THE EARLY VERTEBRATES AND THE FISHES
The lowest of the vertebrates--if indeed it can be called a vertebrate
at all, seeing that it has no vertebræ--is the lancelet, Amphioxus.
The common species of this animal (there are some eight in all) occurs
in the sea off our own coasts, and is usually to be found half buried
in the sand or mud of the sea floor. It is some two inches in length
and has the shape of a laterally flattened cigar, and one of its very
obvious features is the arrangement of the muscles in regular layers
from front to back, in the same manner as those of a fish.
To describe some of its features in detail, the alimentary canal bears
a somewhat striking similarity to that of Balanoglossus. There is a
round, simple mouth, unprovided with jaws, and surrounded by a number
of projecting bristles. This leads into a large pharynx, through the
walls of which, on either side, pass a large number of gill slits. The
pharynx is not divided into an upper and a lower canal, but there is a
shallow groove along the bottom which serves the same purpose as the
food canal in Balanoglossus. The remaining, digestive, part of the gut
is practically a simple tube, with a blind sac attached, representing
the liver. The gill slits do not open directly to the exterior, but
into the so-called peribranchial chamber, formed by the junction below
the body of two flap-like outgrowths, one from the upper part of either
side. This chamber opens to the outside by a single pore.
Above the gut lies a straight, cylindrical rod of cartilage, pointed
at either end. This is the highly important structure known as the
_Notochord_, which is present in all the vertebrates, although in the
higher forms it is replaced during development by the vertebræ, the
bony segments of the backbone. Above the notochord again lies the
main nerve cord, a position which it retains throughout the whole
vertebrate group. The nerve cord is simple in structure, with only a
very slight swelling at the front end, representing the brain. There
are two main blood vessels, an upper and a lower, which expand and
contract alternately throughout their whole length, and thus maintain
the circulation. The blood passes forward in the ventral vein, is
pumped through the fine vessels of the gills, and collected into the
upper artery. From this it is distributed throughout the body by branch
vessels, to be re-collected into the ventral vein. If the reader will
refer to the illustrations in Figs. 61, 62, and 63, the relationships
of these parts will be more easily understood. There is a single small
eye-spot, a single organ of smell, but no hearing organs. It seems
probable that this extremely ill-developed condition of the sensory
system is due in some measure to degeneration, and is not a primitive
characteristic. There are numerous pairs of simple nephridia, which
open into the peribranchial chamber, and bear a close resemblance to
those of the worms.
The lancelet forms a most important link between the lower and the
higher animals. It is in all probability derived from some form similar
to Balanoglossus, and it certainly leads up to the round-mouths, which
form the next step in the ladder.
[Illustration: FIG. 61.--The Lancelet--Amphioxus.]
[Illustration: FIG. 62.--Diagrammatic cross-section of Amphioxus.
_n_, Nerve cord; _ch_, notochord; _mus_, muscle tissue; _ec_, skin;
_bl_, blood vessel; _cöl_, body cavity; _kd_, pharynx; _ld_, liver
sac; _g_, reproductive gland; _p_, peribranchial chamber.]
[Illustration: FIG. 63.--Diagrams of Tunicate (on the left), Amphioxus
(centre), and young Lamprey (right).
_o_, Mouth; _au_, eye; _c_, peribranchial chamber; _ch_, notochord;
_d_, alimentary canal; _g_, ear; _hz_, heart; _k_, gills; _lb_, liver;
_m_, nerve cord; _m_, brain; _mg_, stomach; _mt_, mantle; _z_, tunicate
embryos.]
Before describing these latter, however, we must briefly allude to the
highly remarkable group of the tunicates or sea squirts, one of which
is shown in Fig. 64. They are sedentary creatures found attached to
rocks or weeds on the sea floor, and in appearance they remind one
rather of misshapen potatoes than of higher animals. They are in fact
regarded by the fishermen who bring them to the surface as plants, and
they were for long looked upon by zoologists as akin to the molluscs.
