THE WORMS AND SOME OF THEIR POSTERITY
The somewhat miscellaneous collection of animals that have been thrown
together and termed _worms_ is of the greatest importance for our
theory of descent. Indeed, it seems probable that all of the four great
groups which we have yet to mention have descended directly from worm
ancestors. This, at all events, is the view of Haeckel, although it
must be admitted that many other theories have been proposed. Nor can
it be taken as a matter for surprise that agreement concerning this
part of our history should be hard to reach, for the difficulties which
are met in it are many and perplexing.
The worms comprise many greatly divergent groups, and the difference
between the lowest and the highest of these has been produced by many
important steps in Evolution. Of these groups but few immediately
concern us; the first and lowest of those which do, is that of
the Turbellarians, a section of the Platodes or flat-worms. The
Turbellarians are small or microscopic tongue-shaped organisms, of
which the majority of species live on the sea-floor, others however
being found in fresh water. The surface of the body is covered
uniformly with cilia, which serve, in the smaller forms, as organs
of propulsion, while in the larger they appear to have the function
of maintaining a flow of fresh water over the surface, and thus of
assisting respiration. In some respects there has been little advance
from the condition of the Cœlenterate. The digestive cavity is a
simple or more or less divided sac, communicating with the exterior
only by means of the mouth. Unlike the condition of affairs in the
Cœlenterates and Ctenophora, however, the sex glands do not discharge
the reproductive bodies into the digestive cavity, but directly to
the exterior by means of a special opening. Each individual has a pair
of male and a pair of female reproductive glands, but the eggs are
not self-fertilised; nor is the fertilisation of the eggs trusted to
chance and the sea-water, as in the lower groups. Instead there is
a definite exchange of sperms between two individuals, and the eggs
are fertilised before they leave the body. They are also frequently
supplied with a store of nutritive material by a pair of special yolk
glands. A distinct step of progress can thus be recognised in the
arrangements for reproduction. Between the outer skin and the inner
digestive layer is developed a considerable mass of cells, forming
muscular and connective tissue, etc. It will readily be understood that
the development of such thick tissue masses occasions two distinct
new difficulties in the animal economy; for where cells are in direct
contact neither with the digestive layer nor with the exterior, their
nutrition and the removal of their waste products can no longer be
efficiently carried on without special devices. Hence on the one
hand a circulatory system, for the transport of food materials,
and on the other an excretory system, become necessary. The first
of these new departures was not destined to be made until the next
stage of progress; the Turbellarians seem to have temporarily got
over the difficulty, like the Ctenophora, by developing a complex
and ramifying digestive cavity. An excretory system, however, makes
its appearance here. Indeed, the beginnings of such a system can be
seen in the Ctenophora, in which there are small excretory organs
opening into the digestive cavity. The corresponding organs in the
worms, as in all subsequent types, open directly to the outside. In
the Turbellarians these organs, which are termed nephridia, are two in
number, and consist of long tubes which branch and ramify throughout
the body, the small branches terminating in special excreting cells,
and the whole constituting a complete and thorough drainage system.
The nervous system consists of one or two small masses of nerve cells
termed ganglia in the front region, with a somewhat complex network
of nerves connecting them with various parts of the body. There are
frequently two pairs of sense organs, probably rudimentary eyes and
ears respectively. The main features of the digestive, reproductive,
excretory, and nervous system are shown in the figures in Fig. 40.
[Illustration: FIG. 40.--A simple Turbellarian--Rhabdocœlum
(diagrammatic).
_m_, Mouth; _d_, digestive cavity; _nc_, nephridia; _au_, eyes; _na_,
sense organs; _g_, brain; _n_, nerves; _h_, male, and _e_, female,
reproductive glands.]
The Turbellarians probably arose from Ctenophora or from some nearly
related form, a view that receives support from the occurrence of
several apparently intermediate types. The differences may, in fact,
be partly accounted for as adaptations to meet the change of habitat
from that of the upper waters to that of the sea floor. A spherical, or
pear, or bell shape is suitable enough for a swimming animal, but would
be impossible for one that was to crawl. The first change, then, we may
imagine, was a flattening, which produced a disc-shaped animal, with
the mouth in the centre of the lower aspect and the sense organs in the
middle of the upper. Secondly, a definite mode of progression, by which
one part of the body continually went first, would be an advantage, as
permitting of a better co-ordination of movements, and an elongation of
the body in the line of movement would have the effect of diminishing
resistance and of making progression easier. Finally the sense organs,
like the scouts of an army, would be best in front, and would migrate
thither, and the mouth, in order to get the full benefit of the food
which the sense organs sought out, would gradually shift to a position
beside them. These adaptions, it is obvious, have produced a complete
change in the architecture of the animal. Our sea-anemone, or Medusa,
or Ctenophore is _radially symmetrical_. That is to say, its parts are
arranged like the spokes of a wheel, and it may be divided into two
equal halves by each of several planes passing through the main axis.
