AT THE DOORWAY OF THE "UPPER CLASSES"
PRIME SIGNIFICANCE OF THE NOTOCHORD
We have been considering up to this point one of the two primary and
natural divisions of the animal kingdom--that into Invertebrates and
Vertebrates. Although these are terms made familiar by long usage, and
refer to the absence or presence of a backbone composed of jointed
sections (vertebræ), a truer conception of the distinction is had by
regarding the first as animals whose skeleton, or frame, that gives
support to the muscles and other soft parts of the body, is exterior;
and the second as animals whose skeleton is interior. The one is, in
scientific language, an "exoskeleton," or more or less hardened outer
shell from the inner surface of which the organs grow and maintain
their attachment and leverage for work or protection; and the other an
"endoskeleton," around which the organs and integument are accumulated
by growth, and by means of which the animal's strength is maintained,
the interior bones--of which the chief is the spinal column, or
backbone--giving a firm fulcrum for the operation of the muscles and a
support and protection for the vital organs.
* * * * *
All the vertebrates are included in a single phylum--Chordata. This
term has supplanted in zoölogy the old term Vertebrata (now reserved
as a class name only), because it is more comprehensive and precise.
Professor Harmer says:
"The axis of the backbone of all vertebrates is formed by an elastic
rod known as the 'notochord,' which lasts throughout life in some of
the lowest forms, but in the higher forms appears only in the embryo.
The universal occurrence of this structure has been regarded as the
most important characteristic of the Vertebrata and their allies, which
are accordingly grouped together in the phylum Chordata. The members
of this phylum are further distinguished from other animals by several
important features. Of these one of the most important appears to be
the existence of lateral outgrowths of the pharynx, which unite with
the skin of the neck and form a series of perforations leading to the
exterior. These structures are the gill slits, and in the fishes their
walls give rise to vascular folds or gills. With the assumption of a
terrestrial life the higher vertebrates lost their gills as functional
organs, respiration being then performed by entirely different organs,
the lungs. But even in these cases, the gill slits appear in the
embryo.... Another fundamental characteristic of the Chordata is given
by the central nervous system, which lies entirely above the alimentary
canal, just dorsal to the notochord. Not only does this position of the
nerve centers distinguish the Chordata from the Invertebrates, but a
further point of difference is found in the development."
This definition requires the inclusion of various creatures very unlike
"vertebrates," and the phylum therefore embraces three subdivisions:
1. Adelochorda--marine wormlike creatures having a notochord in
the anterior of the body, and gill slits, both persistent; 2.
Urochorda--the ascidians or tunicates, small marine creatures, some
fixed along shores, others free-swimming and in some cases united into
swimming colonies (e. g., the salpæ), the tadpolelike larvæ of which
show a notochord in the tail; and 3. Vertebrata.
This last great subphylum is divisible into seven grand natural groups
with the rank of classes, namely:
1. _Acrania_--Lancelets (Amphioxus).
2. _Cyclostomata_--Lampreys; hags.
3. _Pisces_--Fishes.
4. _Amphibia_--Amphibians.
5. _Reptilia_--Reptiles.
6. _Aves_--Birds.
7. _Mammalia_--Mammals.
The first of these seven classes, the Acrania, has usually, heretofore,
been set apart as a subdivision equal in rank to the subphyla
Adelochorda and Urochorda, and the remaining six classes were grouped
into a coordinate subphylum Craniata, denoting that they alone have
a distinct head (cranium); the reason was that its members, the
lancelets, have no spine, but only a notochord, which, however, extends
from end to end of the body above the digestive organs, and persists
in the adult and throughout life. The lancelets (amphioxus) are small,
fish-shaped creatures that burrow in the sand of the seashore, usually
leaving only the head exposed, and sucking in a continuous current
of water which brings with it minute food. They breathe through gill
slits. The reproduction is bisexual, and by eggs.
The significance of the Acrania in this phylum is that they represent
a very early ancestral stage of the stock from which the higher
vertebrates (Craniata) have developed, and from which they themselves,
of course, have also diverged to a certain degree; and it is because
they retain many primitive characteristics that the study of their life
histories has engaged the attention of so many eminent zoölogists and
has thrown so much light on the evolutionary history of the "higher
animals," or vertebrates.
THE ROUNDMOUTHS--LAMPREYS AND HAGS
Popularly included among fishes, the lampreys and hags of the class
Cyclostomata (roundmouths) differ from true fishes by the possession of
a suctorial mouth devoid of functional jaws, by the single olfactory
organ, and by the absence of lateral appendages, or paired fins.
They have an eellike form and method of travel, and some species are
a yard in length. They are bisexual, discharging both eggs and milt
into the water to become fertilized by accidental contact. Lampreys
ascend the rivers to spawn, however, and there make little heaps of
pebbles, carried and piled with the mouth, in which the eggs find some
protection from the many egg-eaters in all streams. Most, if not all,
of the migratory parents die after spawning. From the eggs hatch larvæ
that undergo a metamorphosis. Lampreys live on small crustaceans,
worms, and so forth, eat carrion, and also attack living fishes. The
tongue, like the interior of the mouth, is armed with teeth. They
are in the habit of attaching themselves to stones in order to hold
themselves against a river current, breathing meanwhile by taking water
directly into the pouchlike gill chambers and expelling it, instead of
sucking it through the mouth and passing it out of the gill slits. In
ancient Rome the big sea lampreys of the Mediterranean were eaten as a
delicacy, and even cultivated in landlocked ponds, and they are still
highly prized in some parts of Europe.
The hags are an even more primitive group of cyclostomes that live in
the mud of shallow seas and are too abundant on both our coasts, where
they are a pest of the fisheries. Their general habits are similar
to those of lampreys, but wherever possible they attach themselves
to fish on which they feed. The hag is particularly destructive to
fishes caught on "set lines" of hooks, or in nets, and the loss thus
resulting on the coasts of California, in Japan, and in some European
fisheries is very serious. As these cyclostomes have no scales or
other hard parts to be preserved except a few teeth, no fossil remains
are certainly known, but it is the opinion of paleontologists that
otherwise the class might be traced to the earliest Paleozoic time.