appendage has become reduced to the branchiæ, and the intrinsic
appendage-muscles have entirely disappeared, with the possible exception of
the small post-stigmatic muscle; on the other hand, the dorso-ventral
somatic muscles, which are clearly homologous with the corresponding
muscles of Limulus, have remained, and become the essential respiratory
muscles.
Of these two possible types of respiratory movement it is quite conceivable
that in the water-breathing scorpions of olden times and in their allies,
the dorso-ventral muscles of their branchial appendages may have continued
their _rôle_ of respiratory muscles, and so have given origin to the
respiratory muscles of the ancestors of Ammocoetes.
The respiratory muscles of Ammocoetes are three in number, and have been
described by Nestler and Miss Alcock as the adductor muscle, the striated
constrictor muscle, and the tubular constrictor muscle (Fig. 65, _m. add._,
_m.c.s._, and _m.c.t._). Of these, the constrictor muscle (Fig. 71, _m.
con. str._) is in close contact with its cartilaginous bar, while the
adductor (Fig. 71, _m. add._) is attached to the cartilage only at its
origin and insertion, and the tubular muscles (Fig. 71, _m. con. tub._)
have nothing whatever to do with the cartilage at all, being attached
ventrally to the connective tissue in the neighbourhood {172}of the ventral
aorta (_V.A._), and dorsally to the mid-line between the dorsal aorta
(_D.A._) and the notochord.
The close relationship of the constrictor muscle to the cartilaginous
branchial bar does not favour the surmise that this muscle is homologous
with the dorso-ventral somatic muscle of the scorpion. It is, however,
directly in accordance with the view that this muscle is homologous with
one of the dorso-ventral appendage-muscles, such as the posterior
entapophysio-branchial muscle (_m_1_, Fig. 66) of the Limulus appendage,
especially when the homology of the Ammocoetes branchial cartilage with the
Limulus branchial cartilage is borne in mind. I am, therefore, inclined to
look upon the constrictor and adductor muscles of the Ammocoetes branchial
segment as more likely to have been derived from dorso-ventral muscles
which belonged originally to a branchial appendage, such as we see in
Limulus, than from dorso-ventral somatic muscles, such as the vertical
mesosomatic muscles which are found both in Limulus and scorpion. In other
words, I am inclined to hold the view that the somatic dorso-ventral
muscles have disappeared in this region in Ammocoetes, while dorso-ventral
appendage-muscles have been retained, _i.e._ the exact reverse to what has
taken place in the air-breathing scorpion.
I am especially inclined to this view because of the manner in which it
fits in with and explains van Wijhe's results. Ever since Schneider divided
the striated muscles of vertebrates into parietal and visceral, such a
division has received general acceptance and, as far as the head-region is
concerned, has received an explanation in van Wijhe's work; for Schneider's
grouping corresponds exactly to the two segmentations of the
head-mesoblast, discovered by van Wijhe, _i.e._ to the somatic and
splanchnic striated muscles according to my nomenclature. Of these two
groups the splanchnic or visceral striated musculature, innervated by the
Vth, VIIth, IXth, and Xth nerves, which ought on this theory to be derived
from the musculature of the corresponding appendages, is, speaking
generally, dorso-ventral in direction in Ammocoetes and of the same
character throughout; the somatic musculature, on the other hand, is
clearly divisible, in the head region, into two sets--a spinal and a
cranial set. The somatic muscles innervated by the spinal set of nerves,
including in this term the spino-occipital or so-called hypoglossal nerves,
are in Ammocoetes most sharply defined from all the other muscles of the
body. They form the great dorsal and ventral longitudinal
{173}body-muscles, which extend dorsally as far forward as the nose and are
developed embryologically quite distinctly from the others, being formed as
muscle-plates (Kästchen). On the other hand, the cranial somatic muscles
are the eye-muscles, the formation of which resembles that of the visceral
muscles, and not of the spinal somatic. Their direction is not
longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to
the somatic trunk-muscles, and must, therefore, form a separate group to
themselves. Thus the striated musculature of the Ammocoetes must be divided
into (1) the visceral muscles; (2) the longitudinal somatic muscles; and
(3) the dorso-ventral somatic muscles. Of these the 1st, on the view just
stated, represent the original appendage-muscles; the 2nd belong to the
spinal region, and will be considered with that region; the 3rd represent
the original segmental dorso-ventral somatic muscles, which are so
conspicuous in the musculature of the Limulus and the scorpion group.
The discussion of this last statement will be given when I come to deal
with the prosomatic segments of Ammocoetes. I wish, here, simply to point
out that van Wijhe has shown that the eye-muscles develop from his 1st,
2nd, and 3rd dorsal mesoblastic segments, and therefore represent the
somatic muscles belonging to those segments, while no development of any
corresponding muscles takes place in the 4th, 5th, and 6th segments; so
that if the eye-muscles represent a group of dorso-ventral somatic muscles,
such muscles have been lost in the 4th, 5th, and 6th segments. The latter
segments are, however, the glossopharyngeal and vagus segments, the
branchial musculature of which is derived from the ventral segments of the
mesoderm. In other words, van Wijhe's observations mean that the
dorso-ventral somatic musculature has been lost in the branchial or
mesosomatic region, while the dorso-ventral appendage musculature has been
retained, and that, therefore, the mode of respiration in Ammocoetes more
closely resembles that of Limulus than of Scorpio.
In addition to these branchial muscles, another and very striking set of
muscles is found in the respiratory region of Ammocoetes--the so-called
tubular muscles. These muscles are of great interest, but as they are
especially connected with the VIIth nerve, their consideration is best
postponed to the chapter dealing with that nerve.
Also, in connection with the vagus group of nerves, special sense-organs
are found in the skin covering this mesosomatic region, the so-called
epithelial pit-organs (_Ep. pit._, Fig. 71). They, too, are of {174}great
interest, but their consideration may also better be deferred to the
chapter dealing with those special sense-systems known as the lateral line
and auditory systems.
COMPARISON OF THE BRANCHIAL CIRCULATION IN AMMOCOETES AND LIMULUS.
Closely bound up with the respiratory system is the nature of the
circulation of blood through the gills. Before, therefore, proceeding to
the consideration of the segments in front of those which carry branchiæ,
it is worth while to compare the circulation of the blood in the gills of
Limulus and of Ammocoetes respectively.
In all the higher vertebrates the blood circulates in a closed system of
capillaries, which unite the arterial with the venous systems. In all the
higher invertebrates this capillary system can hardly be said to exist; the
blood is pumped from the arterial system into blood spaces or lacunæ, and
thus comes into immediate contact with the tissues. From these it is
collected into veins, and so returned to the heart. There is, in fact, no
separate lymph-system in the higher invertebrates; the blood-system and
lymph-system are not yet differentiated from each other. This also is the
case in Ammocoetes; here, too, in many places the blood is poured into a
lacunar space, and collected thence by the venous system; a capillary
system is only in its commencement and a lymph-system does not yet exist.
In this part of its vascular system Ammocoetes again resembles the higher
invertebrates more than the higher vertebrates.
This resemblance is still more striking when the circulation in the
respiratory organs of the two animals is compared. A branchial appendage is
essentially an appendage whose vascular system is arranged for the special
purpose of aerating blood. In the higher vertebrates such a purpose is
attained by the pulmonary capillaries, in Limulus by the division of the
posterior surface of the basal part of the appendage into thin lamellar
plates, the interior of each of which is filled with blood. The two
surfaces of each lamella are kept parallel to each other by means of
fibrous or cellular strands forming little pillars at intervals, called by
Macleod "colonettes." A precisely similar arrangement is found in the
scorpion gill-lamella, as seen in Fig. 69, A, taken from Macleod. In
Ammocoetes there are no well-defined branchial capillaries, but the blood
circulates, as in {175}the invertebrate gill, in a lamellar space; here,
also, as Nestler has shown, the opposing walls of the gill-lamella are held
in position by little pillar-like cells, as seen in Fig. 69, B, taken from
his paper.
In this representative of the earliest vertebrates the method of
manufacturing an efficient gill out of a lacunar blood-space is precisely
the same as that which existed in Limulus and the scorpion, and, therefore,
as that which existed in the dominant invertebrate group at the time when
vertebrates first appeared. This similarity indicates a close resemblance
between the circulatory systems of the two groups of animals, and
therefore, to the superficial inquirer, would indicate an homology between
the heart of the vertebrate and the heart of the higher invertebrate; but
the former is situated ventrally to the gut and the nervous system, while
the latter is composed of a long vessel which lies in the mid-dorsal line
immediately under the external dorsal covering. Indeed, this ventral
position of the heart in the one group of animals and its dorsal position
in the other, combined with the corresponding positions of the central
nervous system, is one of the principal reasons why all the advocates of
the origin of vertebrates from the Appendiculata, with the single exception
of myself, feel compelled to reverse the dorsal and ventral surfaces in
deriving the vertebrate from the invertebrate. But there is one most
important fact which ought to make us hesitate before accepting the
homology of the dorsal heart of the arthropod with the ventral heart of the
vertebrate--The heart in all invertebrates is a systemic heart, _i.e._
drives the arterial blood to the different organs of the body, and then the
veins carry it back to the respiratory organ, from whence it passes to the
heart.
[Illustration: FIG. 69.--COMPARISON OF BRANCHIAL LAMELLÆ OF LIMULUS AND
SCORPIO WITH BRANCHIAL LAMELLÆ OF AMMOCOETES.
A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of
Ammocoetes (after Nestler).]
