed
of different kinds of rocks: there are, firstly, the volcanic rocks
which were formed directly by the cooling at the surface of the molten
mass of the earth; secondly, there are the sedimentary rocks, that
have been made out of the former by the action of water, and have been
laid in successive strata at the bottom of the sea. Each of these
sedimentary strata was at first a soft layer of mud; but in the course
of thousands of years it condensed into a solid, hard mass of stone
(sandstone, limestone, marl, etc.), and at the same time permanently
preserved the solid and imperishable bodies that had chanced to fall
into the soft mud. Among these bodies, which were either fossilised or
left characteristic impressions of their forms in the soft slime, we
have especially the more solid parts of the animals and plants that
lived and died during the deposit of the slimy strata.
Hence each of the sedimentary strata has its characteristic fossils,
the remains of the animals and plants that lived during that
particular period of the earth's history. When we make a comparative
study of these strata, we can survey the whole series of such periods.
All geologists are now agreed that we can demonstrate a definite
historical succession in the strata, and that the lowest of them were
deposited in very remote, and the uppermost in comparatively recent,
times. However, there is no part of the earth where we find the series
of strata in its entirety, or even approximately complete. The
succession of strata and of corresponding historical periods generally
given in geology is an ideal construction, formed by piecing together
the various partial discoveries of the succession of strata that have
been made at different points of the earth's surface (cf. Chapter
2.18).
We must act in this way in constructing the phylogeny of man. We must
try to piece together a fairly complete picture of the series of our
ancestors from the various phylogenetic fragments that we find in the
different groups of the animal kingdom. We shall see that we are
really in a position to form an approximate picture of the evolution
of man and the mammals by a proper comparison of the embryology of
very different animals--a picture that we could never have framed from
the ontogeny of the mammals alone. As a result of the above-mentioned
cenogenetic processes--those of disturbed and curtailed
heredity--whole series of lower stages have dropped out in the
embryonic development of man and the other mammals especially from the
earliest periods, or been falsified by modification. But we find these
lower stages in their original purity in the lower vertebrates and
their invertebrate ancestors. Especially in the lowest of all the
vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms
completely preserved in the embryonic development. We also find
important evidence in the fishes, which stand between the lower and
higher vertebrates, and throw further light on the course of evolution
in certain periods. Next to the fishes come the amphibia, from the
embryology of which we can also draw instructive conclusions. They
represent the transition to the higher vertebrates, in which the
middle and older stages of ancestral development have been either
distorted or curtailed, but in which we find the more recent stages of
the phylogenetic process well preserved in ontogeny. We are thus in a
position to form a fairly complete idea of the past development of
man's ancestors within the vertebrate stem by putting together and
comparing the embryological developments of the various groups of
vertebrates. And when we go below the lowest vertebrates and compare
their embryology with that of their invertebrate relatives, we can
follow the genealogical tree of our animal ancestors much farther,
down to the very lowest groups of animals.
In entering the obscure paths of this phylogenetic labyrinth, clinging
to the Ariadne-thread of the biogenetic law and guided by the light of
comparative anatomy, we will first, in accordance with the methods we
have adopted, discover and arrange those fragments from the manifold
embryonic developments of very different animals from which the
stem-history of man can be composed. I would call attention
particularly to the fact that we can employ this method with the same
confidence and right as the geologist. No geologist has ever had
ocular proof that the vast rocks that compose our Carboniferous or
Jurassic or Cretaceous strata were really deposited in water. Yet no
one doubts the fact. Further, no geologist has ever learned by direct
observation that these various sedimentary formations were deposited
in a certain order; yet all are agreed as to this order. This is
because the nature and origin of these rocks cannot be rationally
understood unless we assume that they were so deposited. These
hypotheses are universally received as safe and indispensable
"geological theories," because they alone give a rational explanation
of the strata.
Our evolutionary hypotheses can claim the same value, for the same
reasons. In formulating them we are acting on the same inductive and
deductive methods, and with almost equal confidence, as the geologist.
