BRANCH CHORDATA: THE VERTEBRATES,
ASCIDIANS, ETC.
The branch Chordata includes all the backboned animals or vertebrates,
comprising the fishes, salamanders, frogs and toads, lizards,
crocodiles, turtles and snakes, birds, and all the quadrupeds or
mammals, and includes also a few small unfamiliar ocean animals which
do not look at all like the backboned animals, but which agree with
them in possessing a peculiar structure called the notochord. This
notochord consists of a series or cord of cells extending
longitudinally through the body from head to tail, above the
alimentary canal and below the spinal nerve-cord. In all the
vertebrates excepting a few low forms, the notochord while present in
the young, is replaced in the adult by a segmented bony or
cartilaginous axis, the spinal or vertebral column. But in the
ascidians or sea-squirts (called also tunicates) it persists
throughout life. In addition to this characteristic notochord, nearly
all the Chordata are marked by the presence, either in embryonic or
larval stages only, or else persisting throughout life, of a number of
slits or clefts in the walls of the pharynx which serve for breathing,
and which are called gill-slits.
=Structure of the vertebrates.=--As the backboned or vertebrate
animals make up almost the whole of the branch Chordata, and as the
few other chordates are animals the special structures of which we
shall not undertake to study in this book, we may note here some of
the other more obvious structural characteristics of the true
vertebrates. The possession of a backbone or bony (sometimes
cartilaginous) spinal column is the characteristic by which we
distinguish them from the invertebrate or backboneless animals.
Furthermore, all of the vertebrates possess an internal skeleton which
is in most cases composed of bone, and is firm and strong. In some of
the lower fishes, as the sharks and sturgeons, the skeleton is made up
of cartilage, tough but not hard. The vertebrate skeleton consists
typically of an axial portion comprising the spinal column and head,
and of two pairs of appendages or limbs, variously developed as fins,
wings, legs and arms. In some vertebrates these limbs are represented
by mere rudiments, and in the lowest fish-like forms, the lancelets
and lampreys, there is not the slightest trace of limbs. A part of the
central nervous system, the spinal cord, runs longitudinally through
the body on the dorsal side of the alimentary canal; the circulatory
system is closed, the blood being always confined in the heart and in
vessels called arteries, veins, and capillaries, and the blood is red
in color owing to the presence of numerous red corpuscles or
blood-cells. The nervous system is highly developed, with a large
brain in all the typical forms, and with complex and usually highly
efficient special sense-organs. Respiration is carried on by means of
external gills, or by internal lungs which communicate with the
outside through the mouth and nostrils. To the lungs and gills the
blood is brought to be "purified," i.e., to give up its carbonic-acid
gas and to take up oxygen.
=Classification.=--The Chordata are variously divided by zoologists
into eight or ten classes, of which (in the eight-class system) the
five classes[15] Pisces (fishes), Batrachia (batrachians), Reptilia
(reptiles), Aves (birds), and Mammalia (mammals), belong to the true
vertebrates. These classes will be considered in the five following
chapters.
The remaining three classes include a number of strange marine forms
which until recent years were considered as worms, but which are now
known to be the nearest living allies of the earliest or primitive
vertebrates. The relationship of these forms to early types is
manifest, not in the appearance or structure of the adult stage, but
only during embryonic or larval stages.
[Illustration: FIG. 111.--An ascidian or sea-squirt from the coast of
California. (After Jordan and Kellogg.)]
=The ascidians.=--The sea-squirts, or Ascidians, common on the seashore,
compose one class of these primitive chordate animals. They possess a
simple, sac-like body (fig. 111), fastened to the rocks by one end, the
other being provided with two openings, one for the ingress and the
other for the exit of water, a strong current of which flows constantly
through the body. By means of this current the ascidian obtains food.
Usually sea-squirts live together in large colonies, and in some cases a
number of individuals enclose themselves in a common gelatinous mass,
forming what is called a compound ascidian.
The ascidian when born is a tiny, free-swimming, tadpole-like creature
with a slender finned tail. It swims about freely for only a few
hours, however, soon attaching itself to a rock, and in its further
development becoming degenerate. It loses its tail and with it the
short notochord possessed by the larva; the eye and the auditory organ
are lost, and the nervous system and alimentary canal become much
reduced and simplified. Sea-squirts in their adult stage are very
simple degenerate animals, with low functional development, yet their
embryonic and larval conditions show a considerable degree of
structural specialization, and the presence of the notochord in these
early stages reveals their affinity with the backboned animals.
FOOTNOTE:
[15] The animals included by some zoologists in the single class
Pisces, are held by other zoologists to constitute three distinct
classes, thus making a subdivision of the branch into ten classes.