The only exception to this scheme is found in the vertebrate where the
heart is essentially a branchial heart, the blood being {176}driven from
the heart to the ventral aorta, from which by the branchial arteries it is
carried to the gills, and then, after aeration, is collected into the
dorsal aorta, whence it is distributed over the body. The distributing
systemic vessel is the dorsal aorta, not the heart which belongs
essentially to the ventral venous system. This constitutes a very strong
reason for believing that the systemic heart of the invertebrate is not
homologous with the heart of the vertebrate. How, then, did the vertebrate
heart arise?
Let us first see how the blood is supplied to the gills in Limulus.
[Illustration: FIG. 70.--LONGITUDINAL DIAGRAMMATIC SECTION THROUGH THE
MESOSOMATIC REGION OF LIMULUS, TO SHOW THE ORIGIN OF THE BRANCHIAL
ARTERIES. (After BENHAM.)
_L.V.S._, longitudinal venous sinus, or collecting sinus; _a. br._,
branchial arteries; _V.p._, veno-pericardial muscles; _P._, pericardium.]
In Limulus the blood flows into the lamellæ from sinuses or blood-spaces
(_b.s._, Fig. 66) at the base of each of the lamellæ, which sinuses are
filled by a vessel which may be called the branchial artery, since it is
the afferent branchial vessel. On each side of the middle line of the
ventral surface of the body a large longitudinal venous sinus exists,
called by Milne-Edwards the venous collecting sinus, _L.V.S._, (Fig. 70 and
Fig. 58), which gives off to each of the branchial appendages on that side
a well-defined afferent branchial vessel--the branchial artery (_a. br._).
The blood of the branchial artery flows into the blood-spaces between the
anterior and posterior laminæ of the appendage and thence into the
gill-lamellæ, from which it is collected into an efferent vessel or
branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which
carries it back to the dorsal heart. The position of the branchial artery
and vein is shown in Fig. 66, which represents a section through the
branchial appendage of Limulus at right angles to the cartilaginous
branchial bar (_br. cart._), just as Fig. 65 represents a section through
the {177}branchial appendage of Ammocoetes at right angles to the
cartilaginous branchial bar.
Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and
Benham concur in showing that in both Limulus and the scorpion group a
striking and most useful connection exists between the heart and these two
collecting venous sinuses, in the shape of a segmentally arranged series of
muscular bands (_V.p._, Fig. 70 and Fig. 58), attached, on the one hand, to
the pericardium, and on the other to the venous collecting sinus on each
side. These muscular bands, to which Lankester and Benham have given the
name of 'veno-pericardial muscles,' are so different in appearance from the
rest of the muscular substance, that Milne-Edwards did not recognize them
as muscular, but called them 'brides transparentes.' Blanchard speaks of
them in the scorpion as 'ligaments contractiles,' and considers that they
play an important part in assisting the pulmonary circulation; for, he
says, "en mettant a nu une portion du coeur, on remarque que ces battements
se font sentir sur les ligaments contractiles, et determinent sur les
poches pulmonaires une pression qui fait aussitot refluer et remonter le
sang dans les vaisseaux pneumocardiaques." Lankester, in discussing the
veno-pericardial muscles of Limulus and of the scorpions, says that these
muscles probably contract simultaneously with the heart and are of great
importance in assisting the flow through the pulmonary system. More
recently Carlson has investigated the action of these muscles in the living
Limulus and found that they act simultaneously with the muscles of
respiration.
Precisely the same arrangement of veno-pericardial muscles and of
longitudinal venous collecting sinuses occurs in the scorpions. It is one
of the fundamental characters of the group, and we may fairly assume that a
similar arrangement existed in the extinct forms from which I imagine the
vertebrate to have arisen. The further consideration of this group of
muscles will be given in Chapter IX.
Passing now to the condition of the branchial blood-vessels of Ammocoetes,
we see that the blood passes into the gill-lamellæ from a blood-space in
the appendage, which can hardly be dignified by the name of a blood-vessel.
This blood-space is supplied by the branchial artery which arises
segmentally from the ventral aorta (_V.A._), as seen in Fig. 71 (taken from
Miss Alcock's paper). From the gill-lamellæ the blood is collected into an
efferent or branchial vein (_v. br._), which {178}runs, as seen in Fig. 65,
along the free edge of the diaphragm, and terminates in the dorsal aorta.
The ventral aorta is a single vessel near the heart, but at the
commencement of the thyroid it divides into two, and so forms two ventral
longitudinal vessels, from which the branchial arteries arise segmentally.
[Illustration: FIG. 71.--DIAGRAM CONSTRUCTED FROM A SERIES OF TRANSVERSE
SECTIONS THROUGH A BRANCHIAL SEGMENT, SHOWING THE ARRANGEMENT AND RELATIVE
POSITIONS OF THE CARTILAGE, MUSCLES, NERVES, AND BLOOD-VESSELS.
Nerves coloured red are the motor nerves to the branchial muscles. Nerves
coloured blue are the internal sensory nerves to the diaphragms and the
external sensory nerves to the sense-organs of the lateral line system.
_Br. cart._, branchial cartilage; _M. con. str._, striated constrictor
muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor
muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on
diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of
vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial;
_J.v._, jugular vein; _Ep. pit._, epithelial pit.]
From this description it is clear that the vascular supply of the branchial
segment of Ammocoetes would resemble most closely the vascular supply of
the Limulus branchial appendage, if the ventral aorta of the former was
derived from two longitudinal veins, homologous with the paired
longitudinal venous sinuses of the latter.
{179}_A priori_, such a derivation seems highly improbable; and yet it is
precisely the manner in which embryology teaches us that the heart and
ventral aorta of the vertebrate have arisen.
THE ORIGIN OF THE INVERTEBRATE HEART AND THE ORIGIN OF THE VERTEBRATE
HEART.
Not only does the vertebrate heart differ from that of the invertebrate, in
that it is branchial while the latter is systemic, but also it is unique in
its mode of formation in the embryo. In the Appendiculata the heart is
formed as a single organ in the mid-dorsal line by the growth of the two
lateral plates of mesoblast dorsalwards, the heart being formed where they
meet. In Mammalia and Aves, the heart and ventral aorta commence as a pair
of longitudinal veins, one on each side of the commencing notochord.
If the embryo be removed from the yolk, the surface of the embryo covering
these two venous trunks can be spoken of as the ventral surface of the
embryo at that stage, and indeed we find that in the present day there is
an increasing tendency to speak of this surface as the ventral surface of
the embryo. Thus, Mitsukuri, in his studies of chelonian embryos, lays
great stress on the importance of surface views and when the embryo has
been removed from the yolk, figures and speaks of its ventral surface. So,
also, Locy and Neal find that the best method of seeing the early segments
of the embryo is to remove the embryo from the yolk, and examine what they
speak of as a ventral view. At the period, then, before the formation of
the throat, we may say that on the ventral surface of the embryo a pair of
longitudinal venous sinuses are found, one on each side of the mid-ventral
line, which are in the same position with respect to the mid-axis of the
embryo as are the longitudinal venous sinuses in Limulus.
The next step is the formation of the throat by the extension of the layers
of the embryo laterally to meet in the mid-line and so form the pharynx,
with the consequence that a new ventral surface is formed; these two veins,
as is well known, travel round also, and, meeting together in the new
mid-ventral line, form the subintestinal vein, the heart, and the ventral
aorta.
What is true of Mammalia and Aves, has been shown by P. Mayer to be true
universally among vertebrates, so that in all cases the heart and ventral
aorta have arisen by the coalescence in the new mid-ventral {180}line of
two longitudinal venous channels, which were originally situated one on
each side of the notochord, in what was then the ventral surface of this
part of the embryo. This history is especially instructive in showing how
the pharyngeal region is formed by the growing round of the lateral
mesoblast, _i.e._ the muscular and other mesoblastic tissues of the
branchial segments, and how the two longitudinal veins take part in this
process. The phylogenetic interpretation of this embryological fact seems
to be, that the new ventral surface of the vertebrate in this region is
formed, not only by the branchial appendages, but also by the growth
ventrally of that part of the original ventral surface which covered each
longitudinal venous sinus.
The following out of the consecutive clues, which one after the other arise
in harmonious succession as the necessary sequence of the original working
hypothesis, brings even now into view the manner in which the respiratory
portion of the alimentary canal arose, and gives strong hints as to the
position of that part of the arthropod which gave origin to the notochord.
Here I will say no more at present, for the origin of the new alimentary
canal of the vertebrate and of the notochord will be more fittingly
discussed as a whole, after all the other organs of the vertebrate have
been compared with the corresponding organs of the arthropod.
[Illustration: FIG. 72.--DIAGRAM (UPPER HALF OF FIGURE) OF THE ORIGINAL
POSITION OF VEINS (H) WHICH COME TOGETHER TO FORM THE HEART OF A
VERTEBRATE.
_C.N.S._, central nervous system; _nc._, notochord; _m._, myotome.
The lower half of figure shows comparative position of the longitudinal
venous sinus (_L.V.S._) in Limulus. _C.N.S._, central nervous system;
_Al._, alimentary canal; _H._, heart; _m._, body-muscles.]
The strong evidence that the vertebrate heart was formed from a pair of
longitudinal venous sinuses on the ventral side of the central canal,
carries with it the conclusion that the original single median dorsal heart
of the arthropod is not represented in the vertebrate, {181}for the dorsal
aorta cannot by any possibility represent that heart.