We hold them to be correct, and claim the status of "biological
theories" for them, because we cannot understand the nature and origin
of man and the other organisms without them, and because they alone
satisfy our demand for a knowledge of causes. And just as the
geological hypotheses that were ridiculed as dreams at the beginning
of the nineteenth century are now universally admitted, so our
phylogenetic hypotheses, which are still regarded as fantastic in
certain quarters, will sooner or later be generally received. It is
true that, as will soon appear, our task is not so simple as that of
the geologist. It is just as much more difficult and complex as man's
organisation is more elaborate than the structure of the rocks.
When we approach this task, we find an auxiliary of the utmost
importance in the comparative anatomy and embryology of two lower
animal-forms. One of these animals is the lancelet (Amphioxus), the
other the sea-squirt (Ascidia). Both of these animals are very
instructive. Both are at the border between the two chief divisions of
the animal kingdom--the vertebrates and invertebrates. The vertebrates
comprise the already mentioned classes, from the Amphioxus to man
(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and
mammals). Following the example of Lamarck, it is usual to put all the
other animals together under the head of invertebrates. But, as I have
often mentioned already, the group is composed of a number of very
different stems. Of these we have no interest just now in the
echinoderms, molluscs, and articulates, as they are independent
branches of the animal-tree, and have nothing to do with the
vertebrates. On the other hand, we are greatly concerned with a very
interesting group that has only recently been carefully studied, and
that has a most important relation to the ancestral tree of the
vertebrates. This is the stem of the Tunicates. One member of this
group, the sea-squirt, very closely approaches the lowest vertebrate,
the Amphioxus, in its essential internal structure and embryonic
development. Until 1866 no one had any idea of the close connection of
these apparently very different animals; it was a very fortunate
accident that the embryology of these related forms was discovered
just at the time when the question of the descent of the vertebrates
from the invertebrates came to the front. In order to understand it
properly, we must first consider these remarkable animals in their
fully-developed forms and compare their anatomy.
We begin with the lancelet--after man the most important and
interesting of all animals. Man is at the highest summit, the lancelet
at the lowest root, of the vertebrate stem.
It lives on the flat, sandy parts of the Mediterranean coast, partly
buried in the sand, and is apparently found in a number of seas.* (*
See the ample monograph by Arthur Willey, Amphioxus and the Ancestry
of the Vertebrates; Boston, 1894.) It has been found in the North Sea
(on the British and Scandinavian coasts and in Heligoland), and at
various places on the Mediterranean (for instance, at Nice, Naples,
and Messina). It is also found on the coast of Brazil and in the most
distant parts of the Pacific Ocean (the coast of Peru, Borneo, China,
Australia, etc.). Recently eight to ten species of the amphioxus have
been determined, distributed in two or three genera.
(FIGURE 2.210. The lancelet (Amphioxus lanceolatus), twice natural
size, left view. The long axis is vertical; the mouth-end is above,
the tail-end below; a mouth, surrounded by threads of beard; b anus, c
gill-opening (porus branchialis), d gill-crate, e stomach, f liver, g
small intestine, h branchial cavity, i chorda (axial rod), underneath
it the aorta; k aortic arches, l trunk of the branchial artery, m
swellings on its branches, n vena cava, o visceral vein.
FIGURE 2.211. Transverse section of the head of the Amphioxus. (From
Boveri.) Above the branchial gut (kd) is the chorda, above this the
neural tube (in which we can distinguish the inner grey and the outer
white matter); above again is the dorsal fin (fh). To the right and
left above (in the episoma) are the thick muscular plates (m); below
(in the hyposoma) the gonads (g). ao aorta (here double), c corium, ec
endostyl, f fascie, gl glomerulus of the kidneys, k branchial vessel,
ld partition between the coeloma (sc) and atrium (p), mt transverse
ventral muscle, n renal canals, of upper and uf lower canals in the
mantle-folds, p peribranchial cavity, (atrium), sc coeloma (subchordal
body-cavity), si principal (or subintestinal) vein, sk perichorda
(skeletal layer).)
