MORPHOLOGY OF THE FINS
=Origin of the Fins of Fishes.=--One of the most interesting problems
in vertebrate morphology, and one of the most important from its
wide-reaching relations, is that of the derivation of the fins of
fishes. This resolves itself at once into two problems, the origin of
the median fins, which appear in the lancelets, at the very bottom of
the fish-like series, and the origin of the paired fins or limbs, which
are much more complex, and which first appear with the primitive sharks.
In this study the problem is to ascertain not what theoretically should
happen, but what, as a matter of fact, has happened in the early
history of the fish-like groups. That these structures, with the others
in the fish body, have sprung from simple origins, growing more complex
with the demands of varied conditions, and then at times again simple,
through degeneration, there can be no doubt. It is also certain that
each structure must have had some element of usefulness in all its
stages. In such studies we have, as Hæckel has expressed it, "three
ancestral documents, paleontology, morphology, and ontogeny"--the
actual history as shown by fossil remains, the sidelight derived from
comparison of structures, and the evidence of the hereditary influences
shown in the development of the individual. As to the first of these
ancestral documents, the evidence of paleontology is conclusive where
it is complete. But in very few cases are we sure of any series of
details. The records of geology are like a book with half its leaves
torn out, the other half confused, displaced, and blotted. Still
each record actually existing represents genuine history, and in
paleontology we must in time find our final court of appeal in all
matters of biological origins.
The evidence of comparative anatomy is most completely secured, but
it is often indecisive as to relative age and primitiveness of
origin among structures. As to ontogeny, it is, of course, true that
through heredity "the life-history of the individual is an epitome
of the life-history of the race." "Ontogeny repeats phylogeny," and
phylogeny, or line of descent of organisms and structures, is what we
are seeking. But here the repetition is never perfect, never nearly so
perfect in fact as Hæckel and his followers expected to find it. The
demands of natural selection may lead to the lengthening, shortening,
or distortion of phases of growth, just as they may modify adult
conditions. The interpolation of non-ancestral stages is recognized
in several groups. The conditions of the individual development may,
therefore, furnish evidence in favor of certain theories of origins,
but they cannot alone furnish the absolute proof.
In the process of development the median or vertical fins are doubtless
older than the paired fins or limbs, whatever be the origin of the
latter. They arise in a dermal keel which is developed in a web fitting
and accentuating the undulatory motion of the body. In the embryo of
the fish the continuous vertical fin from the head along the back and
around the tail precedes any trace of the paired fins.
In this elementary fin-fold slender supports, the rudiments of
fin-rays, tend to appear at intervals. These are called by Ryder
ray-hairs or actinotrichia. They are the prototype of fin-rays in the
embryo fish, and doubtless similarly preceded the latter in geological
time. In the development of fishes the caudal fin becomes more and more
the seat of propulsion. The fin-rays are strengthened, their basal
supports are more and more specialized, and the fin-fold ultimately
divides into distinct fins, the longest rays developed where most
needed.
That the vertical fins, dorsal, anal, and caudal, have their origin in
a median fold of the skin admits of no question. In the lowest forms
which bear fins these structures are dermal folds, being supported
by very feeble rays. Doubtless at first the vertical fins formed a
continuous fold, extending around the tail, this fold ultimately
broken, by atrophy of parts not needed, into distinct dorsal, anal,
and caudal fins. In the lower fishes, as in the earlier sharks, there
is an approach to this condition of primitive continuity, and in the
embryos of almost all fishes the same condition occurs. Dr. John A.
Ryder points out the fact that there are certain unexplained exceptions
to this rule. The sea-horse, pipefish, and other highly modified forms
do not show this unbroken fold, and it is wanting in the embryo of
the top-minnow, _Gambusia affinis_. Nevertheless the existence of a
continuous vertical fold in the embryo is the rule, almost universal.
The codfish with three dorsals, the Spanish mackerel with dorsal and
anal finlets, the herring with one dorsal, the stickleback with a
highly modified one, all show this character, and we may well regard it
as a certain trait of the primitive fish. This fold springs from the
ectoblast or external series of cells in the embryo. The fin-rays and
bony supports of the fins spring from the mesoblast or middle series
of cells, being thrust upward from the skeleton as supports for the
fin-fold.
=Origin of the Paired Fins.=--The question of the origin of the paired
fins is much more difficult and is still far from settled, although
many, perhaps the majority of recent writers favor the theory that
these fins are parts of a once continuous lateral fold of skin,
corresponding to the vertical fold which forms the dorsal, anal, and
caudal. In this view the lateral fold, at first continuous, became soon
atrophied in the middle, while at either end it is highly specialized,
at first into an organ of direction, then into fan-shaped and later
paddle-shaped organs of locomotion. According to another view, the
paired fins originated from gill structures, originally both close
behind the head, the ventral fin migrating backward with the progress
of evolution of the species.
=Evidence of Paleontology.=--If we had representations of all the early
forms of fishes arranged in proper sequence, we could decide once for
all, by evidence of paleontology, which form of fin appears first and
what is the order of appearance. As to this, it is plain that we do
not know the most primitive form of fin. Sharks of unknown character
must have existed long before the earliest remains accessible to us.
Hence the evidence of paleontology seems conflicting and uncertain. On
the whole it lends most support to the fin-fold theory. In the later
Devonian, a shark, _Cladoselache fyleri_, is found in which the paired
fins are lappet-shaped, so formed and placed as to suggest their
origin from a continuous fold of skin. In this species the dorsal fins
show much the same form. Other early sharks, constituting the order of
_Acanthodei_, have fins somewhat similar, but each preceded by a stiff
spine, which may be formed from coalescent rays.