The only definite external features of the tunicates are two apertures
at the upper end, one in the centre and one somewhat on one side. The
absence of any other definite external characters is due to the fact
that the creature is enclosed in a mantle of cellulose. The central
opening is the mouth, which leads into a large pharynx, the walls
of which are perforated by numerous gill slits. This is surrounded by
the mantle cavity, which connects with the outer water by means of the
second pore. The gut is continued into a simple stomach and intestine,
the latter bending back upon itself and opening into the bottom of the
mantle cavity, as shown in the diagram in Fig. 63. In the adult animal
there is no trace of the notochord, and only a remnant of the nerve
cord; and there are either no special sense organs or only traces of
these. On the other hand, the tunicates possess a centralised heart.
They are hermaphrodite, and, very curiously, a number of forms multiply
like corals, by a simple process of budding.
[Illustration: FIG. 64.--A Tunicate.
_in.ap._, Opening leading to mouth; _ex.ap._, opening of peribranchial
chamber.
From Dendy's _Outlines of Evolutionary Biology_ (Constable).]
[Illustration: FIG. 65.--Larva of Tunicate.
_n_, Nerve cord; _s_, sense organs; _m_, mouth; _kd_, pharynx; _vd_,
alimentary canal; _ch_, notochord.]
Now the remarkable fact has been made out that the young tunicate (see
Fig. 65) bears a most striking resemblance to the immature lancelet. It
is a free-swimming, tadpole-like creature, and possesses a notochord
and nerve cord and in general the same characters that we described
for Amphioxus. It is only later that the creature settles down and
assumes its final degenerate sedentary form. There can be no doubt that
the tunicates have been derived from some lancelet-like form, but the
course of their evolution has been unique. The type is the lost brother
of the vertebrate family, who has chosen a distinctly downward path in
life; yet who has come to no miserable end, but lives on, more or less
successfully, in his lower social sphere.
The round-mouths, including the lampreys and the hag-fish, stand midway
between the lancelet and the fishes, and therefore constitute for us an
important group.
The lamprey is a fairly generally known eel-like creature, of which
there is a smaller fresh-water, and a larger salt-water species, the
latter reaching a length of about a yard. It is found attached to, and
feeding on, the dead bodies of fish, and less frequently on living
specimens. The hag is much more definitely parasitic in its habits,
and often occurs in the body cavity of living fish. These forms were
for long regarded as fishes, and are sometimes even yet included
in that group, but all their characteristics point to a very much
lower position in the animal world than such a classification would
indicate. One of the most striking external differences is that the
round-mouths have nothing to represent the two pairs of fins which
occur uniformly in the fishes, and which are, in a true sense, the
forebears of our own arms and legs. Even more important than this is
the absence of jaws. The mouth in this group is a simple round opening,
whose edges are armed with pointed teeth, the latter, however, bearing
no real resemblance to the teeth of the higher animals. By means of
these teeth, and a pointed, tongue-like organ, and by suction, the
round-mouths are able to bore into the tissues of the animals on which
they prey. The absence of jaws and of extremities is, of course, a
feature which they share with the lancelet. Turning to the internal
structure, we may first observe that there is still no vertebral
column, but only a simple, rod-like notochord, similar in its shape,
and in its relations to other parts, to that of Amphioxus. There
is, however, an additional development of cartilage in the region
of the head, forming, in particular, a sheath-like covering for the
brain and also a kind of basket-work support for the pharynx and
gills. The original tube-like form of the dorsal nerve cord is easily
recognisable, but it is distinctly distended at its front end into a
brain, which shows a division into a series of three distinct portions,
called respectively the fore, mid, and hind brain. This division, it
is interesting to observe, is the first process in the development of
the brain in all the higher animals. There is a pair of well-developed
hearing organs, and in the lamprey a pair of similarly well-developed
eyes. In the hag fish the latter are greatly reduced, a condition which
is explained by the creature's mode of life. The nostril is unpaired,
a condition which is probably primitive. Respiration is carried out by
means of gills, which are situated in a series of six to eight pouches,
each of which opens into the gullet and again directly to the outside,
the external openings being an obvious feature of the animal. There
is a very distinct, simple heart, which pumps the blood to the gills,
from whence it is collected and distributed throughout the body. The
digestive canal is a simple tube, provided, however, with a liver and
a pancreas, the two most important digestive glands in the higher
animals. The sexes are separate, but traces of a previous hermaphrodite
condition seem to persist. Henceforward in the vertebrate group the
sexes are always separate. A character of the sex organs which is to be
regarded as primitive is that they are unconnected with the excretory
system, whereas in the higher vertebrates the two systems are always
strangely interconnected. As in all the higher types, there is but one
pair of male or female reproductive glands. Finally, the round-mouths
differ markedly from the lancelet in the structure of the skin. In the
latter animal the skin is composed of a single layer of cells. In the
former it consists of an epidermis, some three or four layers thick,
and an underlying cuticle or 'true skin'; in other words, the skin has
the same general structure as that of the higher types. The lamprey
and the hag are illustrated in Figs. 66 and 67, and some of the main
anatomical features of the group are shown in Figs. 63, 68, and 69.