Although it is not now functional the original existence of so important an
organ as a dorsal heart may have left traces of its former presence; if so,
such traces would be most likely to be visible in the lowest vertebrates,
just as the median eyes are much more evident in them than in the higher
forms. In Fig. 58 the position of the dorsal heart is shown in Limulus, and
in Fig. 70 the shape and extent of this dorsal heart is shown. It extends
slightly into the prosomatic region, and thins down to a point there, runs
along the length of the animal and finally thins down to a point at the
caudal end.
The heart is surrounded by a pericardium, from which at regular intervals a
number of dorso-ventral muscles pass, to be inserted into the longitudinal
venous sinus on each side. These veno-pericardial muscles are absolutely
segmental with the mesosomatic segments, and are confined to that region,
with the exception of two pairs in the prosomatic region. Their homologies
will be discussed later.
Any trace of a heart such as we have just described must be sought for in
Ammocoetes between the central nervous system and the mid-line dorsally.
Now, in this very position a large striking mass of tissue is found,
represented in section in Fig. 73, _f_. It forms a column of similar tissue
along the whole mid-dorsal region, except at the two extremities; it tapers
away in the caudal region, and headwards grows thinner and thinner, so that
no trace of it is seen anterior to the commencement of the branchial
region. It resembles in its dorsal position, in its shape, and in its size
a dorsal heart-tube such as is seen in Limulus and elsewhere, but it
differs from such a tube in its extension headwards. The heart-tube of
Limulus ceases at the anterior end of the mesosomatic region, this
fat-column of Ammocoetes at the posterior end. In its structure there is
not the slightest sign of anything of the nature of a heart; it is a solid
mass of closely compacted cells, and the cells are all very full of fat,
staining intensely black with osmic acid. Nowhere else in the whole body of
Ammocoetes is such a column of fat to be found. It is not skeletogenous
tissue with cells of the nature of cartilage-cells, as Gegenbaur thought
and as Balfour has depicted (Vol. II., Fig. 315) in his 'Comparative
Embryology,' as though this tissue were a part of the vertebral column, but
is simply fat-cells, such as might easily have taken the place of some
other previously existing organ.
{182}I do not know how to decide the question which thus arises. Supposing,
for the sake of argument, that this column of fat-cells has really taken
the place of the original dorsal heart, what criterion would there be as to
this? The heart _ex hypothesi_ having ceased to function, the muscular
tissue would not remain, and the space would be filled up, presumably with
some form of connective tissue. As likely as not, the connective tissue
might take the form of fatty tissue, the storage of fat being a
physiological necessity to an animal, while at the same time no special
organ has been developed for such a purpose, but fat is being laid down in
all manner of places in the body.
This dorsal fat-column, as it is seen in Ammocoetes, is not found in the
higher vertebrates, so that it possesses, at all events, the significance
of being a peculiarity of ancient times before the vertebrate skeletal
column was formed.
I mention it here in connection with my view as to the origin of
vertebrates, because there it is, in the very place where the dorsal heart
ought to have been. For my own part, I should not have expected that a
muscular organ such as the heart would leave any trace of itself if it
disappeared, so that its absence in the dorsal region of the vertebrate
does not seem to me in the slightest degree to invalidate my theory.
[Illustration: FIG. 73.--SECTION THROUGH THE NOTOCHORD (_NC._), THE SPINAL
CANAL AND THE FAT-COLUMN (_F._), OF AMMOCOETES, DRAWN FROM AN OSMIC
PREPARATION.
_sp. c._, spinal cord; _gl._, glandular tissue filling the spinal canal;
_sk._, Gegenbaur's skeletogenous cells; _p._, pigment.]
{183}SUMMARY.
From the close similarity of structure and position between the branchial
skeleton of Limulus and of Ammocoetes, as given in the preceding chapter,
it logically follows that the branchiæ of Ammocoetes must be homologous
with the branchiæ of Limulus. But the respiratory apparatus of Limulus
consists of branchial appendages. It follows, therefore, that the
branchiæ of Ammocoetes, and consequently of the vertebrates, must have
been derived from branchial appendages, and as they are internal, not
external, such branchial appendages must have been of the nature of
'sunk-in' branchial appendages. Such internal appendages are
characteristic of the scorpion tribe, and of, perhaps, the majority of
the Palæostraca, for no external respiratory appendages have been
discovered in any of the sea-scorpions.
In the vertebrates--and it is especially well shown in Ammocoetes--a
double segmentation exists in the head-region, a body or somatic
segmentation, and a branchial or splanchnic segmentation, respectively
expressed by the terms mesomeric and branchiomeric segmentations. The
nerves which supply the latter segments form a very well-marked group
(Charles Bell's system of lateral or respiratory nerves) which do not
conform to the system of spinal nerves, for they do not arise from
separate motor and sensory roots, but are mixed nerves from the very
beginning.
The system of cranial segmental nerves is older than the spinal system,
and cannot, therefore, be derived from it, but can be arranged as a
system supplying two segments, somatic and splanchnic, which differ in
the following way: Each somatic segment is supplied by two roots, motor
and sensory respectively, as in the spinal cord segments, while each
splanchnic segment possesses only one root, which is mixed in function.
The peculiarities of the grouping of the cranial segmental nerves, which
have hitherto been unexplained, immediately receive a straightforward and
satisfactory explanation if the splanchnic or branchiomeric segments owe
their origin to a system of appendages after the style of those of
Limulus.
In Limulus and all the Arthropoda, the segmentation is double, being
composed of (1) somatic or body-segments, constituting the mesomeric
segmentation; (2) appendage-segments, which, seeing that they carry the
branchiæ, constitute a branchiomeric segmentation. Similarly to the
cranial region of the vertebrate, the nerves which supply the somatic
segments arise from separate sensory and motor roots, while the single
nerve which supplies each appendage contains all the fibres for the
appendage, both motor and sensory.
It follows from this that the branchial segments supplied by the vagus
and glossopharyngeal nerves ought to have arisen from appendages bearing
branchiæ.
Although the evidence of such appendages has entirely disappeared in the
higher vertebrates, together with the disappearance of branchiæ, and is
not strikingly apparent in the higher gill-bearing fishes, yet in
Ammocoetes, so great is the difference here from all other fishes, it is
natural to describe the pharyngeal or respiratory chamber as a chamber
into which a symmetrical series of respiratory appendages, the so-called
diaphragms, are dependent. Each of these appendages possesses its own
mixed nerve, glossopharyngeal or vagus, {184}its own cartilage, its own
set of visceral muscles, its own sense-organs, just as do the respiratory
appendages of Limulus.
The branchial unit in the vertebrate is not the gill-pouch, but the
branchial bar or appendage between the pouches. Embryology shows how each
such appendage grows inwards, how a coelomic cavity is formed in it,
similarly to the ingrowing of the branchial appendage in scorpions.
We do not know how the palæostracan sea-scorpions breathed; they resemble
the scorpion of the present day somewhat in form, but they are in many
respects closely allied to Limulus. The present-day scorpion is a land
animal, and the muscles by which he breathes are dorso-ventral somatic
muscles, while those of Limulus are the appendage-muscles.
The old sea-scorpions very probably used their appendage-muscles after
the Limulus fashion, being water-breathers, even although their
respiratory appendages were no longer free but sunk in below the surface
of the body. The probability that such was the case is increased after
consideration of the method of breathing in Ammocoetes, for the
respiratory muscles of the latter animal are directly comparable with the
muscles of the respiratory appendages of Limulus, and are not somatic.
Even the gills themselves of Ammocoetes are built up in the same fashion
as are those of Limulus and the scorpions. The conception of the
branchial unit as a gill-bearing appendage, not a gill-pouch, immediately
explains the formation of the vertebrate heart, which is so strikingly
different from that of all invertebrate hearts, in that it originates as
a branchial and not as a systemic heart, and is formed by the coalescence
of two longitudinal veins.
The origin of these two longitudinal veins is immediately apparent if the
vertebrate arose from a palæostracan, for in Limulus and the whole
scorpion tribe, in which the heart is a systemic heart, the branchiæ are
supplied with blood from two large longitudinal venous sinuses, situated
on each side of the middle line of the animal in an exactly corresponding
position to that of the two longitudinal veins, which come together to
form the heart and ventral aorta of the vertebrate. The consideration of
the respiratory apparatus and of its blood-supply in the vertebrate still
further points to the origin of vertebrates from the Palæostraca.
{185}CHAPTER V
_THE EVIDENCE OF THE THYROID GLAND_
The value of the appendage-unit in non-branchial segments.--The double
nature of the hyoid segment.--Its branchial part.--Its thyroid part.--The
double nature of the opercular appendage.--Its branchial part.--Its
genital part.--Unique character of the thyroid gland of Ammocoetes--Its
structure.--Its openings.--The nature of the thyroid segment.--The uterus
of the scorpion.--Its glands.--Comparison with the thyroid gland of
Ammocoetes.--Cephalic genital glands of Limulus.--Interpretation of
glandular tissue filling up the brain-case of Ammocoetes.--Function of
thyroid gland.--Relation of thyroid gland to sexual functions.--Summary.
I have now given my reasons why I consider that the glossopharyngeal and
vagus nerves were originally the nerves belonging to a series of
mesosomatic branchial appendages, each of which is still traceable in the
respiratory chamber of Ammocoetes, and gives the type-form from which to
search for other serially homologous, although it may be specially
modified, segments.