[Illustration: FIG. 51.--_Cladoselache fyleri_ (Newberry), restored.
Upper Devonian of Ohio. (After Dean.)]
[Illustration: FIG. 52.--Fold-like pectoral and ventral fins of
_Cladoselache fyleri_. (After Dean.)]
Long after these appears another type of sharks represented by
_Pleuracanthus_ and _Cladodus_, in which the pectoral fin is a jointed
organ fringed with rays arranged serially in one or two rows. This
form of fin has no resemblance to a fold of skin, but accords better
with Gegenbaur's theory that the pectoral limb was at first a modified
gill-arch. In the Coal Measures are found also teeth of sharks
(_Orodontidæ_) which bear a strong resemblance to still existing forms
of the family of _Heterodontidæ_, which originates in the Permian. The
existing _Heterodontidæ_ have the usual specialized form of shark-fin,
with three of the basal segments especially enlarged and placed side
by side, the type seen in modern sharks. Whatever the primitive
form of shark-fin, it may well be doubted whether any one of these
three (_Cladoselache_, _Pleuracanthus_, or _Heterodontus_) actually
represents it. The beginning is therefore unknown, though there is
some evidence that _Cladoselache_ is actually more nearly primitive
than any of the others. As we shall see, the evidence of comparative
anatomy may be consistent with either of the two chief theories, while
that of ontogeny or embryology is apparently inconclusive, and that of
paleontology is apparently most easily reconciled with the theory of
the fin-fold.
[Illustration: FIG. 53.--Pectoral fin of shark, _Chiloscyllium_. (After
Parker and Haswell.)]
=Development of the Paired Fins in the Embryo.=--According to Dr.
John A. Ryder ("Embryography of Osseous Fishes," 1882) "the paired
fins in Teleostei arise locally, as short longitudinal folds, with
perhaps a few exceptions. The pectorals of _Lepisosteus_ originate in
the same way. Of the paired fins, the pectoral or anterior pair seems
to be the first to be developed, the ventral or pelvic pair often not
making its appearance until after the absorption of the yolk-sac has
been completed, in other cases before that event, as in _Salmo_ and
in _Gambusia_. The pectoral fin undergoes less alteration of position
during its evolution than the posterior pair."
In the codfish (_Gadus callarias_) the pectoral fin-fold "appears as a
slight longitudinal elevation of the skin on either side of the body of
the embryo a little way behind the auditory vesicles, and shortly after
the tail of the embryo begins to bud out. At the very first it appears
to be merely a dermal fold, and in some forms a layer of cells extends
out underneath it from the sides of the body, but does not ascend
into it. It begins to develop as a very low fold, hardly noticeable,
and, as growth proceeds, its base does not expand antero-posteriorly,
but tends rather to become narrowed, so that it has a pedunculated
form. With the progress of this process the margin of the fin-fold
also becomes thinner at its distal border, and at the basal part
mesodermal cells make their appearance more noticeably within the inner
contour-line. The free border of the fin-fold grows out laterally and
longitudinally, expanding the portion outside of the inner contour-line
of the fin into a fan-shape. This distal thinner portion is at first
without any evidence of rays; further than that there is a manifest
tendency to a radial disposition of the histological elements of the
fin."
The next point of interest is found in the change of position of the
pectoral fin by a rotation on its base. This is associated with changes
in the development of the fish itself. The ventral fin is also, in most
fishes, a short horizontal fold and just above the preanal part of the
median vertical fold which becomes anal, caudal, and dorsal. But in
the top-minnow (_Gambusia_), of the order Haplomi, the ventral first
appears as "a little papilla and not as a fold, where the body-walls
join the hinder upper portion of the yolk-sac, a very little way in
front of the vent." "These two modes of origin," observes Dr. Ryder,
"are therefore in striking contrast and well calculated to impress us
with the protean character of the means at the disposal of Nature to
achieve one and the same end."
=Current Theories as to Origin of Paired Fins.=--There are three chief
theories as to the morphology and origin of the paired fins. The
earliest is that of Dr. Karl Gegenbaur, supported by various workers
among his students and colleagues. In his view the pectoral and ventral
fins are derived from modifications of primitive gill-arches. According
to this theory, the skeletal arrangements of the vertebrate limb are
derived from modifications of one primitive form, a structure made up
of successive joints, with a series of fin-rays on one or both sides
of it. To this structure Gegenbaur gives the name of archipterygium.
It is found in the shark, _Pleuracanthus_, in _Cladodus_, and in all
the Dipnoan and Crossopterygian fishes, its primitive form being still
retained in the Australian genus of Dipnoans, _Neoceratodus_. This
biserial archipterygium with its limb-girdle is derived from a series
of gill-rays attached to a branchial arch. The backward position of the
ventral fin is due to a succession of migrations in the individual and
in the species.
As to this theory, Mr. J. Graham Kerr observes:
[Illustration: FIG. 54.--Skull and shoulder-girdle of _Neoceratodus
forsteri_ (Günther), showing the archipterygium.]
"The Gegenbaur theory of the morphology of vertebrate limbs thus
consists of two very distinct portions. The first, that the
archipterygium is the ground-form from which all other forms of
presently existing fin skeletons are derived, concerns us only
indirectly, as we are dealing here only with the _origin_ of the limbs,
i.e., their origin from other structures that were not limbs.
"It is the second part of the view that we have to do with, that
deriving the archipterygium, the skeleton of the primitive paired fin,
from a series of gill-rays and involving the idea that the limb itself
is derived from the septum between two gill-clefts.