It has an upper and a lower surface, but no head and tail ends. The
lowest of the worms now can be divided into two halves only in one
direction, that which separates the right and left sides. They are, in
scientific language, _bilaterally symmetrical_. The change to this type
of architecture was a very important step of Evolution, particularly
in relation to locomotion. Bilateral symmetry was destined to remain a
constant feature of three of the four great groups that evolved from
the worms. The star-fishes reverted to the earlier condition.
[Illustration: FIG. 41.--A primitive flat-worm--Aphanostomum (× 50).
_a_, Mouth; _g_, sense organ; _i_, internal digestive tissue; _s_,
male, and _o_, female, reproductive glands; with _m_ and _f_, external
openings.]
The next class of worms with which we have to deal is that of the
Rotifera. In their general structure, and in their excretory and
sensory-nervous systems, the Rotifers do not differ essentially from
the Turbellarians. They do differ, however, in that the digestive
cavity has a second opening to the exterior, at the end opposite to
the mouth. The advantage of this arrangement, which was retained
in the subsequent stages of Evolution, is obvious, for it renders
possible a much more regular and thorough digestive process. Instead
of the food passing in, and the undigested remains passing out, by
the same opening, and instead of the contents of the digestive cavity
being a general mixture of food material in all stages of digestion,
there is now a regular stream of food passing through the cavity in
one direction, and being digested as it goes. A near relative of the
Rotifers is shown in Fig. 42.
[Illustration: FIG. 42.--Chænonotus, a lower worm.
_m_, Mouth; _e_, eye; _ss_, sensory hairs; _œ_, œsophagus; _sk_, skin;
_d_, digestive canal; _n_, nephridia; _ex_, excretory opening; _c_,
cilia; _a_, anus; _b_, brain; _mc_, muscle cells; _n_, nerves; _o_,
ovary.]
Thirdly, we must briefly allude to the Nemertines. These are a group of
flattened thread-like worms of very variable size, found both in fresh
and salt water. The most notable advance in this group is to be seen
in the occurrence of a special circulatory system. It has already been
indicated that the gastric cavity of the lower forms has the double
function of digestion and of the transport of nutritive substances to
the various parts of the body. In the Nemertines the second of these
functions is carried out by the blood system, which consists of two or
three vessels that run parallel throughout the length of the body and
anastomose at either end. There is no indication of any enlarged or
specially contractile portion of any of these, no indication, that is
to say, of a heart. The blood conveys not only nutritive substances,
but also, as in the higher animals, oxygen. Some Nemertines have indeed
red blood, containing true hæmoglobin, which is well known as the
oxygen-carrying material in the vertebrates. A typical Nemertine is
shown in Fig. 44, and a diagram showing some features of the anatomy
in Fig. 45. It will be seen that the nervous system is of the same
type as in the worms already described. There are two pairs of sense
organs, one pair being eyes, and the other probably having the function
of gauging the chemical nature of the water. The Nemertines possess
a peculiar organ in a snout or proboscis, which they can protrude or
withdraw into a special sac. The snout is armed with a sharp sting, and
forms an effective weapon whether against the creature's enemies or its
prey.
[Illustration: FIG. 43.--Nephridium of a Turbellarian.]
About this stage of Evolution, the exact point being somewhat difficult
to fix, there appears the body cavity. This, which is altogether
distinct from the digestive cavity, is a familiar feature of the
anatomy of the higher animals. In it are suspended the heart and lungs
and the whole of the digestive organs and glands. The question of the
origin of the body cavity and the blood system is a very difficult one,
and a thorough theoretical discussion would take us too far.
Before proceeding to the question of the origin of the vertebrates,
we may pause briefly to consider the other groups to which the worms
appear to have given rise. First of these we may take the Echinoderms,
which include the well-known star-fishes and sea-urchins, and the
very beautiful feather stars. As already indicated, it is believed
that the radial symmetry, which is so characteristic of this group,
is not a primitive feature, but that, in fact, the Echinoderms are
descended from bilaterally symmetrical ancestors. One reason for
this view is that the larval or immature form is always markedly
bilaterally symmetrical. In an ordinary star-fish, which we may take as
typical of the group, the mouth is in the middle of the lower aspect,
and the excretory opening of the digestive cavity in the upper side
just opposite. There is no blood system, or excretory organs, and no
concentration of nerve cells into any form of brain. Eyes, however,
are present, and sensitiveness to light may be easily demonstrated.
The most remarkable feature of the group is the water-vascular system,
consisting of a series of radial canals, one in each ray, which join a
circular one situated in the central portion of the body. The system of
canals communicates with the exterior by means of a sieve-like plate
on the upper surface, and it is kept full of water by the continual
pumping action of cilia on the walls of the tube which leads down from
the sieve plate.