As long as the branchial unit consisted of the gill-pouch the segments of
the head-region were always referred to such units, hence we find Dohrn and
Marshall picturing to themselves the ancestor of vertebrates as possessing
a series of branchial pouches right up to the anterior end of the body.
Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the
mouth as formed by the coalescence of gill-slits, of the trigeminal nerve
as supplying modified branchial segments, etc.; thus a picture of an animal
is formed such as never lived on this earth, or could be reasonably
imagined to have lived on it. Yet Dohrn's conceptions of the segmentation
were sound, his interpretation only was in fault, because he was obliged to
express his segments in terms of the gill-pouch unit. Once abandon that
point of view and take as the unit a branchial appendage, then immediately
we see that in the region in front of the branchiæ we may still have
segments {186}homologous to the branchial segments, originally
characterized by the presence of appendages, but that such appendages need
never have carried branchiæ. The new mouth may have been formed by such
appendages, which would express Dohrn's suggestion of its formation by
coalesced gill-slits; the olfactory organ may have been the sense-organ
belonging to an antennal appendage, which would be what Marshall really
meant in calling it a branchial sense-organ.
THE FACIAL NERVE AND THE FOREMOST RESPIRATORY SEGMENT.
This simple alteration of the branchiomeric unit from a gill-pouch to an
appendage, which may or may not bear branchiæ, immediately sheds a flood of
light on the segmentation of the head-region, and brings to harmony the
chaos previously existing. Let us, then, follow out its further teachings.
Next anteriorly to the glossopharyngeal and vagus nerves comes the facial
nerve; a nerve which supplies the hyoid segment, or, rather, according to
van Wijhe the two hyoid segments, for embryologically there is evidence of
two segments. As already mentioned, the facial nerve is usually included in
the trigeminal or pro-otic group of nerves, the opisthotic group being
confined to the glossopharyngeal and vagus. This inclusion of the facial
nerve into the pro-otic group of nerves forms one of the main reasons why
this group has been supposed to have originally supplied gill-pouch
segments, for the hyoid segment is clearly associated with branchiæ.
When, however, we examine Ammocoetes (_cf._ Figs. 63 and 64) it is clear
that the foremost of the segments forming the respiratory chamber, which
must be classed with the rest of the mesosomatic or opisthotic segments, is
that supplied by the facial nerves.
An examination of this respiratory chamber shows clearly that there are six
pairs of branchial appendages or diaphragms, which are all exactly similar
to each other. These are those already considered, the foremost of which
are supplied by the IXth or glossopharyngeal nerves. Immediately anterior
to this glossopharyngeal segment is seen in the figures the segment
supplied by the VIIth or facial nerves. It is so much like the segments
belonging to the glossopharyngeal and vagus nerves as to make it certain
that we are dealing here with a branchial segment, composed of a pair of
branchial appendages similar to those in the other cases, except that the
cartilaginous bar is here replaced by a bar of muco-cartilage and the
branchiæ are confined to the posterior part of each appendage. The anterior
portion is, as is seen in Fig. 74, largely occupied by blood-spaces, but in
addition carries the ciliated groove (_ps. br._) called by Dohrn
'pseudo-branchiale Rinne.' This groove leads directly into the thyroid
gland, which is a large bilateral organ situated in the middle line, as
seen in Fig. 80 and Fig. 85. As shown by Miss Alcock, the facial nerve
supplies this thyroid gland, as well as the posterior hyoid branchial
segment, and, as pointed out by Dohrn, there is every reason to consider
this thyroid gland as indicative of a separate segment, especially when van
Wijhe's statement that the hyoid segment is in reality double is taken into
account.
{187}[Illustration: FIG. 74.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.
Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
Tubular constrictor muscles distinguished from striated constrictor muscles
by simple hatching. _Tent._, tentacles; _Tent. m.c._, muco-cartilage of
tentacles; _Vel. m.c._, muco-cartilage of the velum; _Hy. m.c._,
muco-cartilage of the hyoid segment; _Ps. br._, pseudo-branchial groove;
_Br. cart._, branchial cartilages; _Sp._, space between somatic and
splanchnic muscles; _Th. op._, orifice of thyroid; _H._, heart.]
{188}The evidence, then, of Ammocoetes points directly to this conclusion:
The facial nerves represent the foremost of the mesosomatic group of
nerves, and supply two segments, which have amalgamated with each other.
The most posterior of these, the hyoid segment, is a branchial segment of
the same character as those supplied by the vagus and glossopharyngeal
nerves; represents, therefore, the foremost pair of branchial appendages.
The anterior or thyroid segment, on the other hand, differs from the rest
in that, instead of branchiæ, it carries the thyroid gland with its two
ciliated grooves. If this segment, which is the foremost of the mesosomatic
segments, also indicates a pair of appendages which carry the thyroid gland
instead of branchiæ, then it follows that this pair of appendages has
joined together in the mid-line ventrally and thus formed a single median
organ--the thyroid gland. If, then, we find that the foremost of the
mesosomatic appendages in the Palæostraca was really composed of two pairs
of appendages, of which the most posterior carried branchiæ, while the
anterior pair had amalgamated in the mid-line ventrally, and carried some
special organ instead of branchiæ, then the accumulation of coincidences is
becoming so strong as to amount to proof of the correctness of our line of
investigation.
THE FIRST MESOSOMATIC SEGMENT IN LIMULUS AND ITS ALLIES.
What, then, is the nature of the foremost pair of mesosomatic appendages in
Limulus. They differ from the rest of the mesosomatic appendages in that
they do not carry branchiæ, and instead of being {189}separate are joined
together in the mid-line ventrally to form a single terminal plate-like
appendage known as the operculum. On its posterior surface the operculum
carries the genital duct on each side.
So also in the scorpion group, the operculum is always found and always
carries the genital ducts.
A survey of the nature of the opercular appendage demonstrates the
existence of three different types--
1. That of Limulus, in which the operculum is free, and carries only the
terminations of the genital ducts. In this type the duct on each side opens
to the exterior separately (Fig. 75).
[Illustration: FIG. 75.--OPERCULUM OF LIMULUS TO SHOW THE TWO SEPARATE
GENITAL DUCTS.]
[Illustration: FIG. 76.--OPERCULUM OF MALE SCORPION.
_Ut._, terminal chamber, or uterus.]
2. The type of Scorpio, Androctonus, Buthus, etc., in which the operculum
is not free, but forms part of the ventral surface of the body-wall, but,
like Limulus, carries only the terminations of the genital ducts. In this
type the duct on each side terminates in a common chamber (vagina or
uterus), which communicates with the exterior by a single external median
opening. This common chamber, or uterus (_Ut._), extends the whole breadth
of the operculum (as seen in Fig. 76), and is limited to that segment.
3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the
Pedipalpi, in which the operculum forms a part of the ventral surface of
the body wall, but no longer covers only the termination of the genital
apparatus. It really consists of two parts, a median anterior, which covers
the terminal genital apparatus, {190}and a lateral posterior, which covers
the first pair of gills, or lung-books, as they are called. In this type
(Fig. 77) the genital ducts terminate in a common chamber or uterus, the
nature of which will be further considered.
As has been pointed out by Blanchard, the terminal genital organs of the
scorpions and the Pedipalpi vary considerably in the different genera,
especially the male genital organs. The general type of structure is the
same, and consists in both male and female of vasa deferentia, which come
together to form a common chamber before the actual opening to the
exterior. This common chamber has been called in the female scorpion the
vagina, or in Thelyphonus the uterus. I shall use the latter term, in
accordance with Tarnani's work, and the corresponding chamber in the male
will be the _uterus masculinus_.
A considerable discussion has taken place about the method of action of the
external genital organs in the members of the scorpion tribe, into which it
is hardly necessary to enter here. The evidence points to the conclusion
that in all these forms the operculum covers a median single chamber or
uterus, into which the genital ducts open on each side, the main channels
of emission being provided with a massive chitinous internal framework. We
may feel certain that in the old extinct sea-scorpions, Eurypterus, etc., a
similar arrangement existed, and that therefore in them also the median
portion of the operculum covered a median chamber or uterus composed of the
amalgamation of the terminations of the two genital ducts, which were
originally separate, as in Limulus.
[Illustration: FIG. 77.--OPERCULUM AND FOLLOWING SEGMENTS OF MALE
THELYPHONUS.
Opercular segment is marked out by thick black line. _Ut. Masc._, uterus
masculinus; _Int. Op._, internal opening of uterus into genital chamber;
_Ext. Op._, common external opening to genital chamber (_Gen. Ch._) and
pulmonary chamber.]
The observations of Schmidt, Zittel, and others show that the
{191}operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus,
etc., belonged to the type of Thelyphonus, rather than to that of Limulus
or Scorpio. In Fig. 78 I give a picture from Schmidt of the ventral aspect
of Eurypterus, and by the side of it a picture of the isolated operculum.
Schmidt considers that there were five branchiæ-bearing segments
constituting the mesosoma, the foremost of which formed the operculum. Such
operculum is often found isolated, and is clearly composed of two lateral
appendages fused together in the middle line, of such a nature as to form a
median elongated tongue, which lies between and separates the first three
pairs of branchial segments. This median tongue, together with the anterior
and median portion of the operculum, concealed, in all probability,
according to Schmidt, the terminal parts of the genital organs, just as the
median part of the operculum in Phrynus and Thelyphonus conceals the
complicated terminal portions of the genital organs. The posterior part of
the operculum, like that of Phrynus and Thelyphonus, carried the first pair
of branchiæ, so Schmidt thinks from the evidence of markings on some
specimens.