Johannes Muller classed the lancelet with the fishes, although he
pointed out that the differences between this simple vertebrate and
the lowest fishes are much greater than between the fishes and the
amphibia. But this was far from expressing the real significance of
the animal. We may confidently lay down the following principle: The
Amphioxus differs more from the fishes than the fishes do from man and
the other vertebrates. As a matter of fact, it is so different from
all the other vertebrates in its whole organisation that the laws of
logical classification compel us to distinguish two divisions of this
stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the
Craniota (man and the other vertebrates). The first and lower division
comprises the vertebrates that have no vertebrae or skull (cranium).
Of these the only living representatives are the Amphioxus and
Paramphioxus, though there must have been a number of different
species at an early period of the earth's history.
Opposed to the Acrania is the second division of the vertebrates,
which comprises all the other members of the stem, from the fishes up
to man. All these vertebrates have a head quite distinct from the
trunk, with a skull (cranium) and brain; all have a centralised heart,
fully-formed kidneys, etc. Hence they are called the Craniota. These
Craniotes are, however, without a skull in their earlier period. As we
already know from embryology, even man, like every other mammal,
passes in the earlier course of his development through the important
stage which we call the chordula; at this lower stage the animal has
neither vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even
after the formation of the primitive vertebrae has begun, the
segmented foetus of the amniotes still has for a long time the simple
form of a lyre-shaped disk or a sandal, without limbs or extremities.
When we compare this embryonic condition, the sandal-shaped foetus,
with the developed lancelet, we may say that the amphioxus is, in a
certain sense, a permanent sandal-embryo, or a permanent embryonic
form of the Acrania; it never rises above a low grade of development
which we have long since passed.
The fully-developed lancelet (Figure 2.210) is about two inches long,
is colourless or of a light red tint, and has the shape of a narrow
lancet-formed leaf. The body is pointed at both ends, but much
compressed at the sides. There is no trace of limbs. The outer skin is
very thin and delicate, naked, transparent, and composed of two
different layers, a simple external stratum of cells, the epidermis,
and a thin underlying cutis-layer. Along the middle line of the back
runs a narrow fin-fringe which expands behind into an oval tail-fin,
and is continued below in a short anus-fin. The fin-fringe is
supported by a number of square elastic fin-plates.
In the middle of the body we find a thin string of cartilage, which
goes the whole length of the body from front to back, and is pointed
at both ends (Figure 2.210 i). This straight, cylindrical rod
(somewhat compressed for a time) is the axial rod or the chorda
dorsalis; in the lancelet this is the only trace of a vertebral
column. The chorda develops no further, but retains its original
simplicity throughout life. It is enclosed by a firm membrane, the
chorda-sheath or perichorda. The real features of this and of its
dependent formations are best seen in the transverse section of the
Amphioxus (Figure 2.211). The perichorda forms a cylindrical tube
immediately over the chorda, and the central nervous system, the
medullary tube, is enclosed in it. This important psychic organ also
remains in its simplest shape throughout life, as a cylindrical tube,
terminating with almost equal plainness at either end, and enclosing a
narrow canal in its thick wall. However, the fore end is a little
rounder, and contains a small, almost imperceptible bulbous swelling
of the canal. This must be regarded as the beginning of a rudimentary
brain. At the foremost end of it there is a small black pigment-spot,
a rudimentary eye; and a narrow canal leads to a superficial
sense-organ. In the vicinity of this optic spot we find at the left
side a small ciliated depression, the single olfactory organ. There is
no organ of hearing. This defective development of the higher
sense-organs is probably, in the main, not an original feature, but a
result of degeneration.