"This view is based on the skeletal structures within the fin. It
rests upon (1) the assumption that the archipterygium is the primitive
type of fin, and (2) the fact that amongst the Selachians is found a
tendency for one branchial ray to become larger than the others, and,
when this has happened, for the base of attachment of neighboring rays
to show a tendency to migrate from the branchial arch on to the base
of the larger or, as we may call it, primary ray; a condition coming
about which, were the process to continue rather farther than it is
known to do in actual fact, would obviously result in a structure
practically identical with the archipterygium. Gegenbaur suggests that
the archipterygium actually has arisen in this way in phylogeny."
[Illustration: FIG. 55.--_Acanthoessus wardi_ (Egerton). Carboniferous.
Family _Acanthoessidæ_. (After Woodward.)]
[Illustration: FIG. 56.--Shoulder-girdle of _Acanthoessus_. (After
Dean.)]
[Illustration: FIG. 57.--Pectoral fin of _Pleuracanthus_. (After Dean.)]
The fin-fold theory of Balfour, adopted by Dohrn, Weidersheim, Thacher,
Mivart, Ryder, Dean, Boulenger, and others, and now generally accepted
by most morphologists as plausible, is this: that "The paired limbs
are persisting and exaggerated portions of a fin-fold once continuous,
which stretched along each side of the body and to which they bear an
exactly similar phylogenetic relation as do the separate dorsal and
anal fins to the once continuous median fin-fold."
"This view, in its modern form, was based by Balfour on his observation
that in the embryos of certain Elasmobranchs the rudiments of the
pectoral and pelvic fins are at a very early period connected together
by a longitudinal ridge of thickened epiblast--of which indeed they
are but exaggerations. In Balfour's own words referring to these
observations: 'If the account just given of the development of the limb
is an accurate record of what really takes place, it is not possible
to deny that some light is thrown by it upon the first origin of the
vertebrate limbs. The facts can only bear one interpretation, viz.,
that the limbs are the remnants of continuous lateral fins.'
[Illustration: FIG. 58.--Shoulder-girdle of _Polypterus bichir_.
Specimen from the White Nile.]
"A similar view to that of Balfour was enunciated almost synchronously
by Thacher and a little later by Mivart--in each case based on
anatomical investigation of Selachians--mainly relating to the
remarkable similarity of the skeletal arrangements in the paired and
unpaired fins."
A third theory is suggested by Mr. J. Graham Kerr (_Cambridge Philos.
Trans._, 1899), who has recently given a summary of the theories on
this subject. Mr. Kerr agrees with Gegenbaur as to the primitive nature
of the archipterygium, but believes that it is derived, not from the
gill-septum, but from an external gill. Such a gill is well developed
in the young of all the living sharks, Dipnoans and Crossopterygians,
and in the latter types of fishes it has a form analogous to that of
the archipterygium, although without bony or cartilaginous axis.
We may now take up the evidence in regard to each of the different
theories, using in part the language of Kerr, the paragraphs in
quotation-marks being taken from his paper. We may first consider
Balfour's theory of the lateral fold.
=Balfour's Theory of the Lateral Fold.=--"The evidence in regard to
this view may be classed under three heads, as ontogenetic, comparative
anatomical, and paleontological. The ultimate fact on which it was
founded was Balfour's discovery that in certain Elasmobranch embryos,
but especially in _Torpedo_ (_Narcobatus_), the fin rudiments were,
at an early stage, connected by a ridge of epiblast. I am not able
to make out what were the other forms in which Balfour found this
ridge, but subsequent research, in particular by Mollier, a supporter
of the lateral-fold view, is to the effect that it does not occur in
such ordinary sharks as _Pristiurus_ and _Mustelus_, while it is to
be gathered from Balfour himself that it does not occur in _Scyllium_
(_Scyliorhinus_).
"It appears to me that the knowledge we have now that the longitudinal
ridge is confined to the rays and absent in the less highly specialized
sharks greatly diminishes its security as a basis on which to rest a
theory. In the rays, in correlation with their peculiar mode of life,
the paired fins have undergone (in secondary development) enormous
extension along the sides of the body, and their continuity in the
embryo may well be a mere foreshadowing of this.
[Illustration: FIG. 59.--Arm of a frog.]
"An apparently powerful support from the side of embryology came
in Dohrn and Rabl's discoveries that in _Pristiurus_ all the
interpterygial myotomes produce muscle-buds. This, however, was
explained away by the Gegenbaur school as being merely evidence of the
backward migration of the hind limb--successive myotomes being taken
up and left behind again as the limb moved farther back. As either
explanation seems an adequate one, I do not think we can lay stress
upon this body of facts as supporting either one view or the other.
The facts of the development of the skeleton cannot be said to support
the fold view; according to it we should expect to find a series of
metameric supporting rays produced which later on become fused at
their bases. Instead of this we find a _longitudinal_ bar of cartilage
developing quite continuously, the rays forming as projections from its
outer side.
"The most important evidence for the fold view from the side of
comparative anatomy is afforded by (1) the fact that the limb derives
its nerve supply from a large number of spinal nerves, and (2) the
extraordinary resemblance met with between the skeletal arrangements
of paired and unpaired fins. The believers in the branchial arch
hypothesis have disposed of the first of these in the same way as they
did the occurrence of interpterygial myotomes, by looking on the nerves
received from regions of the spinal cord anterior to the attachment of
the limb as forming a kind of trail marking the backward migration of
the limb.