[Illustration: FIG. 44.--A Nemertine--Tetrastemma.
Actual length about 1-1/2 inch.]
[Illustration: FIG. 45.--Diagram of Nemertine--Nemertopsis.
_cg_ and _sg_, Sense organ; _a_, eyes; _gh_, brain; _bl_, blood
vessels; _n_, nerve cord; _d_, alimentary canal; _ex_, nephridia; _r_,
snout, withdrawn.]
The ordinary star-fish is carnivorous, and lives largely on ordinary
mussels, which it bridges over with its arms, and opens by a steady
and long-continued pulling, the soft parts being then sucked up by the
partially protruded stomach. A few types of Echinoderms are shown in
Figs. 46, 47, 48.
[Illustration: FIG. 46.--Star-fishes.]
[Illustration: FIG. 47.--Feather Star.
Photo: Harold Bastin.]
The group of the Mollusca includes such common forms as cuttle-fishes,
whelks, slugs, snails, mussels, and oysters. These, it will be
observed, comprise marine, freshwater, and land forms. The molluscs,
like the next two groups with which we have to deal, have made a
conquest of the land, though in the present instance it cannot be
regarded as very complete. The anatomy of the group shows much
variation, and only a few of the leading features can be alluded to.
The digestive system is highly developed. The mouth is provided with
a jaw or jaws, and with a tongue-like ribbon, which is covered with
rows of teeth, like a file, and by whose action the food is torn and
disintegrated. A gullet leads from the mouth to a stomach, which is
followed by an intestine. Salivary glands and a large hepatic gland
or liver are present. Respiration occurs partly through the skin, but
special organs also exist for this function, gills in the water forms,
and a lung cavity in those which breathe air. There is a well-developed
blood system, and generally a heart; the blood is pumped direct from
the heart to the general body tissues, and returns to it by way of
the kidneys or nephridia, which purify it of waste materials, and the
respiratory organs, where it is freed of carbon dioxide and supplied
with oxygen. The nervous system varies greatly, but a pair of cerebral
ganglia--a brain--is usually present. There is a particularly keen
sense of smell, and taste and hearing may also easily be shown to
exist. Some forms are blind, from which condition there is a regular
series of stages of development of the eye, up to forms in which it
becomes a highly perfected organ, with cornea, iris, lens, and retina.
The close similarity between this and the ordinary vertebrate eye,
which must have evolved quite separately, is one of the strangest
coincidences of Evolution. Thus in many ways the molluscs are to be
regarded as highly specialised types. But in two important directions,
in intelligence and in their arrangements for locomotion, they stand as
a group on a low plane of development. Figs. 49, 50, and 51 illustrate
some of the forms met with in the group. The origin of the molluscs,
as well as that of the Echinoderms, is wrapped in obscurity. That
each group is derived from some form of worm is probable, yet some
zoologists hold even such a general statement as this to be lacking in
support.
[Illustration: FIG. 48.--Sea Urchin.
1, With spines broken off; 2, with spines on.]
[Illustration: FIG. 49.--Molluscs--Univalves.]
[Illustration: FIG. 50.--Molluscs--Bivalves.]
[Illustration: FIG. 51.--Molluscs--Cuttle-fish, with eggs.]
Our third great group is that of the Arthropods (literally 'jointed
footed'), including the Crustaceans (crabs, lobsters, shrimps, etc.),
spiders and mites, centipedes and insects. The Arthropods are sometimes
classed together with their ancestors, the ringed worms (such as the
common earth-worm), as Articulata, a name which refers to a very
obvious feature, the repetition of similar segments in a regular series
from front to rear. This is perhaps most apparent in the ringed worms
and centipedes, but it is to be seen in all members of the group. This
same tendency to reduplication of parts in a regular series may be
observed in the vertebrates, as we shall see. Slight indications of it
are also to be found in the Nemertines. Numerous theories have been
proposed which derive the vertebrates from some of the Articulata--from
the ringed worms or the Crustaceans, and even from the air-breathing
members; and at first sight such theories seem attractive, for in some
of their more obvious characters there is a certain resemblance between
the two groups. But there are also many and fundamental differences,
and few zoologists have accepted any hypothesis of this type. We may
briefly allude to some of these differences.
[Illustration: FIG. 52.
1, Marine swimming ringed worm; 2, giant centipede; 3, Peripatus.
Photo: Martin Duncan, Berridge, and Bastin.]