Underneath the axial rod or chorda runs a very simple alimentary
canal, a tube that opens on the ventral side of the animal by a mouth
in front and anus behind. The oval mouth is surrounded by a ring of
cartilage, on which there are twenty to thirty cartilaginous threads
(organs of touch, Figure 2.210 a). The alimentary canal divides into
sections of about equal length by a constriction in the middle. The
fore section, or head-gut, serves for respiration; the hind section,
or trunk-gut, for digestion. The limit of the two alimentary regions
is also the limit of the two parts of the body, the head and the
trunk. The head-gut or branchial gut forms a broad gill-crate, the
grilled wall of which is pierced by numbers of gill-clefts (Figure
2.210 d). The fine bars of the gill-crate between the clefts are
strengthened with firm parallel rods, and these are connected in pairs
by cross-rods. The water that enters the mouth of the Amphioxus passes
through these clefts into the large surrounding branchial cavity or
atrium, and then pours out behind through a hole in it, the
respiratory pore (porus branchialis, Figure 2.210 c). Below, on the
ventral side of the gill-crate, there is in the middle line a ciliated
groove with a glandular wall (the hypobranchial groove), which is also
found in the Ascidia and the larvae of the Cyclostoma. It is
interesting because the thyroid gland in the larynx of the higher
vertebrates (underneath the "Adam's apple") has been developed from
it.
(FIGURE 2.212. Transverse section of an Amphioxus-larva, with five
gill-clefts, through the middle of the body.
FIGURE 2.213. Diagram of the preceding. (From Hatschek.) A epidermis,
B medullary tube, C chorda, C1 inner chorda-sheath, D visceral
epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome),
3 skeletal plate (sclerotome), 4 coeloseptum (partition between dorsal
and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocoel
(dorsal body-cavity), II splanchnocoel (ventral body-cavity).)
Behind the respiratory part of the gut we have the digestive section,
the trunk or liver (hepatic) gut. The small particles that the
Amphioxus takes in with the water--infusoria, diatoms, particles of
decomposed plants and animals, etc.--pass from the gill-crate into the
digestive part of the canal, and are used up as food. From a somewhat
enlarged portion, that corresponds to the stomach (Figure 2.210 e), a
long, pouch-like blind sac proceeds straight forward (f); it lies
underneath on the left side of the gill-crate, and ends blindly about
the middle of it. This is the liver of the Amphioxus, the simplest
kind of liver that we meet in any vertebrate. In man also the liver
develops, as we shall see, in the shape of a pouch-like blind sac,
that forms out of the alimentary canal behind the stomach.
The formation of the circulatory system in this animal is not less
interesting. All the other vertebrates have a compressed, thick,
pouch-shaped heart, which develops from the wall of the gut at the
throat, and from which the blood-vessels proceed; in the Amphioxus
there is no special centralised heart, driving the blood by its
pulsations. This movement is effected, as in the annelids, by the thin
blood-vessels themselves, which discharge the function of the heart,
contracting and pulsating in their whole length, and thus driving the
colourless blood through the entire body. On the under-side of the
gill-crate, in the middle line, there is the trunk of a large vessel
that corresponds to the heart of the other vertebrates and the trunk
of the branchial artery that proceeds from it; this drives the blood
into the gills (Figure 2.210 l). A number of small vascular arches
arise on each side from this branchial artery, and form little
heart-shaped swellings or bulbilla (m) at their points of departure;
they advance along the branchial arches, between the gill-clefts and
the fore-gut, and unite, as branchial veins, above the gill-crate in a
large trunk blood-vessel that runs under the chorda dorsalis. This is
the principal artery or primitive aorta (Figure 2.214 D). The branches
which it gives off to all parts of the body unite again in a larger
venous vessel at the underside of the gut, called the subintestinal
vein (Figures 1.210 o and 2.212 E). This single main vessel of the
Amphioxus goes like a closed circular water-conduit along the
alimentary canal through the whole body, and pulsates in its whole
length above and below. When the upper tube contracts the lower one is
filled with blood, and vice versa. In the upper tube the blood flows
from front to rear, then back from rear to front in the lower vessel.
The whole of the long tube that runs along the ventral side of the
alimentary canal and contains venous blood may be called the
"principal vein," and may be compared to the ventral vessel in the
worms. On the other hand, the long straight vessel that runs along the
dorsal line of the gut above, between it and the chorda, and contains
arterial blood, is clearly identical with the aorta or principal
artery of the other vertebrates; and on the other side it may be
compared to the dorsal vessel in the worms.
(FIGURE 2.214. Transverse section of a young Amphioxus, immediately
after metamorphosis, through the hindermost third (between the
atrium-cavity and the anus).