"The similarity in the skeleton is indeed most striking, though
its weight as evidence has been recently greatly diminished by the
knowledge that the apparently metameric segmentation of the skeletal
and muscular tissues of the paired fins is quite secondary and does not
at all agree with the metamery of the trunk. What resemblance there is
may well be of a homoplastic character when we take into account the
similarity in function of the median and unpaired fins, especially in
such forms as _Raja_, where the anatomical resemblances are especially
striking. There is a surprising dearth of paleontological evidence in
favor of this view."
The objection to the first view is its precarious foundation. Such
lateral folds are found only in certain rays, in which they may be
developed as a secondary modification in connection with the peculiar
form of these fishes. Professor Kerr observes that this theory must be
looked upon and judged: "Just as any other view at the present time
regarding the nature of the vertebrate limb, rather as a speculation,
brilliant and suggestive though it be, than as a logically constructed
theory of the now known facts. It is, I think, on this account
allowable to apply to it a test of a character which is admittedly very
apt to mislead, that of 'common sense.'
"If there is any soundness in zoological speculation at all, I think
it must be admitted that the more primitive vertebrates were creatures
possessing a notochordal axial skeleton near the dorsal side, with the
main nervous axis above it, the main viscera below it, and the great
mass of muscle lying in myotomes along its sides. Now such a creature
is well adapted to movements of the character of lateral flexure, and
not at all for movements in the sagittal plane--which would be not only
difficult to achieve, but would tend to alternately compress and extend
its spinal cord and its viscera. Such a creature would swim through the
water as does a Cyclostome, or a _Lepidosiren_, or any other elongated
vertebrate without special swimming organs. Swimming like this,
specialization for more and more rapid movement would mean flattening
of the tail region and is extension into an at first not separately
mobile median tail-fold. It is extremely difficult to my mind to
suppose that a new purely _swimming_ arrangement should have arisen
involving up-and-down movement, and which, at its first beginnings,
while useless as a swimming organ itself, must greatly detract from the
efficiency of that which already existed."
=Objections to Gegenbaur's Theory.=--We now return to the Gegenbaur
view--that the limb is a modified gill-septum.
"Resting on Gegenbaur's discovery already mentioned, that the gill-rays
in certain cases assume an arrangement showing great similarity to that
of the skeletal elements of the archipterygium, it has, so far as I
am aware, up to the present time received no direct support whatever
of a nature comparable with that found for the rival view in the fact
that, in certain forms at all events, the limbs actually do arise in
the individual in the way that the theory holds they did in phylogeny.
No one has produced either a form in which a gill-septum becomes the
limb during ontogeny, or the fossil remains of any form which shows an
intermediate condition.
"The portion of Gegenbaur's view which asserts that the biserial
archipterygial fin is of an extremely primitive character is supported
by a large body of anatomical facts, and is rendered further probable
by the great frequency with which fins apparently of this character
occur amongst the oldest known fishes. On the lateral-fold view we
should have to regard these as independently evolved, which would
imply that fins of this type are of a very perfect character, and in
that case we may be indeed surprised at their so complete disappearance
in the more highly developed forms, which followed later on."
[Illustration: FIG. 60.--_Pleuracanthus decheni_ (Goldfuss). (After
Dean.)]
As to Gegenbaur's theory it is urged that no form is known in which a
gill-septum develops into a limb during the growth of the individual.
The main thesis, according to Professor Kerr, "that the archipterygium
was derived from gill-rays, is supported only by evidence of an
indirect character. Gegenbaur in his very first suggestion of his
theory pointed out, as a great difficulty in the way of its acceptance,
the position of the limbs, especially of the pelvic limbs, in a
position far removed from that of the branchial arches. This difficulty
has been entirely removed by the brilliant work of Gegenbaur's
followers, who have shown from the facts of comparative anatomy and
embryology that the limbs, and the hind limbs especially, actually
have undergone, and in ontogeny do undergo, an extensive backward
migration. In some cases Braus has been able to find traces of this
migration as far forward as a point just behind the branchial arches.
Now, when we consider the numbers, the enthusiasm, and the ability of
Gegenbaur's disciples, we cannot help being struck by the fact that the
_only_ evidence in favor of this derivation of the limbs has been that
which tends to show that a migration of the limbs backwards has taken
place from a region somewhere near the last branchial arch, and that
they have failed utterly to discover any intermediate steps between
gill-rays and archipterygial fin. And if for a moment we apply the test
of common sense we cannot but be impressed by the improbability of the
evolution of a gill-septum, which in all the lower forms of fishes is
fixed firmly in the body-wall, and beneath its surface, into an organ
of locomotion.
[Illustration: FIG. 61.--Embryos of _Heterodontus japonicus_ Maclay and
Macleay, a Cestraciont shark, showing the backward migration of the
gill-arches and the forward movement of the pectoral fin. _a_, _b_,
_c_, representing different stages of growth. (After Dean.)]
"May I express the hope that what I have said is sufficient to show in
what a state of uncertainty our views are regarding the morphological
nature of the paired fins, and upon what an exceedingly slender basis
rest both of the two views which at present hold the field?"
As to the backward migration of the ventral fins, Dr. Bashford Dean has
recently brought forward evidence from the embryo of a very ancient
type of shark (_Heterodontus japonicus_) that this does not actually
occur in that species. On the other hand, we have a forward migration
of the pectoral fin, which gradually takes its place in advance
of the hindmost gill-arches. The accompanying cut is from Dean's
paper, "Biometric Evidence in the Problem of the Paired Limbs of the
Vertebrates" (American Naturalist for November, 1902). Dean concludes
that in _Heterodontus_ "there is no evidence that there has ever been a
migration of the fins in the Gegenbaurian sense." "The gill region, at
least in its outer part, shows no affinity during proportional growth
with the neighboring region of the pectoral fin. In fact from an early
stage onward, they are evidently growing in opposite directions."