[Illustration: FIG. 78.--_Eurypterus._
The segments and appendages on the right are numbered in correspondence
with the cranial system of lateral nerve-roots as found in vertebrates.
_M._, metastoma. The surface ornamentation is represented on the first
segment posterior to the branchial segments. The opercular appendage is
marked out by dots.]
Apparently an opercular appendage of this kind is in reality the result of
a fusion of the genital operculum with the first branchial appendage in
forms such as the scorpion; for, in order that the tergal plates may
correspond in number with the sternal in Eurypterus, etc., it is necessary
to consider that the operculum is composed of two sternites joined
together. Similarly in Thelyphonus, Phrynus, etc., this numerical
correspondence is only observed if the operculum is looked upon as double.
A restoration of the mesosomatic region of Eurypterus, viewed {192}from the
internal surface, might be represented by Fig. 79, in which the thick line
represents the outline of the opercular segment, and the fainter lines the
succeeding branchial segments. The middle and anterior part of the
opercular segment carried the terminations of the genital organs; these I
have represented, in accordance with our knowledge of the nature of these
organs in the present-day scorpions, as a median elongated uterus,
bilaterally formed, from which the genital ducts passed, probably as in
Limulus, towards a mass of generative gland in the cephalic region, and not
as in Scorpio or Thelyphonus, tailwards to the abdominal region.
[Illustration: FIG. 79.--DIAGRAM TO INDICATE THE PROBABLE NATURE OF THE
MESOSOMATIC SEGMENTS OF EURYPTERUS.
The opercular segment is marked out by the thick black line. The segments
_II.-VI._ bear branchiæ, and segment _I._ is supposed in the male to carry
the uterus masculinus (_Ut. Masc._) and the genital ducts.]
It is possible that in Holm's representation of Eurypterus, Fig. 104, the
genital duct on each side is indicated.
THE THYROID GLAND OF AMMOCOETES.
If we compare this mesosomatic region of Eurypterus with that of
Ammocoetes, the resemblance is most striking, and gives a meaning to the
facial nerve which is in absolute accordance with the interpretation
already given of the glossopharyngeal and vagus nerves. In both cases the
foremost respiratory or mesosomatic segment is double, the posterior
lateral part alone bearing the branchiæ, while the median and anterior part
bore in the one animal the uterus and genital ducts, in the other the
thyroid gland and ciliated grooves. We are driven, therefore, to the
conclusion that this extraordinary and unique organ, the so-called thyroid
gland of Ammocoetes, which exists only in the larval condition and is got
rid of as soon as the adult sexual organs are formed, shows the very form
and position of the uterus of this invertebrate ancestor of Ammocoetes.
What, then, is the nature of the thyroid gland in Ammocoetes?
{193}Throughout the vertebrate kingdom it is possible to compare the
thyroid gland of one group of animals with that of another without coming
across any very marked difference of structure right down to and including
Petromyzon. When, however, we examine Ammocoetes, we find that the thyroid
has suddenly become an organ of much more complicated structure, covering a
much larger space, and bearing no resemblance to the thyroid glands of the
higher forms. At transformation the thyroid of Ammocoetes is largely
destroyed, and what remains of the gland in Petromyzon becomes limited to a
few follicles resembling those of other fishes. The structure and position
of this gland in Ammocoetes is so well known that it is unnecessary to
describe it in detail. For the purpose, however, of making my points clear,
I give in Fig. 80 the position and appearance of the thyroid gland (_Th._)
when the skin and underlying laminated layer has been removed by the action
of hypochlorite of soda. On the one side the ventral somatic muscles have
been removed to show the branchial cartilaginous basket-work.
[Illustration: FIG. 80.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES.
_Th._, thyroid gland; _M._, lower lip, with its muscles.]
The series of transverse sections in Fig. 81 represents the nature of the
organ at different levels in front of and behind the opening into the
respiratory chamber; and in Fig. 82 I have sketched the appearance of the
whole gland, viewed so as to show its opening into the respiratory chamber
and its posterior curled-up termination.
{194}[Illustration: FIG. 81.--SAMPLES FROM A COMPLETE SERIES OF TRANSVERSE
SECTIONS THROUGH THE THYROID GLAND OF AMMOCOETES.
Sections 1 and 2 are anterior to the thyroid opening, _Th. o._; sections 3,
4, and 5 are through the thyroid opening; and section 6 is posterior to the
thyroid opening before the commencement of the curled portion.]
{195}The series of transverse sections (1-6, Fig. 81) show that we are
dealing here with a central glandular chamber, C (Fig. 81 (6) and Fig. 82),
which opens by the thyroid duct (_Th. o._) into the pharyngeal chamber, and
is curled upon itself in its more posterior part. This central chamber
divides, anteriorly to the thyroid orifice, into two portions, A, A[prime]
(Fig. 82), giving origin to two tubes, B, B[prime], which lie close
alongside of, and extend further back than, the posterior limit of the
curled portion of the central chamber, C. The structure of the central
chamber, C, and, therefore, of the separate coils, is given in both
Schneider's and Dohrn's pictures, and is represented in Fig. 81 (6), which
shows the peculiar arrangement and character of the glandular cells typical
of this organ, and also the nature of the central cavity, with the
arrangement of the ciliated epithelium. The structure of each of the
lateral tubes, B, is different from that of the central chamber, in that
only half the central chamber is present in them, as is seen by the
comparison of the tube B with the tube C in Fig. 81 (5 and 6), so that we
may look upon the central chamber, C, as formed of two tubes, similar in
structure to the tubes B, which have come together to form a single chamber
by the partial absorption of their walls, the remains of the wall being
still visible as the septum, which partially divides the chamber, C, into
halves.
In the walls of each of these tubes is situated a continuous glandular
line, the structure of the glandular elements being specially characterized
by the length of the cells, by the large spherical nucleus situated at the
very base of each cell, and by the way in which the cells form a
wedge-shaped group, the thin points of all the wedge-shaped cells coming
together so as to form a continuous line along the chamber wall. This free
termination of the cells of the gland in the lumen of the chamber
constitutes the whole method for the secretion of the gland; there is no
duct, no alveolus, nothing but this free termination of the cells.
Moreover, sections through the portion A, A[prime] (Fig. 82) show that
here, as in the central chamber, C, four of these glandular lines open into
a common chamber, but they are not the same four as in the case of the
central chamber, for if we name these glandular lines on the left side _a
b, a[prime] b[prime]_ (Fig. 81), and on the right side _c d, c[prime]
d[prime]_, then the central chamber has opening into it the glands _a b, c
d_, while the chambers of A and A[prime] have opening into them
respectively _a b, a[prime] b[prime]_, and _c d, c[prime] d[prime]_.
Further, the same series of sections shows that the glands _a_ and _b_ are
continuous with the glands _a[prime]_ and _b[prime]_ respectively across
the apex of A, and similarly on the other side, so that the two glandular
rows _a b_ are continuous with the two glandular rows _a[prime] b[prime]_,
and we see that the {196}cavity of the portion A or A[prime] is formed by
the bending over of the tube or horn, B or B[prime], with the partial
absorption of the septum so formed between the tube and its bent-over part.
If, then, we uncoil the curled-up part of C, and separate the portion, B,
on each side from the chamber, C, we see that the so-called thyroid of
Ammocoetes may be represented as in Fig. 83, _i.e._ it consists of a long,
common chamber, C, which, for reasons apparent afterwards, I will call the
_palæo-hysteron_, which opens, by means of a large orifice, into the
respiratory or pharyngeal chamber. The anterior end of this chamber
terminates in two tubes, or horns, B, B[prime], the structure of which
shows that the median chamber, C, is the result of the amalgamation of two
such tubes, and consequently in this chamber, or _palæo-hysteron_, the
glandular lines are symmetrically situated on each side.
[Illustration: FIG. 82.--DIAGRAMMATIC REPRESENTATION OF THE SO-CALLED
THYROID GLAND OF AMMOCOETES.
_C_, central chamber; _A, A[prime]_, anterior extremity; _B, B[prime]_,
posterior extremity; _Th. o._, thyroid opening into respiratory chamber;
_Ps. br., Ps. br[prime]._, ciliated grooves, Dohrn's pseudo-branchial
grooves.]
[Illustration: FIG. 83.--THYROID GLAND AS IT WOULD APPEAR IF THE CENTRAL
CHAMBER WERE UNCURLED AND THE TWO HORNS, _B_, _B[prime]_, SEPARATED FROM
THE CENTRAL CHAMBER.]
Any explanation, then, of the thyroid gland of Ammocoetes, must {197}take
into account the clear evidence that it is composed of two tubes, which
have in part fused together to form an elongated central chamber, in part
remain as horns to that chamber, and that in its walls there exist lines of
gland-cells of a striking and characteristic nature.
Further, this central chamber, with its horns, is not a closed chamber, but
is in communication with the pharyngeal or respiratory chamber by three
ways. In the first place, the central chamber, as is well known, opens into
the respiratory chamber by a funnel-shaped opening--the so-called thyroid
duct (_Th. o._). In the second place, there exist two ciliated grooves
(_Ps. br._, _Ps. br[prime]._), the pseudo-branchial grooves of Dohrn, which
have direct communication with the thyroid chamber. The manner in which
these grooves communicate with the thyroid chamber has never, to my
knowledge, been described previously to my description in the _Journal of
Physiology and Anatomy_; it is very instructive, for, as I have there
shown, each groove enters into the corresponding lateral horn, so that, in
reality, there are three openings into the thyroid chamber or
palæo-hysteron--a median opening into the central chamber, and a separate
opening into each lateral horn.