FIGURE 2.215. Diagram of preceding. (From Hatschek.) A epidermis, B
medullary tube, C chorda, D aorta, E visceral epithelium, F
subintestinal vein. 1 corium-plate, 2 muscle-plate, 3 fascie-plate, 4
outer chorda-sheath, 5 myoseptum, 6 skin-fibre plate, 7 gut-fibre
plate, I myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.)
The coeloma or body-cavity has some very important and distinctive
features in the Amphioxus. The embryology of it is most instructive in
connection with the stem-history of the body-cavity in man and the
other vertebrates. As we have already seen (Chapter 1.10), in these
the two coelom-pouches are divided at an early stage by transverse
constrictions into a double row of primitive segments (Figure 1.124),
and each of these subdivides, by a frontal or lateral constriction,
into an upper (dorsal) and lower (ventral) pouch.
These important structures are seen very clearly in the trunk of the
amphioxus (the latter third, Figures 2.212 to 2.215), but it is
otherwise in the head, the foremost third (Figure 2.216). Here we find
a number of complicated structures that cannot be understood until we
have studied them on the embryological side in the next chapter (cf.
Figure 1.81). The branchial gut lies free in a spacious cavity filled
with water, which was wrongly thought formerly to be the body-cavity
(Figure 2.216 A). As a matter of fact, this atrium (commonly called
the peribranchial cavity) is a secondary structure formed by the
development of a couple of lateral mantle-folds or gill-covers (M1,
U). The real body-cavity (Lh) is very narrow and entirely closed,
lined with epithelium. The peribranchial cavity (A) is full of water,
and its walls are lined with the skin-sense layer; it opens outwards
in the rear through the respiratory pore (Figure 2.210 c).
On the inner surface of these mantle-folds (M1), in the ventral half
of the wide mantle cavity (atrium), we find the sex-organs of the
Amphioxus. At each side of the branchial gut there are between twenty
and thirty roundish four-cornered sacs, which can clearly be seen from
without with the naked eye, as they shine through the thin transparent
body-wall. These sacs are the sexual glands they are the same size and
shape in both sexes, only differing in contents. In the female they
contain a quantity of simple ova (Figure 2.219 g); in the male a
number of much smaller cells that change into mobile ciliated cells
(sperm-cells). Both sacs lie on the inner wall of the atrium, and have
no special outlets. When the ova of the female and the sperm of the
male are ripe, they fall into the atrium, pass through the gill-clefts
into the fore-gut, and are ejected through the mouth.
(FIGURE 2.216. Transverse section of the lancelet, in the fore half.
(From Ralph.) The outer covering is the simple cell-layer of the
epidermis (E). Under this is the thin corium, the subcutaneous tissue
of which is thickened; it sends connective-tissue partitions between
the muscles (M1) and to the chorda-sheath. N medullary tube, Ch
chorda, Lh body-cavity, A atrium, L upper wall of same, E1 inner wall,
E2 outer wall, Lh1 ventral remnant of same, Kst gill-reds, M ventral
muscles, R seam of the joining of the ventral folds (gill-covers), G
sexual glands.)
Above the sexual glands, at the dorsal angle of the atrium, we find
the kidneys. These important excretory organs could not be found in
the Amphioxus for a long time, on account of their remote position and
their smallness; they were discovered in 1890 by Theodor Boveri
(Figure 2.217 x). They are short segmented canals; corresponding to
the primitive kidneys of the other vertebrates (Figure 2.218 B). Their
internal aperture (Figure 2.217 B) opens into the body-cavity; their
outer aperture into the atrium (C). The prorenal canals lie in the
middle of the line of the head, outwards from the uppermost section of
the gill-arches, and have important relations to the branchial vessels
(H). For this reason, and in their whole arrangement, the primitive
kidneys of the Amphioxus show clearly that they are equivalent to the
prorenal canals of the Craniotes (Figure 2.218 B). The prorenal duct
of the latter (Figure 2.218 C) corresponds to the branchial cavity or
atrium of the former (Figure 2.217 C).