[Illustration: FIG. 66.--The Lamprey--Petromyzon.]
[Illustration: FIG. 67.--The Hag-fish--Myxine.]
There remain, even after the most thorough investigation of
Balanoglossus, the lancelet, and the round-mouths, some questions with
regard to the origin of the vertebrates that are still unanswered. It
must, however, be regarded as an extremely fortunate circumstance that
representatives have come down to us of the three ancient groups to
which these three types respectively belong. This is the more fortunate
in that the groups in question are known only from their few living
members, a circumstance which is of course easily accounted for by the
absence of any hard parts capable of being preserved as fossils. From
this point onwards there is a skeleton, and we are consequently enabled
to draw valuable information from fossils. Partly in consequence of
this, the Evolution chain from this point onwards is much more complete
than the portion that we have dealt with thus far.
[Illustration: FIG. 68.--Diagrammatic cross-section of lamprey larva.
_n_, Nerve cord; _ch_, notochord; _ar_, artery; _v_, vein; _g_,
reproductive body; _d_, alimentary canal.]
[Illustration: FIG. 69.--Mouth of lamprey.]
We have already observed that the true fishes, to which we must
now direct our attention, differ from the round-mouths in several
important characters. They possess two pairs of extremities, the
pectoral and pelvic fins; they have a pair of nostrils; there is also
a well-developed skull, which includes a series of cartilaginous or
bony arches situated in the wall of the gut and between the successive
gill clefts. These branchial arches bear a certain resemblance to
the basket-work arrangement of cartilage in the round-mouths, but
for various reasons are not regarded as having been derived from the
latter. It is from the first pair of these arches that the jaws are
formed, organs which make their first appearance in the lower fishes.
The skeleton shows great development in other directions. The notochord
is present in its primitive condition during the earlier stages of
development, but it becomes surrounded, and in many cases largely
suppressed, by the portions of the vertebræ. Each vertebra consists
of an upper and a lower portion, the upper forming an arch round the
nerve cord and the lower bearing lateral processes or ribs. In the
higher forms the two portions become united round the notochord, and
the resulting vertebra may encroach inwards until it becomes solid, the
notochord then remaining only as a series of small pieces of cartilage
between the successive units of the vertebral column. There is also,
of course, a skeleton in connection with the limbs, but this does not
yet correspond in detail to that of the other classes of vertebrates.
The brain is much more highly developed than in the round-mouths; in
many forms, particularly, there is a considerable development of the
cerebral hemispheres, a portion of the fore brain, and the seat of the
higher intelligence. The eyes and ears show the same main features as
in the higher groups. The ear has three semicircular canals, the same
number as in man, as against two in the lamprey and one in the hag.
Fishes are possessed of a peculiar 'sixth sense,' the organs for which
are situated in two lines running along the sides of the body, the
latter forming a familiar feature of a cod or whiting. The nature of
this sense is not definitely known, but it appears to be of the nature
of a very refined appreciation of wave motions in the water. It is
probably by means of these 'lateral line' sense organs, for instance,
that fishes are so easily able to avoid obstacles when swimming in the
dark.
