THE PROTOCHORDATA
=The Chordate Animals.=--Referring to our metaphor of the tree with
its twigs as used in the chapter on classification we find the fishes
with the higher vertebrates as parts of a great branch from which the
lower twigs have mostly perished. This great branch, phylum, or line of
descent is known in zoology as _Chordata_, and the organisms associated
with it or composing it are chordate animals.
The chordate animals are those which at some stage of life possess a
notochord or primitive dorsal cartilage which divides the interior of
the body into two cavities. The dorsal cavity contains the great nerve
centers or spinal cord; the ventral cavity contains the heart and
alimentary canal. In all other animals which possess a body cavity,
there is no division by a notochord, and the ganglia of the nervous
system if existing are placed on the ventral side or in a ring about
the mouth.
=The Protochordates.=--Modern researches have shown that besides the
ordinary back-boned animals certain other creatures easily to be
mistaken for mollusks or worms and being chordate in structure must be
regarded as offshoots from the vertebrate branch. These are degenerate
allies, as is shown by the fact that their vertebrate traits are shown
in their early or larval development and scarcely at all in their adult
condition. As Dr. John Sterling Kingsley has well said: "Many of the
species start in life with the promise of reaching a point high in the
scale, but after a while they turn around and, as one might say, pursue
a downward course, which results in an adult which displays but few
resemblances to the other vertebrates." In the Tunicates or Ascidians
(sea-squirts, sea-pears, and salpas), which constitute the class known
as _Tunicata_ or _Urochordata_, there is no brain, the notochord is
confined to the tail and is usually present only in the larval stage
of the animal when it has the form of a tadpole. In later life the
animal usually becomes quiescent, attached to some hard object, fixed
or floating. It loses its form and has the appearance of a hollow,
leathery sac, the body organs being developed in a tough tunic. There
are numerous families of Tunicates and the species are found in nearly
all seas. They suggest no resemblance to fishes and look like tough
clams without shells. The internal cavity being usually filled with
water it is squirted out through the two apertures when the animal is
handled. The class _Enteropneusta_ (_Adelochorda_, or _Hemichordata_),
includes the rather rare worm-like forms related to _Balanoglossus_.
Bateson has shown that these animals possess a notochord which is
developed in the anterior part of the body. They have no fins and
before the mouth is a long proboscis. Gill-slits are found in the
larval tunicate. In _Balanoglossus_ these persist through life as in
the fishes.
The remaining chordate forms constitute the vertebrates proper, not
worm-like nor mollusk-like, the notochord not disappearing with age,
except as it gives way, by specialized segmentation to the complex
structures of the vertebral column. These vertebrates, which are
permanently aquatic, are known in a popular sense as fishes. The fish,
in the broad sense, is a back-boned animal which retains the homologue
of the back-bone throughout life, which does not develop jointed
limbs, its locomotive members, if present, being developed as fins,
and which breathes through life the air contained in water by means of
gills. This definition excludes the Tunicates and Enteropneusta on the
one hand and the Amphibia or Batrachia with the reptiles, birds, and
mammals on the other. The Amphibia are much more closely related to
certain fishes than the classes of fishes are to each other. Still for
purposes of systematic study, the frogs and salamanders are left out
of the domain of ichthyology, while the Tunicata and the Enteropneusta
might well be included in it.
The known branchiferous or gill-bearing chordates living and extinct
may be first divided into eight classes--the _Enteropneusta_, the
_Tunicata_, the _Leptocardii_, or lancelets, the _Cyclostomi_, or
lampreys, the _Elasmobranchii_, or sharks, the _Ostracophori_ the
_Arthrodira_, and the _Teleostomi_, or true fishes. The first two
groups, being very primitive and in no respect fish-like in appearance,
are sometimes grouped together as _Protochordata_, the others with the
higher Chordates constituting the _Vertebrata_.
=Other Terms used in Classification.=--The Leptocardii are sometimes
called Acraniata (without skull), as distinguished from the higher
groups, Craniota, in which the skull is developed. The _Leptocardii_,
_Cyclostomi_, and _Ostracophori_ are sometimes called _Agnatha_
(without jaws) in contradistinction to the _Gnathostomi_ (jaw mouths),
which include the sharks and true fishes with the higher vertebrates.
The sharks and Teleostomes are sometimes brought together as _Pisces_,
or fishes, as distinguished from other groups not true fishes. To the
sharks and true fishes the collective name of _Lyrifera_ has been
given, these fishes having the harp-shaped shoulder-girdle, its parts
united below. The _Ostracophores_ and _Arthrodires_ agreeing in the
bony coat of mail, and both groups now extinct and both of uncertain
relationship, have been often united under the name of _Placoderms_,
and these and many other fishes have been again erroneously confounded
with the Ganoids. Again, the Teleostomi have been frequently divided
into three classes--_Crossopterygii_, _Dipneusti_ or _Dipnoi_, and
_Actinopterygii_. The latter may be again divided into _Ganoidei_
and _Teleostei_ and all sorts of ranks have been assigned to each of
these groups. For our purposes a division into eight classes is most
convenient, and lowest among these we may place the _Enteropneusta_.
=The Enteropneusta.=--Most simple, most worm-like, and perhaps most
primitive of all the Chordates is the group of worm-shaped forms,
forming the class of _Enteropneusta_. The class of _Enteropneusta_,
also called _Adelochorda_ or _Hemichordata_, as here recognized,
consists of a group of small marine animals allied to the genus
_Balanoglossus_, or acorn-tongues (~balanos~, acorn; ~glôssa~, tongue).