(FIGURE 2.217. Transverse section through the middle of the Amphioxus.
(From Boveri.) On the left a gill-rod has been struck, and on the
right a gill-cleft; consequently on the left we see the whole of a
prorenal canal (x), on the right only the section of its fore-leg. A
genital chamber (ventral section of the gonocoel), x pronephridium, B
its coelom-aperture, C atrium, D body-cavity, E visceral cavity, F
subintestinal vein, G aorta (the left branch connected by a branchial
vessel with the subintestinal vein), H renal vessel.
FIGURE 2.218. Transverse section of a primitive fish embryo
(Selachii-embryo, from Boveri.). To the left pronephridia (B), the
right primitive kidneys (A). The dotted lines on the right indicate
the later opening of the primitive kidney canals (A) into the prorenal
duct (C). D body-cavity, E visceral cavity, F subintestinal vein, G
aorta, H renal vessel.)
If we sum up the results of our anatomic study of the Amphioxus, and
compare them with the familiar organisation of man, we shall find an
immense distance between the two. As a fact, the highest summit of the
vertebrate organisation which man represents is in every respect so
far above the lowest stage, at which the lancelet remains, that one
would at first scarcely believe it possible to class both animals in
the same division of the animal kingdom. Nevertheless, this
classification is indisputably just. Man is only a more advanced stage
of the vertebral type that we find unmistakably in the Amphioxus in
its characteristic features. We need only recall the picture of the
ideal Primitive Vertebrate given in a former chapter, and compare it
with the lower stages of human embryonic development, to convince
ourselves of our close relationship to the lancelet. (Cf. Chapter
1.11.)
It is true that the Amphioxus is far below all other living
vertebrates. It is true that it has no separate head, no developed
brain or skull, the characteristic feature of the other vertebrates.
It is (probably as a result of degeneration) without the auscultory
organ and the centralised heart that all the others have; and it has
no fully-formed kidneys. Every single organ in it is simpler and less
advanced than in any of the others. Yet the characteristic connection
and arrangement of all the organs is just the same as in the other
vertebrates. All these, moreover, pass, during their embryonic
development, through a stage in which their whole organisation is no
higher than that of the Amphioxus, but is substantially identical with
it.
(FIGURE 2.219. Transverse section of the head of the Amphioxus (at the
limit of the first and second third of the body). (From Boveri) a
aorta (here double), b atrium, c chorda, co umlaut coeloma
(body-cavity), e endostyl (hypobranchial groove), g gonads (ovaries),
kb gill-arches, kd branchial gut, l liver-tube (on the right,
one-sided), m muscles, n renal canals, r spinal cord, sn spinal
nerves, sp gill-clefts.)
In order to see this quite clearly, it is particularly useful to
compare the Amphioxus with the youthful forms of those vertebrates
that are classified next to it. This is the class of the Cyclostoma.
There are to-day only a few species of this once extensive class, and
these may be distributed in two groups. One group comprises the
hag-fishes or Myxinoides. The other group are the Petromyzontes, or
lampreys, which are a familiar delicacy in their marine form. These
Cyclostoma are usually classified with the fishes. But they are far
below the true fishes, and form a very interesting connecting-group
between them and the lancelet. One can see how closely they approach
the latter by comparing a young lamprey with the Amphioxus. The chorda
is of the same simple character in both; also the medullary tube, that
lies above the chorda, and the alimentary canal below it. However, in
the lamprey the spinal cord swells in front into a simple pear-shaped
cerebral vesicle, and at each side of it there are a very simple eye
and a rudimentary auditory vesicle. The nose is a single pit, as in
the Amphioxus. The two sections of the gut are also just the same and
very rudimentary in the lamprey. On the other hand, we see a great
advance in the structure of the heart, which is found underneath the
gills in the shape of a centralised muscular tube, and is divided into
an auricle and a ventricle. Later on the lamprey advances still
further, and gets a skull, five cerebral vesicles, a series of
independent gill-pouches, etc. This makes all the more interesting the
striking resemblance of its immature larva to the developed and
sexually mature Amphioxus.