The system of ciliated grooves on the inner ventral surface of the
respiratory chamber of Ammocoetes was originally described by Schneider as
consisting of a single median groove, which extends from the opening of the
thyroid to the posterior extremity of the branchial chamber, and a pair of
grooves, or semi-canals, which, starting from the region of the thyroid
orifice, run headwards and diverge from each other, becoming more and more
lateral, and more and more dorsal, till they come together in the
mid-dorsal pharyngeal line below the auditory capsules. The latter are the
pseudo-branchial grooves of Dohrn, of which I have already spoken.
Schneider looked upon the whole of this system as a single system, for he
speaks of "a ciliated groove, which extends from the orifice of the stomach
(_i.e._ anterior intestine) to the orifice of the thyroid, then divides
into two, and runs forward right and left of the median ridge, etc." Dohrn
rightly separates the median ciliated groove posterior to the thyroid
orifice (seen in Fig. 81 (6)) from the paired pseudo-branchial grooves; the
former is a shallow depression which opens into the rim of the thyroid
orifice, while the latter has a much more intimate connection with the
thyroid gland itself.
{198}A series of sections, such as is given in Fig. 81, shows the relation
of this pair of ciliated grooves to the thyroid better than any elaborate
description. In the first place, it is clear that they remain separate up
to their termination--they do not join in the middle line to open into the
thyroid duct; in the second place, they are separate from the thyroid
orifice--they do not terminate at the rim of the orifice, as is the case
with the median groove just mentioned, but continue on each side on the
wall of the thyroid duct (Fig. 81 (2)), gradually moving further and
further away from the actual opening of the duct into the pharyngeal
chamber. During the whole of their course on the wall of the funnel-shaped
duct they retain the character of grooves, and are therefore open to the
lumen of the duct. The direction of the groove (_Ps. br._) shifts as it
passes deeper and deeper towards the thyroid, until at last, as seen in
Fig. 81 (3 and 4), it is continuous with the narrow diverticulum of the
turned-down single part of the thyroid (B), or turned-down horn, as I have
called it. In other words, the median chamber opens into the pharyngeal or
respiratory chamber by a single large, funnel-shaped opening, and, in
addition, the two ciliated grooves terminate in the lateral horns on each
side, and only indirectly into the central chamber, owing to their being
semi-canals, and not complete canals. If they were originally canals, and
not grooves, then the thyroid of Ammocoetes would be derived from an organ
composed of a large, common glandular chamber, which opened into the
respiratory chamber by means of an extensive median orifice, and possessed
anteriorly two horns, from each of which a canal or duct passed headwards
to terminate somewhere in the region of the auditory capsule.
Dohrn has pointed out that a somewhat similar structure and topographical
arrangement is found in Amphioxus and the Tunicata, the gland-cells being
here arranged along the hypobranchial groove to form the endostyle and not
shut off to form a closed organ, as in the thyroid of Ammocoetes. Dohrn
concludes, in my opinion rightly, that the endostyle in the Tunicata and in
Amphioxus represents the remnants of the more elaborate organ in
Ammocoetes, and that, therefore, in order to explain the meaning of these
organs in the former animals, we must first find out their meaning in
Ammocoetes. Dohrn, however, goes further than this; for just as he
considers Amphioxus and the Tunicata to have arisen by degeneration from an
Ammocoetes-like form, so he considers Ammocoetes to have arisen {199}from a
degenerated Selachian; therefore, in order to be logical, he ought to show
that the thyroid of Ammocoetes is an intermediate downward step between the
thyroid of Selachians and that of Amphioxus and the Tunicates. Here, it
seems to me, his argument utterly breaks down; it is so clear that the
thyroid of Petromyzon links on to that of the higher fishes, and that the
Ammocoetes thyroid is so immeasurably more complicated and elaborate a
structure than is that of Petromyzon, as to make it impossible to believe
that the Ammocoetes thyroid has been derived by a process of degeneration
from that of the Selachian. On the contrary, the manner in which it is
eaten up at transformation and absolutely disappears in its original form
is, like the other instances mentioned, strong evidence that we are dealing
here with an ancestral organ, which is confined to the larval form, and
disappears when the change to the higher adult condition takes place.
Dohrn's evidence, then, points strongly to the conclusion that the
starting-point of the thyroid gland in the vertebrate series is to be found
in the thyroid of Ammocoetes, which has given rise, on the one hand, to the
endostyle of Amphioxus and the Tunicata, and on the other, to the thyroid
gland of Petromyzon and the rest of the Vertebrata.
The evidence which I have just given of the intimate connection of the two
pseudo-branchial grooves with the thyroid chamber shows, to my mind,
clearly that Dohrn is right in supposing that morphologically these two
grooves and the thyroid must be considered together. His explanation is
that the whole system represents a modified pair of branchial segments
distinct from those belonging to the VIIth and IXth nerves. The cavity of
the thyroid and the pseudo-branchial grooves are, therefore, according to
him, the remains of the gill-pouches of this fused pair of branchial
segments, which no longer open to the surface, and the glandular tissue of
the thyroid is derived from the modified gill-epithelium. This view of
Dohrn's, which he has urged most strongly in various papers, is, I think,
right in so far as the separateness of the thyroid segment is concerned,
but is not right, and is not proven, in so far as concerns the view that
the thyroid gland is a modified pair of gills.
We may distinctly, on my view, look upon the thyroid segment, with its
ciliated grooves and its covering plate of muco-cartilage, as a distinct
paired segment, homologous with the branchial segments, without any
necessity of deriving the thyroid gland from a pair of gills.
{200}The evidence that such a median segment has been interpolated
ventrally between the foremost pairs of branchial segments is remarkably
clear, for the limits ventrally of the branchial segments are marked out on
each side by the ventral border of the cartilaginous basket-work; and it is
well known, as seen in Fig. 80, that whereas this cartilaginous framework
on the two sides meets together in the middle ventral line in the posterior
branchial region, it diverges in the anterior region so as to form a
tongue-shaped space between the branchial segments on the two sides. This
space is covered over with a plate of muco-cartilage which bears on its
inner surface the thyroid gland.
[Illustration: FIG. 84.--DIAGRAM OF (A) VENTRAL SURFACE AND (B) LATERAL
SURFACE OF AMMOCOETES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PITS ON
THE BRANCHIAL REGION, AND THEIR INNERVATION BY _VII._, THE FACIAL, _IX._,
THE GLOSSOPHARYNGEAL, AND _X^1_-_X^6_, THE VAGUS NERVES.]
In addition to this evidence that we are dealing here with a ventral
tongue-like segment belonging to the facial nerve which is interpolated
between the foremost branchial segments, we find the most striking fact
that at transformation the whole of this muco-cartilaginous plate
disappears, the remarkable thyroid gland of the {201}Ammocoetes is eaten
up, and nothing is left except a small, totally different glandular mass;
and now the cartilaginous basket-work meets together in the middle line in
this region as well as in the more posterior region. In other words, the
striking characteristic of transformation here is the destruction of this
interpolated segment, and the resulting necessary drawing together
ventrally of the branchial segments on each side.
[Illustration: FIG. 85.--FACIAL SEGMENT OF AMMOCOETES MARKED OUT BY
SHADING.
_VII._ 1, thyroid part of segment; _VII._ 2, hyoid or branchial part; 3-9,
succeeding branchial segments belonging to IXth and Xth nerves; _V_, the
velar folds; _Ps. br._, Dohrn's pseudo-branchial groove; _Th. o._, thyroid
opening; _C_, curled portion of thyroid.]
Moreover, another most instructive piece of evidence pointing in the same
direction is afforded by the behaviour of the ventral epithelial {202}pits,
as determined by Miss Alcock. Although there is no indication on the
ventral surface of the skin of any difference between the anterior and
posterior portions of the respiratory region, yet when the ventral rows of
the epithelial pits supplied by each branchial nerve are mapped out, we see
how the most anterior ones diverge more and more from the mid-ventral line,
following out exactly the limits of the underlying muco-cartilaginous
thyroid plate (Fig. 84).
The whole evidence strongly leads to the conclusion that the thyroid
portion of the facial segment was inserted as a median tongue between the
foremost branchial segments on each side, and that, therefore, the whole
facial segment, consisting as it does of a thyroid part and a hyoid or
branchial part, may be represented as in Fig. 85, which is obtained by
splitting an Ammocoetes longitudinally along the mid-dorsal line, so as to
open out the pharyngeal chamber and expose the whole internal surface. The
facial segment is marked out by shading lines, the glosso-pharyngeal and
vagus segments and the last of the trigeminal segments being indicated
faintly. The position of the thyroid gland is indicated by oblique lines, C
being the curled portion.
THE UTERUS OF THE SCORPION GROUP.
Seeing how striking is the arrangement and the structure of the glandular
tissue of this thyroid, how large the organ is and how absolutely it is
confined to Ammocoetes, disappearing entirely as such at transformation, we
may feel perfectly certain that a corresponding, probably very similar,
organ existed in the invertebrate ancestor of the vertebrate; for the
transformation process consists essentially of the discarding of
invertebrate characteristics and the putting on of more vertebrate
characters; also, so elaborate an organ cannot possibly have been evolved
as a larval adaptation during the life of Ammocoetes. We may therefore
assert with considerable confidence that the thyroid gland was the
_palæo-hysteron_, and was derived from the uterus of the ancient
palæostracan forms. If, then, it be found that a glandular organ of this
very peculiar structure and arrangement is characteristic of the uterus of
any living member of the scorpion group, then the confidence of this
assertion is greatly increased.