[Illustration: FIG. 70.--Diagrammatic dissection of dog-fish (Scyllium).
_sk_, Skull; _gh_, brain; _n_, nerve cord; _ch_, notochord; _ho_,
reproductive gland; _un_, kidney; _wk_, vertebræ; _k_, jaw; _z_,
tongue; _ks_, gill slits; _ar_, arteries; _ph_, pharynx; _h_, heart;
_lb_, liver; _m_, stomach; _mil_, spleen; _pan_, pancreas; _vd_,
intestine, with spiral fold; _cöl_, body cavity; _r_, rectum.]
The heart has one auricle and one ventricle, except in a single group
which we shall afterwards mention. The heart is situated immediately
behind the gills, to which the blood is pumped directly by the
ventricle. From the gills, the blood is collected and distributed
throughout the body, is re-collected and returned to the auricle. The
circulatory system is provided with a set of blood-glands, essentially
similar to those in man himself. There is a spleen, a thymus and a
thyroid gland, and a pair of suprarenal bodies. The several functions
of these glands form an extremely difficult chapter of physiology,
but, broadly speaking, they are concerned in the formation of the
white blood corpuscles, the removal of worn-out red corpuscles, and in
certain obscure but important chemical changes in the composition of
the blood. The blood itself consists of a fluid plasma in which float
white and red blood corpuscles, the latter being flat and oval, and
containing the same oxygen-carrying substance, hæmoglobin, as is found
in mammals.
The alimentary canal is simple. The mouth cavity is succeeded by the
pharynx, the walls of which are perforated by the gill clefts. Next
follow the gullet, the stomach, and the intestine, the division into
the three portions being apparent often only after close examination.
There are generally gastric glands, of simple form, a large liver, and
almost always a pancreas. The kidneys and the reproductive organs open
to the exterior by a common duct. A further characteristic feature of
the fishes is their external covering of scales. True teeth, comparable
to those of the higher vertebrates, appear first in this group. Some of
the main features that we have mentioned are illustrated in Fig. 70.
Careful study of the fishes makes it evident that they have very much
in common with the higher groups of vertebrates. It is not too much to
say, with Haeckel, that there is far more difference between Amphioxus
and the fishes than between the fishes and man.
There are four main divisions of the fish group. The first, that of the
Elasmobranchs, comprises the sharks and dog-fishes, the skates and the
rays. The second group, the Ganoids, includes the sturgeon and a few
less well-known forms. The third, the so-called bony or food fishes,
includes the vast majority of ordinary species, such as the salmon
and trout, the cod, herring, eel, and all our ordinary freshwater
species. The fourth, the 'lung fishes,' consists of three very
remarkable species, which we shall later describe in detail. The mutual
relationships of these groups is well understood, and it is possible to
make fairly definite statements regarding their evolution.
The Elasmobranchs are at once the most primitive and, so far as is
known, the oldest of the four. From these evolved the lower Ganoids,
which then divided into two main branches, the first of which led up to
the higher Ganoids and through them, at a comparatively late date, to
the bony fishes. The second led to the lung fishes and, either through
them or along a somewhat parallel line, to the amphibians and the land
vertebrates generally. It is with the second line, therefore, that we
shall be mainly concerned.
The Elasmobranchs are characterised by the fact that the gill slits
open individually to the exterior, there being no gill cover, such
as is found in the other groups. Their scales are simple, tooth-like
projections, and in fact there is no essential difference between them
and the teeth. The skull is more primitive than in the other groups,
but a discussion of its details would necessarily be very involved. The
living members of the group show a fairly high stage of development
of the vertebræ--considerably higher, in fact, than that found in the
lung fishes--but some extinct members showed a very primitive condition
with regard to this point. In the fossil skeleton shown in Fig. 71, for
instance, it is apparent that the notochord was present as a simple
continuous rod. The skeleton in question is from the Permian and
belongs to what is regarded as the most primitive type of fish known.