=Kerr's Theory of Modified External Gills.=--"It is because I feel
that in the present state of our knowledge neither of the two views I
have mentioned has a claim to any higher rank than that of extremely
suggestive speculations that I venture to say a few words for the third
view, which is avowedly a mere speculation.
"Before proceeding with it I should say that I assume the serial
homology of fore and hind limbs to be beyond dispute. The great and
deep-seated resemblances between them are such as to my mind seem not
to be adequately explicable except on this assumption.
"In the Urodela (salamanders) the external gills are well-known
structures--serially arranged projections from the body-wall near the
upper ends of certain of the branchial arches. When one considers the
ontogenetic development of these organs, from knob-like outgrowth
from the outer face of the branchial arch, covered with ectoderm and
possessing a mesoblastic core, and which frequently if not always
appear before the branchial clefts are open, one cannot but conclude
that they are morphologically projections of the outer skin and
that they have nothing whatever to do with the gill-pouches of the
gut-wall. Amongst the Urodela one such gill projects from each of the
first three branchial arches. In _Lepidosiren_ there is one on each
of the branchial arches I-IV. In _Polypterus_ and _Calamoichthys_
(_Erpetoichthys_) there is one on the hyoid arch. Finally, in many
Urodelan larvæ we have present at the same time as the external gills
a pair of curious structures called balancers. At an early stage of my
work on _Lepidosiren_, while looking over other vertebrate embryos
and larvæ for purposes of comparison, my attention was arrested by
these structures, and further examinations, by section or otherwise,
convinced me that there were serial homologues of the external gills,
situated on the mandibular arch. On then looking up the literature,
I found that I was by no means first in this view. Rusconi had long
ago noticed the resemblance, and in more recent times both Orr and
Maurer had been led to the same conclusion as I had been. Three
different observers having been independently led to exactly the same
conclusions, we may, I think, fairly enough regard the view I have
mentioned of the morphological nature of the balancers as probably a
correct one.
"Here, then, we have a series of homologous structures projecting
from each of the series of visceral arches. They crop up on the
Crossopterygii, the Dipnoi, and the Urodela, i.e., in three of the most
archaic of the groups of Gnathostomata. But we may put it in another
way. The groups in which they do not occur are those whose young
possess a very large yolk-sac (or which are admittedly derived from
such forms). Now wherever we have a large yolk-sac we have developed on
its surface a rich network of blood-vessels for purposes of nutrition.
But such a network _must necessarily_ act as an extraordinarily
efficient organ of respiration, and did we not know the facts we
might venture to prophesy that in forms possessing it any other small
skin-organ of respiration would tend to disappear.
"No doubt these external gills are absent also in a few of the
admittedly primitive forms such as, e.g., (_Neo-_) _Ceratodus_.
But I would ask that in this connection one should bear in mind
one of the marked characteristics of external gills--their great
regenerative power. This involves their being extremely liable to
injury and consequently a source of danger to their possessor. Their
absence, therefore, in certain cases may well have been due to natural
selection. On the other hand, the _presence_ in so many lowly forms
of these organs, the general close similarity in structure that runs
through them in different forms, and the exact correspondence in their
position and relations to the body can, it seems to me, _only_ be
adequately explained by looking on them as being homologous structures
inherited from a common ancestor and consequently of great antiquity in
the vertebrate stem."
As to the third theory, Professor Kerr suggests tentatively that the
external gill may be the structure modified to form the paired limbs.
Of the homology of fore and hind limbs and consequently of their like
origin there can be no doubt.
The general gill-structures have, according to Kerr, "the primary
function of respiration. They are also, however, provided with an
elaborate muscular apparatus comprising elevators, depressors, and
adductors, and larvæ possessing them may be seen every now and then
to give them a sharp backward twitch. They are thus _potentially_
motor organs. In such a Urodele as _Amblystoma_ their homologues on
the mandibular arch are used as supporting structures against a solid
substratum exactly as are the limbs of the young _Lepidosiren_.
[Illustration: FIG. 62.--_Polypterus congicus_, a _Crossopterygian_
fish from the Congo River. Young, with external gills. (After
Boulenger.)]
"I have, therefore, to suggest that the more ancient Gnathostomata
possessed a series of potentially motor, potentially supporting
structures projecting from their visceral arches; it was inherently
extremely probable that these should be made use of when actual
supporting, and motor appendages had to be developed in connection with
clambering about a solid substratum. If this had been so, we should
look upon the limb as a modified external gill; the limb-girdle, with
Gegenbaur, as a modified branchial arch.
"This theory of the vertebrate paired limb seems to me, I confess, to
be a more plausible one on the face of it than either of the two which
at present hold the field. If untrue, it is so dangerously plausible
as to surely deserve more consideration than it appears to have had.
One of the main differences between it and the other two hypotheses
is that, instead of deriving the swimming-fin from the walking and
supporting limb, it goes the other way about. That this is the safer
line to take seems to me to be shown by the consideration that a very
small and rudimentary limb could _only_ be of use if provided with a
fixed _point d'appui_. Also on this view, the pentadactyle limb and the
swimming-fin would probably be evolved independently from a simple form
of limb. This would evade the great difficulties which have beset those
who have endeavored to establish the homologies of the elements of the
pentadactyle limb with those of any type of fully formed fin."