In Limulus, as already stated, the genital ducts open separately {203}on
each side of the operculum, and do not combine to form a uterus; I have
examined them and was unable to find any glandular structure at all
resembling that of the thyroid gland of Ammocoetes. I then turned my
attention to the organs of the scorpion, in which the two ducts have fused
to form a single uterus.
[Illustration: FIG. 86.--SECTION THROUGH THE TERMINAL CHAMBER OR UTERUS OF
THE MALE SCORPION.
_C_, cavity of chamber. A portion of the epithelial lining of the channels
of emission is drawn above the section of the uterus.]
{204}[Illustration: FIG. 87.--LONGITUDINAL SECTION THROUGH THREE OF THE
CONES OF THE UTERINE GLANDS OF THE SCORPION.]
[Illustration: FIG. 88.--SAGITTAL SECTION THROUGH THE UTERINE GLAND OF
SCORPION, SHOWING THE INTERNAL CHITINOUS SURFACE (_b_) AND THE GLANDULAR
CONES (_a_) CUT THROUGH AT VARIOUS DISTANCES FROM THE INTERNAL SURFACE.]
I there found that both in the male and in the female the genital ducts on
each side terminate in a common chamber or uterus, which underlies the
whole length of the operculum, and opens to the exterior in the middle
line, as shown in Fig. 76. In transverse section, this uterus has the
appearance shown in Fig. 86, _i.e._ it is a large tube, evidently
expansible, lined with a chitinous layer and epithelial cells belonging to
the chitinogenous layer, except in two symmetrical places, where the
uniformity of the uterine wall is interrupted by two large, remarkable
glandular structures. The structure of these glands is better shown by
means of sagittal sections. They are composed of very long, wedge-shaped
cells, each of which possesses a large, round nucleus at the basal end of
the cell (Fig. 87). These cells are arranged in bundles of about eight to
ten, which are separated from each other by connective tissue, the apex of
each conical bundle being directed into the cavity of the uterus; where
this brush-like termination of the cells reaches the surface, the chitinous
layer is absent, so that this layer is, on surface view, seen (Fig. 88
(_b_)) to be pitted with round holes over that part of the internal surface
of the uterus where these glands are situated. Each of these holes
represents the termination of one of these cone-shaped wedges of cells. If
the section is cut across at right angles to the axis of these cones, then
its appearance is represented in Fig. 88 (_a_), and shows well the
arrangement of the blocks of cells, separated from each other by connective
tissue. When the section passes through the basal part of the cones, and
only in that case, then the nuclei of the cells appear, often in
considerable numbers in one section, as {205}is seen in Fig. 89. In Fig. 88
the section shows at _b_ the holes in the chitin in which the cones
terminate, and then a series of layers of sections through the cones
further and further away from their apices.
These conical groups of long cells, represented in Fig. 87, form on each
side of the uterus a gland, which is continuous along its whole length, and
thus forms a line of secreting surface on each side, just as in the
corresponding arrangement of the glandular structures in the thyroid of
Ammocoetes. This uterus and glandular arrangement is found in both sexes;
the gland is, however, more developed in the male than in the female
scorpion.
[Illustration: FIG. 89.--TRANSVERSE SECTION THROUGH THE BASAL PART OF THE
UTERINE GLANDS OF THE SCORPION.]
The resemblance between the structure of the thyroid of Ammocoetes and the
uterus of the scorpion is most striking, except in two respects, viz. the
nature of the lining of the non-glandular part of the cavity--in the one
case ciliated, in the other chitinous--and the place of exit of the cavity,
the thyroid of Ammocoetes opening into the respiratory chamber, while the
uterus of Scorpio opens direct to the exterior.
[Illustration: FIG. 90.--SECTION OF CENTRAL CHAMBER OF THYROID OF
AMMOCOETES AND SECTION OF UTERUS OF SCORPION.]
With respect to the first difference, the same difficulty is met {206}with
in the comparison of the ciliated lining of the tube in the central nervous
system of vertebrates with the chitinous lining of the intestine in the
arthropod. Such a difference does not seem to me either unlikely or
unreasonable, seeing that cilia are found instead of chitin in the
intestine of the primitive arthropod Peripatus. Also the worm-like
ancestors of the arthropods almost certainly possessed a ciliated
intestine. Finally, the researches of Hardy and McDougall on the intestine
of Daphnia point directly to the presence of a ciliated rather than a
chitinous epithelial lining of the intestine in this animal--all evidence
pointing to the probability that in the ancient arthropod forms, derived as
they were from the annelids, the intestine was originally ciliated and not
chitinous. It is from such forms that I suppose vertebrates to have sprung,
and not from forms like the living king-crabs, scorpions, Apus, Branchipus,
etc. I only use them as illustrations, because they are the only living
representatives of the great archaic group, from which the Crustacea,
Arachnida, and Vertebrata all took origin.
The second difference is more important, and is at first sight fatal to any
comparison between the two organs. How is it possible to compare the uterus
of the scorpion, which opens on the surface by an _external_ genital
opening, with the thyroid of Ammocoetes, which opens by an _internal_
opening into the respiratory chamber? However close may be the histological
resemblance of structure in the two cases, surely such a difference is too
great to be accounted for.
It is, however, to be remembered that the operculum of Scorpio covers only
the terminal genital apparatus, and does not, therefore, resemble the
operculum of the presumed ancestor of Ammocoetes, which, as already argued,
must have resembled the operculum of Thelyphonus with its conjoint
branchial and genital apparatus, rather than that of Scorpio. Before,
therefore, making too sure of the insuperable character of this difficulty,
we must examine the uterus of the Pedipalpi, and see the nature of its
opening.
The nature of the terminal genital organs in Thelyphonus has been described
to some extent by Blanchard, and more recently by Tarnani. The ducts of the
generative organs terminate, according to the latter observer, in the large
uterus, which is found both in the male and female; he describes the walls
of the uterus in the female as formed of elongated glandular epithelium,
with a strongly-developed porous, chitinized intima. In the male, he says
that the {207}epithelium of the uterus masculinus and its processes is
extraordinarily elongated, the chitin covering being thick. In these
animals, then, the common chamber or uterus into which the genital ducts
empty, which, like the corresponding chamber in the scorpion, occupies the
middle region of the operculum, is a large and conspicuous organ. Further,
and this is a most striking fact, the _uterus masculinus_ does not open
direct to the exterior, but into the genital cavity, "which lies above the
uterus, so that the latter is situated between the lower wall of the
genital cavity and the outer integument." The opening, therefore, of the
uterus is not external but _internal_, into the large internal space known
as the genital cavity. The arrangement is shown in Fig. 91, taken from
Tarnani's paper, which represents a diagrammatic sagittal section through
the exit of the male genital duct. Yet another most striking fact is
described by Tarnani. This genital cavity is continuous with the pulmonary
or gill cavities on each side, so that instead of a single opening for the
genital products and one on each side for each gill-pouch, as would be the
case if the arrangement was of the same kind as in the scorpion, there is a
single large chamber, the genital chamber, common to both respiratory and
genital organs.
[Illustration: FIG. 91.--SAGITTAL MEDIAN DIAGRAMMATIC SECTION THROUGH THE
OPERCULUM OF THE MALE THELYPHONUS. (From TARNANI.)
The thick line is the operculum, composed of two segments, _I._ and _II._
_Ut. Masc._, uterus masculinus; _Gen. Ch._, genital chamber; _Int. Op._,
internal opening; _Ext. Op._, external opening common to the genital and
respiratory organs.]
This genital chamber, according to Tarnani, opens to the exterior by a
single median opening between the operculum and the succeeding segment;
similarly, a communication from side to side exists between the second pair
of gill-pouches. I have been able to examine _Hypoctonus formosus_ and
_Thelyphonus caudatus_, and in both cases, in both male and female, the
opening to the exterior of the common chamber for respiration and for the
genital products was {208}not a single opening, as described by Tarnani in
_Thelyphonus asperatus_, but on each side of the middle line, a round
orifice closed by a lid, like the nest of the trapdoor spider, led into the
common genital chamber (_Gen. Ch._) into which both uterus and gills
opened. In Fig. 77 I have endeavoured to represent the arrangement of the
genital and respiratory organs in the male Thelyphonus according to
Tarnani's and my own observations.
If we may take Thelyphonus as a sample of the arrangement in those
scorpions in which the operculum was fused with the first branchial
appendage, among which must be included the old sea-scorpions, then it is
most significant that their uterus should open internally into a cavity
which was continuous with the respiratory cavity. Thus not only the
structure of the gland, but also the arrangement of the internal opening
into the respiratory, or, as it became later, the pharyngeal cavity, is in
accordance with the suggestion that the thyroid of Ammocoetes represents
the uterus of the extinct Eurypterus-like ancestor.
Into this uterus the products of the generative organs were poured by means
of the _vasa deferentia_, so that there was not a single median opening or
duct in connection with it, but also two side openings, the terminations of
the _vasa deferentia_. These are described by Tarnani in Thelyphonus as
opening into the two horns of the uterus, which thus shows its bilateral
character, although the body of the organ is median and single; these ducts
then pass within the body of the animal, dorsal to the uterus, towards the
testes or ovaries as the case may be, organs which are situated in these
animals, as in other scorpions, in the abdomen, so that the direction of
the ducts from the generative glands to the uterus is headwards. If,
however, we examine the condition of affairs in Limulus, we find that the
main mass of the generative material is cephalic, forming with the liver
that dense glandular mass which is packed round the supra-oesophageal and
prosomatic ganglia, and round the stomach and muscles of the head-region.