While the Amphioxus is thus connected through the Cyclostoma with the
fishes, and so with the series of the higher vertebrates, it is, on
the other hand, very closely related to a lowly invertebrate marine
animal, from which it seems to be entirely remote at first glance.
This remarkable animal is the sea-squirt or Ascidia, which was
formerly thought to be closely related to the mussel, and so classed
in the molluscs. But since the remarkable embryology of these animals
was discovered in 1866, there can be no question that they have
nothing to do with the molluscs. To the great astonishment of
zoologists, they were found, in their whole individual development, to
be closely related to the vertebrates. When fully developed the
Ascidiae are shapeless lumps that would not, at first sight, be taken
for animals at all. The oval body, frequently studded with knobs or
uneven and lumpy, in which we can discover no special external organs,
is attached at one end to marine plants, rocks, or the floor of the
sea. Many species look like potatoes, others like melon-cacti, others
like prunes. Many of the Ascidiae form transparent crusts or deposits
on stones and marine plants. Some of the larger species are eaten like
oysters. Fishermen, who know them very well, think they are not
animals, but plants. They are sold in the fish markets of many of the
Italian coast-towns with other lower marine animals under the name of
"sea-fruit" (frutti di mare). There is nothing about them to show that
they are animals. When they are taken out of the water with the net
the most one can perceive is a slight contraction of the body that
causes water to spout out in two places. The bulk of the Ascidiae are
very small, at the most a few inches long. A few species are a foot or
more in length. There are many species of them, and they are found in
every sea. As in the case of the Acrania, we have no fossilised
remains of the class, because they have no hard and fossilisable
parts. However, they must be of great antiquity, and must go back to
the primordial epoch.
The name of "Tunicates" is given to the whole class to which the
Ascidiae belong, because the body is enclosed in a thick and stiff
covering like a mantle (tunica). This mantle--sometimes soft like
jelly, sometimes as tough as leather, and sometimes as stiff as
cartilage--has a number of peculiarities. The most remarkable of them
is that it consists of a woody matter, cellulose--the same vegetal
substance that forms the stiff envelopes of the plant-cells, the
substance of the wood. The tunicates are the only class of animals
that have a real cellulose or woody coat. Sometimes the cellulose
mantle is brightly coloured, at other times colourless. Not
infrequently it is set with needles or hairs, like a cactus. Often we
find a mass of foreign bodies--stone, sand, fragments of
mussel-shells, etc.--worked into the mantle. This has earned for the
Ascidia the name of "the microcosm."
(FIGURE 2.220. Organisation of an Ascidia (left view); the dorsal side
is turned to the right and the ventral side to the left, the mouth (o)
above; the ascidia is attached at the tail end. The branchial gut
(br), which is pierced by a number of clefts, continues below in the
visceral gut. The rectum opens through the anus (a) into the atrium
(cl), from which the excrements are ejected with the respiratory water
through the mantle-hole or cloaca (a); m mantle. (From Gegenbaur.)
FIGURE 2.221. Organisation of an Ascidia (as in Figure 2.220, seen
from the left). sb branchial sac, v stomach, i small intestine, c
heart, t testicle, vd sperm-duct, o ovary, o apostrophe ripe ova in
the branchial cavity. The two small arrows indicate the entrance and
exit of the water through the openings of the mantle. (From
Milne-Edwards.))
The hind end, which corresponds to the tail of the Amphioxus, is
usually attached, often by means of regular roots. The dorsal and
ventral sides differ a good deal internally, but frequently cannot be
distinguished externally. If we open the thick tunic or mantle in
order to examine the internal organisation, we first find a spacious
cavity filled with water--the mantle-cavity or respiratory cavity
(Figure 2.220 cl). It is also called the branchial cavity and the
cloaca, because it receives the excrements and sexual products as well
as the respiratory water. The greater part of the respiratory cavity
is occupied by the large grated branchial sac (br). This is so like
the gill-crate of the Amphioxus in its whole arrangement that the
resemblance was pointed out by the English naturalist Goodsir, years
ago, before anything was known of the relationship of the two animals.