Two specimens of Elasmobranchs are shown in Figs. 72 and 73, and the
teeth of a shark in Fig. 74.
[Illustration:
FIG. 71.--Fossil skeleton of _Pleuracanthus Decheri_, a primitive
Elasmobranch.]
[Illustration:
Photo: Thiele.
FIG. 72.--Spotted dog-fish.]
[Illustration:
Photo: Thiele.
FIG. 73.--Blue Shark.]
[Illustration: FIG. 74.--Teeth of Shark.]
In the Ganoids and bony fishes there is a gill cover, and in all
but a few Ganoids there is some formation of true bone, whereas in
the Elasmobranchs the skeleton is wholly cartilaginous. One of the
most striking anatomical features of these groups, and one which
distinguishes them from the Elasmobranchs, is the presence of a swim
bladder, a large sac-like outgrowth from the upper part of the gut. The
function of the swim bladder is that of regulating the specific gravity
of the fish, which becomes greater or less according as air is expelled
or taken in. The Ganoids and bony fishes are illustrated in Figs. 75 to
79.
[Illustration:
Photo: Underwood.
FIG. 75.--Ganoid fishes--The Sturgeon.]
The lung fishes or Dipnoi present a curious mixture of primitive and
of highly advanced characters. In their persistent notochord and their
inconsiderable formation of bone, they are much more primitive than
the food fishes. On the other hand, an extremely important departure
is seen in the adaption of the swim bladder as a respiratory organ. In
one of the three existing species this organ is single, in the others
it is double. The wall of the swim bladder is thick, and contains
considerable muscle tissue. Its inner surface is covered with a complex
system of pits and blind sacs, the walls of which contain numerous
capillary vessels.
[Illustration: FIG. 76.--Ganoid fishes--_Polypterus bihir_.]
[Illustration: FIG. 77.--Ganoid fishes--American bony pike,
Lepidosteus.]
[Illustration:
Photo: Thiele.
FIG. 78.--Bony- or food-fishes--The Salmon.]
[Illustration: FIG. 79.--Bony- or food-fishes--The Common Eel.
Photo: Underwood.]
[Illustration: FIG. 80.--Lung fishes--Australian lung fish, Ceratodus.]
[Illustration: FIG. 81.--Lung fishes--Protopterus (Africa).]
[Illustration: FIG. 82.--Lung fishes--Lepidosiren (South America).]
[Illustration:
FIG. 83.--Swim bladder (lung) of Protopterus.]
There are three living species of lung fishes, one of which is found in
Australia, another in Tropical Africa, and a third in the tributaries
of the Amazon. All live under conditions which make ordinary
respiration by gills difficult. The Australian species inhabits
rivers which become reduced, in the dry season, to stagnant pools of
foul water, in which ordinary fish frequently fail to survive. Under
such circumstances the creature comes periodically to the surface to
breathe. The other two species live in rivers which actually dry up
in summer, and the fishes bury themselves in the mud, and remain
in a torpid condition, breathing air by their lungs, until the rainy
season comes round, perhaps four or even six months later. Correlated
with the special method of respiration is a special type of blood
system, whereby part of the blood is pumped direct to the lungs, and
returns direct to the heart. There are two auricles, to receive the
blood from the lungs and the general circulation respectively, but only
one ventricle, in which the two streams become mixed. Figs. 80 to 82
illustrate the three existing Dipnoi, and the structure of the lung is
shown in Fig. 83.
It is obvious, from the distribution of the lung fishes, and also
from geological evidence, that the group was once very plentifully
represented, and has only been preserved from total extinction by
peculiar circumstances.
Regarding the position of the group, some zoologists regard them as the
direct ancestors of the Amphibians. Others believe that the group had a
common origin with the bony fishes and the Amphibia in some early form
of Ganoid. In any case, the Dipnoi possess an extraordinary interest as
showing the beginnings of adaption to a life out of the water.