=Uncertain Conclusions.=--In conclusion we may say that the evidence
of embryology in this matter is inadequate, though possibly favoring
on the whole the fin-fold theory; that of morphology is inconclusive,
and probably the final answer may be given by paleontology. If the
records of the rocks were complete, they would be decisive. At
present we have to decide which is the more primitive of two forms
of pectoral fin actually known among fossils. That of _Cladoselache_
is a low, horizontal fold of skin, with feeble rays, called by Cope
_ptychopterygium_. That of _Pleuracanthus_ is a jointed paddle-shaped
appendage with a fringe of rays on either side. In the theory of
Gegenbaur and Kerr _Pleuracanthus_ must be, so far as the limbs are
concerned, the form nearest the primitive limb-bearing vertebrate. In
Balfour's theory _Cladoselache_ is nearest the primitive type from
which the other and with it the archipterygium of later forms may be
derived.
Boulenger and others question even this, believing that the
archipterygium in _Pleuracanthus_ and other primitive sharks and
that in _Neoceratodus_ and its Dipnoan and Crossopterygian allies
and ancestors have been derived independently, not the latter
from the former. In this view there is no real homology between
the archipterygium in the sharks possessing it and that in the
_Dipnoans_ and _Crossopterygians_. In the one theory the type of
_Pleuracanthus_ would be ancestral to the other sharks on the one
hand, and to Crossopterygians and all higher vertebrates on the other.
With the theory of the origin of the pectoral from a lateral fold,
_Pleuracanthus_ would be merely a curious specialized offshoot from the
primitive sharks, without descendants and without special significance
in phylogeny.
As elements bearing on this decision we may note that the tapering
unspecialized diphycercal tail of _Pleuracanthus_ seems very primitive
in comparison with the short heterocercal tail of _Cladoselache_. This
evidence, perhaps deceptive, is balanced by the presence on the head of
_Pleuracanthus_ of a highly specialized serrated spine, evidence of a
far from primitive structure. Certainly neither the one genus nor the
other actually represents the primitive shark. But as _Cladoselache_
appears in geological time, long before _Pleuracanthus_, _Cladodus_,
or any other shark with a jointed, archipterygial fin, the burden
of proof, according to Dean, rests with the followers of Gegenbaur.
If the remains found in the Ordovician at Cañon City referred to
Crossopterygians are correctly interpreted, we must regard the shark
ancestry as lost in pre-Silurian darkness, for in sharks of some sort
the Crossopterygians apparently must find their remote ancestry.
[Illustration: FIG. 63.--Heterocercal tail of Sturgeon, _Acipenser
sturio_ (Linnæus). (After Zittel.)]
=Forms of the Tail in Fishes.=--In the process of development the
median or vertical fins are, as above stated, older than the paired
fins or limbs, whatever be the origin of the latter. They arise in a
dermal keel, its membranes fitting and accentuating the undulatory
motion of the body.
In this elementary fin-fold slender supports (actinotrichia), the
rudiments of fin-rays, appear at intervals. In those fins of most
service in the movement of the fish, the fin-rays are strengthened, and
their basal supports specialized.
Dean calls attention to the fact that in fishes which swim, when
adult, by an undulatory motion, the paired fins tend to disappear, as
in the eel and in all eel-like fishes, as blennies and eel-pouts.
The form of the tail at the base of the caudal fin varies in the
different groups. In most primitive types, as in most embryonic
fishes, the vertebræ grow smaller to the last (diphycercal). In
others, also primitive, the end of the tail is directed upward, and
the most of the caudal fin is below it. Such a tail is seen in most
sharks, in the sturgeon, garpike, bowfin, and in the Ganoid fishes.
It is known as heterocercal, and finally in ordinary fishes the tail
becomes homocercal or fan-shaped, although usually some trace of the
heterocercal condition is traceable, gradually growing less with the
process of development.
Since Professor Agassiz first recognized, in 1833, the distinction
between the heterocercal and homocercal tail, this matter has been the
subject of elaborate investigation and a number of additional terms
have been proposed, some of which are in common use.
A detailed discussion of these is found in a paper by Dr. John A.
Ryder "On the Origin of Heterocercy" in the Report of the U. S. Fish
Commissioner for 1884. In this paper a dynamic or mechanical theory
of the causes of change of form is set forth, parts of this having a
hypothetical and somewhat uncertain basis.
Dr. Ryder proposes the name _archicercal_ to denote the cylindroidal
worm-like caudal end of the larva of fishes and amphibians before they
acquire median fin-folds. The term _lophocercal_ is proposed by Ryder
for the form of caudal fin which consists of a rayless fold of skin
continuous with the skin of the tail, the inner surfaces of this fold
being more or less nearly in contact. To the same type of tail Dr.
Jeffries Wyman in 1864 gave the name _protocercal_. This name was used
for the tail of the larval ray when it acquires median fin-folds. The
term implies, what cannot be far from true, that this form of tail is
the first in the stages of evolution of the caudal fin.
To the same type of tail Mr. Alexander Agassiz gave, in 1877, the name
of _leptocardial_, on the supposition that it represented the adult
condition of the lancelet. In this creature, however, rudimentary
basal rays are present, a condition differing from that of the early
embryos.
The diphycercal tail, as usually understood, is one in which the
end of the vertebral column bears "not only hypural but also epural
intermediary pieces which support rays." The term is used for the
primitive type of tail in which the vertebræ, lying horizontally, grow
progressively smaller, as in _Neoceratodus_, _Protopterus_, and other
Dipnoans and Crossopterygians. The term was first applied by McCoy to
the tails of the Dipnoan genera _Diplopterus_ and _Gyroptychius_, and
for tails of this type it should be reserved.
[Illustration: FIG. 64.--Heterocercal tail of Bowfin, _Amia calva_
(Linnæus). (After Zittel.)]