As a fact, even in the Ascidia the mouth (o) opens first into this
wide branchial sac. The respiratory water passes through the
lattice-work of the branchial sac into the branchial cavity, and is
ejected from this by the respiratory pore (a apostrophe). Along the
ventral side of the branchial sac runs a ciliated groove--the
hypobranchial groove which we have previously found at the same spot
in the Amphioxus. The food of the Ascidia also consists of tiny
organisms, infusoria, diatoms, parts of decomposed marine plants and
animals; etc. These pass with the water into the gill-crate and the
digestive part of the gut at the end of it, at first into an
enlargement of it that represents the stomach. The adjoining small
intestine usually forms a loop, bends forward, and opens by an anus
(Figure 2.220 a), not directly outwards, but first into the mantle
cavity; from this the excrements are ejected by a common outlet (a
apostrophe) together with the used-up water and the sexual products.
The outlet is sometimes called the branchial pore, and sometimes the
cloaca or ejection-aperture. In many of the Ascidiae a glandular mass
opens into the gut, and this represents the liver. In some there is
another gland besides the liver, and this is taken to represent the
kidneys. The body-cavity proper, or coeloma, which is filled with
blood and encloses the hepatic gut, is very narrow in the Ascidia, as
in the Amphioxus, and is here also usually confounded with the wide
atrium, or peribranchial cavity, full of water.
There is no trace in the fully-developed Ascidia of a chorda dorsalis,
or internal axial skeleton. It is the more interesting that the young
animal that emerges from the ovum HAS a chorda, and that there is a
rudimentary medullary tube above it. The latter is wholly atrophied in
the developed Ascidia, and looks like a small nerve-ganglion in front
above the gill-crate. It corresponds to the upper "gullet-ganglion" or
"primitive brain" in other vermalia. Special sense-organs are either
wanting altogether or are only found in a very rudimentary form, as
simple optic spots and touch-corpuscles or tentacles that surround the
mouth. The muscular system is very slightly and irregularly developed.
Immediately under the thin corium, and closely connected with it, we
find a thin muscle tube, as in the worms. On the other hand, the
Ascidia has a centralised heart, and in this respect it seems to be
more advanced than the Amphioxus. On the ventral side of the gut, some
distance behind the gill-crate, there is a spindle-shaped heart. It
retains permanently the simple tubular form that we find temporarily
as the first structure of the heart in the vertebrates. This simple
heart of the Ascidia has, however, a remarkable peculiarity. It
contracts in alternate directions. In all other animals the beat of
the heart is always in the same direction (generally from rear to
front); it changes in the Ascidia to the reverse direction. The heart
contracts first from the rear to the front, stands still for a minute,
and then begins to beat the opposite way, now driving the blood from
front to rear; the two large vessels that start from either end of the
heart act alternately as arteries and veins. This feature is found in
the Tunicates alone.
Of the other chief organs we have still to mention the sexual glands,
which lie right behind in the body-cavity. All the Ascidiae are
hermaphrodites. Each individual has a male and a female gland, and so
is able to fertilise itself. The ripe ova (Figure 2.221 o apostrophe)
fall directly from the ovary (o) into the mantle-cavity. The male
sperm is conducted into this cavity from the testicle (t) by a special
duct (vd). Fertilisation is accomplished here, and in many of the
Ascidiae developed embryos are found. These are then ejected with the
breathing-water through the cloaca (q), and so "born alive."
If we now glance at the entire structure of the simple Ascidia
(especially Phallusia, Cynthia, etc.) and compare it with that of the
Amphioxus, we shall find that the two have few points of contact. It
is true that the fully-developed Ascidia resembles the Amphioxus in
several important features of its internal structure, and especially
in the peculiar character of the gill-crate and gut. But in most other
features of organisation it is so far removed from it, and is so
unlike it in external appearance, that the really close relationship
of the two was not discovered until their embryology was studied. We
will now compare the embryonic development of the two animals, and
find to our great astonishment that the same embryonic form develops
from the ovum of the Amphioxus as from that of the Ascidia--a typical
chordula.