In the Arthropods, where the body consists of hard and soft parts, the
'skeleton' is an external one, and encloses the soft parts. Respiration
occurs by means of the skin or of gills, or, in air-breathing forms,
by 'trachea,' which are small branching tubes opening on the sides
of the body. But in no case has the mouth or the digestive tract any
connection with the respiratory system, a condition of affairs very
different from that obtaining in the vertebrates. The nervous system
consists of a brain, situated above the gullet, a nerve ring round
the latter, and a double nerve cord running along the body, _below_
the digestive canal. This is obviously the opposite position to that
occupied by the main nerve cord in the vertebrates, an important point
of difference.
The Arthropods are an extraordinarily successful group. A multitude
of forms of Crustaceans populate the waters, and they are excelled in
numbers and variety only by the insects upon land. While the individual
size appears to be somewhat strictly limited, probably by the nature
of the respiratory and blood systems, many types show exceedingly high
development in various directions--in intelligence, in social and
parental instincts, etc. The insects are of course to be regarded as
the highest Articulata, and have, like the highest vertebrates, the
mammals and birds, almost completely forsaken the water for the dry
land and the air. An interesting member of the Articulata, from the
standpoint of the Evolution theory, is the Peripatus, shown with a
ringed worm on Fig. 52 (3). It gains its interest for us from the fact
that, while classed as an Arthropod, it stands very nearly half-way
between the ringed worms (Fig. 52 (1)) and the true Arthropoda, and
thus forms a solitary link between the two types. In Fig. 52 (2) and
Figs. 53 to 58 are shown a number of types of Arthropods.
[Illustration: FIG. 53.--Arthropods--The Lobster.]
[Illustration: FIG. 54.--Arthropods--Scorpion.
Photo: Leonard Bastin.]
[Illustration: FIG. 55.--Arthropods (insects)--Stag-horn beetle.
Photo: Harold Bastin.]
[Illustration: FIG. 56.--Arthropods (insects)--Dragon-fly.
Photo: Harold Bastin.]
[Illustration: FIG. 57.--Arthropods (insects)--Mantis.
Photo: Harold Bastin.]
[Illustration: FIG. 58.--Arthropods (insects)--Swallow-tail butterfly
and larva.
Photos: Harold Bastin.]
[Illustration: FIG. 59.--Balanoglossus.]
We must now go back and take up the main thread of our story. The next
stage that falls to be described is that of a highly interesting group
of worms known as Enteropneusta, a name signifying 'gut breathers.'
This group contains a very small number of worm forms, which are to
be found burrowing in the sand of the sea floor. A typical example is
the Balanoglossus, a worm of some four inches in length, whose general
appearance is illustrated in Fig. 59. The creature has, as will be
observed, a large muscular snout or proboscis, behind which follows a
small portion called the collar, and behind this again the long body.
The most noteworthy feature of the group, as the name implies, is the
respiratory system. The mouth, which is situated in the region of the
'collar,' leads into a gullet, which is partially divided into an upper
and a lower canal by means of two inwardly projecting longitudinal
folds, one on either side. Only the lower of these canals is used as a
food passage; the upper communicates with the outside by means of
a large number of transverse slits on its sides. Water is continually
being taken in by the mouth and passed along this upper canal, to reach
the outside by way of the gill slits, and in doing so it passes over
the gills, where the blood is circulating in fine capillary vessels.
Here the blood is supplied with oxygen from the water, and is at the
same time relieved of carbonic oxide. This, it will be observed, is
the same method of respiration as that of the fishes. Behind the last
gill slit the digestive canal becomes a simple tube, with two digestive
glands or liver sacs. There are two main blood vessels, the larger
running along above the digestive canal, and the smaller below it, the
two being connected by means of numerous branches. There is a swelling
of the dorsal vessel--a heart--at its forward extremity, in the base
of the proboscis. The nervous system is peculiar; it consists of two
nerve cords, the smaller below the gut and the larger above it--the
latter therefore occupying a position similar to that of the spinal
cord in the vertebrates. Thus in two respects, in its respiratory
and nervous systems, Balanoglossus must be regarded as a highly
extraordinary member of the worm group, and in both its peculiarities
it shows an approach to the vertebrate. There can be little doubt as
to the position of this group as an important connecting link between
the ordinary worms and the vertebrates. Fig. 60 illustrates the main
features of the anatomy.
[Illustration: FIG. 60.--Section of front end of Balanoglossus.
_e_, Snout; _m_, mouth; _h_, heart; _cö_, body cavity; _d_, alimentary
canal; _n_, _n_, nerve cords; _vg_, ventral blood vessel; _dg_, dorsal
blood vessel; _f_, fold dividing the alimentary canal; _vd_, food
canal; _k_, gill slits.]
As to the origin of the Enteropneusta, opinions are somewhat divided.
Their blood system and their development would seem to suggest a
descent from the ringed worms. On the other hand, their possession of
a snout, and their very slight indication of division into segments,
would seem to separate them from the group mentioned and to connect
them rather with the Nemertines. The latter view is perhaps the more
probable.