[Illustration: FIG. 65.--Heterocercal tail of Garpike, _Lepisosteus
osseus_ (Linnæus).]
The heterocercal tail is one in which the hindmost vertebræ are bent
upwards. The term is generally applied to those fishes only in which
this bending is considerable and is externally evident, as in the
sharks and Ganoids. The character disappears by degrees, changing
sometimes to diphycercal or leptocercal by a process of degeneration,
or in ordinary fishes becoming _homocercal_. Dr. Ryder uses the term
heterocercal for all cases in which any up-bending of the axis takes
place, even though it involves the modification of but a single
vertebra. With this definition, the tail of salmon, herring, and
even of most bony fishes would be considered heterocercal, and most
or all of these pass through a heterocercal stage in the course of
development. The term is, however, usually restricted to those forms in
which the curving of the axis is evident without dissection.
[Illustration: FIG. 66.--_Coryphænoides carapinus_ (Goode and Bean),
showing leptocercal tail. Gulf Stream.]
The homocercal tail is the fan-shaped or symmetrical tail common
among the Teleosts, or bony fishes. In its process of development
the individual tail is first archicercal, then lophocercal, then
diphycercal, then heterocercal, and lastly homocercal. A similar order
is indicated by the sequence of fossil fishes in the rocks, although
some forms of diphycercal tail may be produced by degeneration of the
heterocercal tail, as suggested by Dr. Dollo and Dr. Boulenger, who
divide diphycercal tails into primitive and secondary.
The peculiar tapering tail of the cod, the vertebræ growing
progressively smaller behind, is termed _isocercal_ by Professor Cope.
This form differs little from diphycercal, except in its supposed
derivation from the homocercal type. A similar form is seen in eels.
[Illustration: FIG. 67.--Heterocercal tail of Young Trout, _Salmo
fario_ (Linnæus). (After Parker and Haswell.)]
The term _leptocercal_ has been suggested by Gaudry, 1883, for those
tails in which the vertebral column ends in a point. We may, perhaps,
use it for all such as are attenuate, ending in a long point or whip,
as in the _Macrouridæ_, or grenadiers, the sting-rays, and in various
degenerate members of almost every large group.
The term _gephyrocercal_ is devised by Ryder for fishes in which the
end of the vertebral axis is aborted in the adult, leaving the caudal
elements to be inserted on the end of this axis, thus bridging over
the interval between the vertical fins, as the name (~gephyros~,
bridge; ~kerkos~, tail) is intended to indicate. Such a tail has been
recognized in four genera only, _Mola_, _Ranzania_, _Fierasfer_, and
_Echiodon_, the head-fishes and the pearl-fishes.
[Illustration: FIG. 68.--Isocercal tail of Hake, _Merluccius productus_
(Ayres).]
[Illustration: FIG. 69.--Homocercal tail of a Flounder, _Paralichthys
californicus_.]
The part of the body of the fish which lies behind the vent is known
as the urosome. The urostyle is the name given to a modified bony
structure, originally the end of the notochord, turned upward in most
fishes. The term _opisthure_ is suggested by Ryder for the exserted tip
of the vertebral column, which in some larvæ (_Lepisosteus_) and in
some adult fishes (_Fistularia_, _Chimæra_) projects beyond the caudal
fin. The urosome, or posterior part of the body, must be regarded as
a product of evolution and specialization, its function being largely
that of locomotion. In the theoretically primitive fish there is no
urosome, the alimentary canal, as in the worm, beginning at one end of
the body and terminating at the other.
[Illustration: FIG. 70.--Gephyrocercal tail of _Mola mola_ (Linnæus).
(After Ryder.)]
=Homologies of the Pectoral Limb.=--Dr. Gill has made an elaborate
attempt to work out the homologies of the bones of the pectoral
limb.[4] From his thesis we take the following:
"The following are assumed as premises that will be granted by all
zootomists:
"1. Homologies of parts are best determinable, _ceteris paribus_, in
the most nearly related forms.
"2. Identification should proceed from a central or determinate point
outwards.
"The applications of these principles are embodied in the following
conclusions:
"1. The forms that are best comparable and that are most nearly related
to each other are the Dipnoi, an order of fishes at present represented
by _Lepidosiren_, _Protopterus_, and _Ceratodus_, and the Batrachians
as represented by the _Ganocephala_, Salamanders, and Salamander-like
animals.
"2. The articulation of the anterior member with the shoulder-girdle
forms the most obvious and determinable point for comparison in the
representatives of the respective classes.
[Illustration: FIG. 71.--Shoulder-girdle of _Amia calva_ (Linnæus).]
[Illustration: FIG. 72.--Shoulder-girdle of a Sea Catfish, _Selenaspis
dowi_.]
=The Girdle in Dipnoans.=--"The proximal element of the anterior limb
in the Dipnoi has almost by common consent been regarded as homologous
with the _humerus_ of the higher vertebrates.
"The humerus of Urodele Batrachians, as well as the extinct Ganocephala
and Labyrinthodontia, is articulated chiefly with the coracoid.
Therefore the element of the shoulder-girdle with which the humerus
of the Dipnoi is articulated must also be regarded as the _coracoid_
(subject to the proviso hereinafter stated), unless some specific
evidence can be shown to the contrary. No such evidence has been
produced.
"The scapula in the Urodele and other Batrachians is entirely or almost
wholly excluded from the glenoid foramen, and above the coracoid.
Therefore the corresponding element in Dipnoi must be the _scapula_.