These are worm-like creatures with fragile bodies buried in the sand or
mud, or living under rocks of the seashore and in shallow waters, where
they lie coiled in a spiral, with little or no motion. From the surface
of the body a mucous substance is secreted, holding together particles
by which are formed tubes of sand. The animal has a peculiar odor like
that of iodoform. At the front is a long muscular proboscis, very
sensitive, capable of great extension and contraction, largely used
in burrowing in the ground, and of a brilliant orange color in life.
Behind this is a collar which overlaps the small neck and conceals the
small mouth at the base of the proboscis. The gill-slits behind the
collar are also more or less concealed by it.
The body, which is worm-like, extends often to the length of two or
three feet. The gill-slits in the adult are arranged in regular pairs,
there being upwards of fifty in number much like the gill-slits of the
lancelet. As the animal grows older the slits become less conspicuous,
their openings being reduced to small slit-like pores.
In the interior of the proboscis is a rod-like structure which arises
as an outgrowth of the alimentary canal above the mouth. In development
and structure this rod so resembles the notochord of the lancelet
that it is regarded as a true notochord, though found in the anterior
region only. From the presence of gill-slits and notochord and from the
development and structure of the central nervous system _Balanoglossus_
was recognized by William Bateson, who studied an American species,
_Dolichoglossus kowalevskii_, at Hampton Roads in Virginia in 1885,
and at Beaufort in North Carolina, as a member of the Chordate series.
Unlike the Tunicates it represents a primitively simple, not a
degenerate, type. It seems to possess real affinities with the worms,
or possibly, as some have thought, with the sea-urchins.
[Illustration: FIG. 275.--"Tornaria" Larva of _Glossobalanus minutus_.
(After Minot.)]
A peculiar little creature, known as _Tornaria_, was once considered
to be the larva of a starfish. It is minute and transparent, floating
on the surface of the sea. It has no visible resemblance to the adult
_Balanoglossus_, but it has been reared in aquaria and shown to pass
into the latter or into the related genus _Glossobalanus_. No such
metamorphosis was found by Bateson in the more primitive genus
_Dolichoglossus_, studied by him. This adult animal may be, indeed,
a worm as it appears, but the presence of gill-slits, the existence
of a rudimentary notochord, and the character of the central nervous
system are distinctly fish-like and therefore vertebrate characters.
With the Chordates, and not with the worms, this class, _Enteropneusta_
(~enteron~, intestine; ~pnein~, to breathe), must be placed if its
characters have been rightly interpreted. It is possibly a descendant
of the primitive creatures which marked the transition from the archaic
worms, or possibly archaic Echinoderms, to the archaic Chordate type.
[Illustration: FIG. 276.--_Glossobalanus minutus_, one of the higher
Enteropneustans. (After Minot.)]
It is perhaps not absolutely certain that the notochord of
_Balanoglossus_ and its allies is a true homologue of the notochord
of the lancelet. There may be doubt even of the homologies of the
gill-slits themselves. But the balance of evidence seems to throw
_Balanoglossus_ on the fish side of the dividing line which separates
the lower Chordates from the worms.
It may be noticed that Hubrecht regards the proboscis of various marine
Nemertine worms as a real homologue of the notochord, and other writers
have traced with more or less success other apparent or possible
homologies between the Chordate and the Annelid series.
=Classification of Enteropneusta.=--Until recently the _Enteropneusta_
have been usually placed in a single family or even in a single genus.
The recent researches of Professor J. W. Spengel of Giessen and of
Professor William Emerson Ritter of the University of California, have
shown clearly that the group is much larger than had been generally
supposed, with numerous species in all the warm seas. In Spengel's
recent paper, "Die Benennung der Enteropneusten-Gattungen," three
families are recognized with nine genera and numerous species. At least
seven species are now known from the Pacific Coast of North America.
=Family Harrimaniidæ.=--In _Harrimania maculosa_, lately described by
Dr. Ritter from Alaska, the eggs are large, with much food yolk, and
the process of development is probably, without _Tornaria_ stage. A
second species of _Harrimania_ (_H. kupferi_) is now recognized from
Norway and Greenland. This genus is the simplest in structure among
all the Enteropneustans and may be regarded as the lowest of known
Chordates, the most worm-like of back-boned animals.
[Illustration: FIG. 277.--_Harrimania maculosa_ (Ritter), the lowest of
chordate animals. An Enteropneustan from Alaska. (After Ritter.)]
In _Dolichoglossus kowalevskii_ the species studied by Bateson on the
Virginia coast, the same simplicity of development occurs. This genus,
with a third, _Stereobalanus_ (_canadensis_), constitutes in Spengel's
system the family of _Harrimaniidæ_.
=Balanoglossidæ.=--The family _Glandicepitidæ_ contains the genera
_Glandiceps_, _Spengelia_, and _Schizocardium_. In the _Balanoglossidæ_
(_Ptychoderidæ_ of Spengel) the eggs are very small and numerous,
with little food yolk. The species in this family pass through the
Tornaria stage above described, a condition strikingly like that of the
larval starfish. This fact has given rise to the suggestion that the
Enteropneusta have a real affinity with the Echinoderms.
The _Balanoglossidæ_ include the genera _Glossobalanus_,
_Balanoglossus_, and _Ptychodera_, the latter the oldest known member
of the group, its type, _Ptychodera flava_, having been described
by Eschscholtz from the Pacific Coast in 1825, while _Balanoglossus
clavigerus_ was found by Della Chiaje in 1829.
=Low Organization of Harrimaniidæ.=--Apparently the _Harrimaniidæ_,
with simpler structure, more extensive notochord, and direct
development, should be placed at the bottom as the most primitive
of the Enteropneustan series. Dr. Willey, however, regards its
characters as due to degeneration, and considers the more elaborate
_Balanoglossidæ_ as nearest the primitive type. The case in this view
would have something in common with that of the _Larvacea_, which seems
to be the primitive Tunicates, but which may have been produced by the
degeneration of more complex forms.