"The other elements must be determined by their relation to the
preceding, or to those parts from or in connection with which they
originate. All those elements in _immediate_ connection with the
pectoral fin and the scapula must be homologous as a whole with the
coraco-scapular plate of the Batrachians; that is, it is infinitely
more probable that they represent, as a whole or as dismemberments
therefrom, the coraco-scapular element than that they independently
originated. But the homogeneity of that coraco-scapular element
forbids the identification of the several elements of the fish's
shoulder-girdle with regions of the Batrachian's coraco-scapular plate.
[Illustration: FIG. 73.--Clavicles of a Sea Catfish, _Selenaspis dowi_
(Gill).]
"And it is equally impossible to identify the fish's elements with
those of the higher reptiles or other vertebrates which have developed
from the Batrachians. The elements in the shoulder-girdles of the
distantly separated classes _may_ be (to use the terms introduced
by Dr. Lankester) homoplastic, but they _are not_ homogenetic.
Therefore they must be named accordingly. The element of the Dipnoan's
shoulder-girdle, continuous downward from the scapula, and to which the
coracoid is closely applied, may be named _ectocoracoid_.
"Neither the scapula in Batrachians nor the cartilaginous extension
thereof, designated suprascapula, is dissevered from the coracoid.
Therefore there is an _a priori_ improbability against the homology
with the scapula of any part having a distant and merely ligamentous
connection with the humerus-bearing element. Consequently, as an
element better representing the scapula exists, the element named
scapula (by Owen, Günther, etc.) cannot be the homologue of the scapula
of Batrachians. On the other hand, its more intimate relations with the
skull and the mode of development indicate that it is rather an element
originating and developed in more intimate connection with the skull.
It may therefore be considered, with Parker, as a _post-temporal_.
"The shoulder-girdle in the Dipnoi is connected by an azygous
differentiated cartilage, swollen backwards. It is more probable that
this is the homologue of the _sternum_ of Batrachians, and that in the
latter that element has been still more differentiated and specialized
than that it should have originated _de novo_ from an independently
developed nucleus."
[Illustration: FIG. 74.--Shoulder-girdle of a Batfish, _Ogcocephalus
radiatus_ (Mitchill).]
=The Girdle in Fishes Other than Dipnoans.=--"Proceeding from the
basis now obtained, a comparative examination of other types of fishes
successively removed by their affinities from the Lepidosirenids may be
instituted.
"With the humerus of the Dipnoans, the element of the Polypterids
(single at the base, but immediately divaricating and with its limbs
bordering an intervening cartilage which supports the pectoral and
its basilar ossicles) must be homologous. But it is evident that the
external elements of the so-called carpus of the teleosteoid Ganoids
are homologous with that element in Polypterids. Therefore those
elements cannot be carpal, but must represent the humerus.
[Illustration: FIG. 75.--Shoulder-girdle of a Threadfin, _Polydactylus
approximans_ (Lay and Bennett).]
"The element with which the homologue of the humerus, in Polypterids,
is articulated must be homologous with the analogous element in
Dipnoans, and therefore with the _coracoid_. The coracoid of
Polypterids is also evidently homologous with the corresponding
element in the other Ganoids, and the latter consequently must be
also _coracoid_. It is equally evident, after a detailed comparison,
that the single coracoid element of the Ganoids represents the three
elements developed in the generalized Teleosts (Cyprinids, etc.) in
connection with the basis of the pectoral fin, and, such being the
case, the nomenclature should correspond. Therefore the upper element
may be named _hypercoracoid_; the lower, _hypocoracoid_; and the
transverse or median, _mesocoracoid_.
"The two elements of the arch named by Parker, in _Lepidosiren_,
'supraclavicle' (scapula) and 'clavicle' (ectocoracoid) seem to be
comparable together, and as a whole, with the single element carrying
the humerus and pectoral fin in the Crossopterygians (_Polypterus_
and _Calamoichthys_) and other fishes, and therefore not identical
respectively with the 'supraclavicle' and 'clavicle' (except in part)
recognized by him in other fishes. As this compound bone, composed of
the scapula and ectocoracoid fused together, has received no name which
is not ambiguous or deceptive in its homologous allusions, it may be
designated as _proscapula_.
"The post-temporal of the Dipnoans is evidently represented by the
analogous element in the Ganoids generally, as well as in the typical
fishes. The succeeding elements (outside those already alluded to)
appear from their relations to be developed from or in connection
with the post-temporal, and not from the true scapular apparatus;
they may therefore be named _post-temporal_, _posterotemporal_, and
_teleo-temporal_. It will be thus seen that the determinations here
adopted depend mainly (1) on the interpretation of the homologies of
the elements with which the pectoral limbs are articulated, and (2) on
the application of the term 'coracoid.' The name 'coracoid,' originally
applied to the process so called in the human scapula and subsequently
extended to the independent element homologous with it in birds
and other vertebrates, has been more especially retained (e.g., by
Parker in mammals, etc.) for the region including the glenoid cavity.
On the assumption that this may be preferred by some zootomists,
the preceding terms have been applied. But if the name should be
restricted to the proximal element, nearest the glenoid cavity, in
which ossification commences, the name _paraglenal_ given by Dugès
to the cartilaginous glenoid region can be adopted, and the coracoid
would then be represented (in part) rather by the element so named by
Owen. That eminent anatomist, however, reached his conclusion (only in
part the same as that here adopted) by an entirely different course of
reasoning, and by a process, as it may be called, of elimination; that
is, recognizing first the so-called 'radius' and 'ulna,' the 'humerus,'
the 'scapula,' and the 'coracoid' were successively identified from
their relations to the elements thus determined and because they were
numerically similar to the homonymous parts among higher vertebrates."
FOOTNOTES:
[4] Catalogue of the Families of Fishes, 1872.