From this cephalic region the duct passes out on each side at the junction
of the prosomatic and mesosomatic carapace to open separately on the
posterior surface of the operculum, near the middle line, as is indicated
in Fig. 75.
We have, therefore, two distinct possible positions for the genital ducts
among the group of extinct scorpion-like animals, the one from the cephalic
region to the operculum, and the other from the abdominal region to the
operculum.
{209}THE GENERATIVE GLANDS OF LIMULUS AND ITS ALLIES.
The whole argument, so far, has in every case ended with the conclusion
that the original scorpion-like form with which I have been comparing
Ammocoetes resembled in many respects Limulus rather than the present-day
scorpions, and therefore in the case also of the generative organs, with
which the thyroid gland or palæo-hysteron was in connection, it is more
probable that they were cephalic in position rather than abdominal. If this
were so, then the duct on each side, starting from the median ventral
uterus, would take a lateral and dorsal course to reach the huge mass of
generative gland lying within the prosomatic carapace, just as I have
represented in the figure of Eurypterus (Fig. 79), a course which would
take much the same direction as the ciliated groove in Ammocoetes.
We ought, therefore, on this supposition, to expect to find the remains of
the invertebrate generative tissue, the ducts of which terminated in the
thyroid, in the head-region, and not in the abdomen.
Upon removal of the prosomatic carapace of Limulus, a large brownish
glandular-looking mass is seen, in which, if it happens to be a female,
masses of ova are very conspicuous. This mass is composed of two separate
glands, the generative glands and the hepatico-pancreatic glands--the
so-called liver--and surrounds closely the central nervous system and the
alimentary canal. From the generative glands proceed the genital ducts to
terminate on the posterior surface of the operculum. From the liver ducts
pass to the pyloric end of the cephalic stomach, and carry the fluid by
means of which the food is digested, for, in all these animals, the active
digesting juices are formed in the so-called liver, and not in the cells of
the stomach or intestine.
It is a very striking fact that the brain of Ammocoetes is much too small
for the brain-case, and that the space between brain and brain-case is
filled up with a very peculiar glandular-looking tissue, which is found in
Ammocoetes and not elsewhere. Further, it is also striking that in the
brain of Ammocoetes there should still exist the remains of a tube
extending from the IVth ventricle to the surface at the _conus
post-commissuralis_, which can actually be traced right into this tissue on
the outside of the brain (see Fig. 13, _a-e_, Pl. XXVI., in my paper in the
_Quarterly Journal of Microscopical Science_). {210}This, in my opinion, is
the last remnant of one of the old liver-ducts which extended from the
original stomach and intestine into the cephalic liver-mass. This
glandular-looking material is shown surrounding the pineal eye and its
nerve, in Fig. 31, also in Fig. 22, and separately in Fig. 92. It is
composed of large cells, with a badly staining nucleus, closely packed
together with lines of pigment here and there between the cells; this
pigment is especially congregated at the spot where the so-called
liver-duct loses itself in this tissue. The protoplasm in these large cells
does not stain well, and with osmic acid gives no sign of fat, so that
Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue
is not true; peculiar it certainly is, but fatty it is not.
[Illustration: FIG. 92.--DRAWING OF THE TISSUE WHICH SURROUNDS THE BRAIN OF
AMMOCOETES.]
This tissue has been largely described as a peculiar kind of connective
tissue, which is there as packing material, for the purpose of steadying a
brain too small for its case. On the face of it such an explanation is
unscientific; certainly for all those who really believe in evolution, it
is out of the question to suppose that a brain-case has been laid down in
the first instance too large for the brain, in order to provide room for a
subsequent increase of brain; just as it is out of the question to suppose
that the nervous system was laid down originally as an epithelial tube in
order to provide for the further development of the nervous system by the
conversion of more and more of that tube into nervous matter. Yet this
latter proposition has been seriously put forward by professed believers in
evolution and in natural selection.
This tissue bears no resemblance whatever to any form of connective tissue,
either fatty or otherwise. By every test this tissue tells as plainly as
possible that it is a vestige of some former organ, presumably glandular,
which existed in that position; that it is not there as packing material
because the brain happened to be too small for its case, but that, on the
contrary, the brain is too small for its case, because the case, when it
was formed, included this organ as well as the brain; in other words, this
tissue {211}is there because it is the remnant of the great glandular mass
which so closely surrounds the brain and alimentary canal in animals such
as Limulus. In my paper in the _Quarterly Journal of Microscopical
Science_, in which I was comparing the tube of the vertebrate nervous
system with the alimentary canal of the invertebrate, I spoke of this
tissue as being the remnant of the invertebrate liver. At the same time the
whole point of my argument was that the glandular material surrounding the
brain of Limulus was made up of two glands--liver and generative gland--so
that this tissue might be the remnant of either one or the other, or both.
All I desired, at that time, was to point out the glandular appearance of
this so-called packing tissue, which surrounded the brain-region of
Ammocoetes, in connection with the fact that the brain and alimentary canal
of Limulus were closely surrounded with a glandular mass composed partly of
liver, partly of the generative gland. At present, I think these large
cells found round the brain in Ammocoetes are much more likely to be the
remnant of the generative gland than of the liver; the size of the cells
and their arrangement recalls Owen's picture of the generative gland in
Limulus, and seeing how important all generative glands are in their
capacity of internal secreting glands, apart entirely from the extrusion of
the ripe generative products, and how unimportant is an hepato-pancreas
when the alimentary canal is closed, it is much more likely that of the two
glands the former would persist longer than the latter. It may be that all
that is left of the old hepato-pancreas consists of the pigment so markedly
found in between these cells, especially at the place where the old
liver-duct reaches the surface of the brain; just as the only remnant of
the two pineal eyes in the higher vertebrates is the remains of the
pigment, known as brain-sand, which still exists in the pineal gland of
even the highest vertebrate. This, however, is a mere speculation of no
importance. What is important is the recognition of this tissue round the
brain as the remnant of the glandular mass round the brain of animals such
as Limulus. Still further confirmation of the truth of this comparison will
be given when the origin of the auditory organ comes up for discussion.
I conclude, therefore, from the evidence of Ammocoetes, that the generative
glands in the ancestral form were situated largely in the cephalic region,
and suggest that the course and direction of the ciliated pseudo-branchial
grooves on each side indicate the direction of the {212}original opercular
ducts by which the generative products were conveyed to the uterine
chamber, i.e. to the chamber of the thyroid gland, and thence to the common
genital and respiratory cavity, and so to the exterior.
It is easy to picture the sequence of events. First, the generative glands,
chiefly confined to the cephalic region, communicating with the exterior by
separate ducts on the inner surface of the operculum as in Limulus. Then,
in connection with the viviparous habit, these two oviducts fused together
to form a single chamber, covered by the operculum, which opened out to the
exterior by a single opening as in Scorpio: or, in forms such as
Eurypterus, in which the operculum had amalgamated with the first branchial
appendage and possessed a long, tongue-like ventral projection, the
amalgamated ducts formed a long uterine chamber which opened internally
into the genital chamber--a chamber which, as in Thelyphonus, was common
with that of the two gill-chambers, while at the same time the genital
ducts from the cephalic generative material opened into two uterine horns
which arose from the anterior part of the uterus, as in Thelyphonus.
Such an arrangement would lead directly to the condition found in
Ammocoetes, if the generative material around the brain lost its function,
owing to a new exit for generative products being formed in the posterior
part of the body. The connection of the genital duct with this cephalic
gland being then closed and cut off by the brain-case, the position of the
oviducts would still be shown by the ciliated grooves opening into the
folded-down thyroid tube, _i.e._ the folded-down horns of the uterus; the
uterus itself would remain as the main body of the thyroid and still open
by a conspicuous orifice into the common respiratory chamber. Next, in the
degeneration process, we may suppose that not only the oviducts opened out
to form the ciliated groove, but that the uterine chamber itself also
opened out, and thus formed the endostyle of Amphioxus and of the Tunicata.
It might seem at first sight improbable that a closed tube should become an
open groove, although the reverse phenomenon is common enough; the
difficulty, however, is clearly not considered great, for it is precisely
what Dohrn imagines to have taken place in the conversion of the thyroid of
Ammocoetes into the endostyle of Amphioxus and the Tunicata; it is only
carrying on the same idea a stage further to see in the open, ciliated
groove of Ammocoetes the remains of the closed genital duct of Limulus and
its allies.
{213}Such is the conclusion to which the study of the thyroid gland in
Ammocoetes seems to me to lead, and one cannot help wondering why such an
unused and rudimentary organ should have remained after its original
function had gone. Is it possible to find out its function in Ammocoetes?
THE FUNCTION OF THE THYROID GLAND IN AMMOCOETES.
The thyroid gland has been supposed to secrete mucus into the respiratory
chamber for the purpose of entangling the particles of food, and so aiding
in digestion. I see no sign of any such function; neither by the thionin
method, nor by any other test, have Miss Alcock and myself ever been able
to see any trace of mucous secretion in the thyroid, and, indeed, the
thyroid duct is always remarkably free from any sign of any secretion
whatever. Not only is there no evidence of any mucous secretion in the
thyroid of the fully developed Ammocoetes, but also no necessity for such
secretion from Dohrn's point of view, for so copious a supply of mucus is
poured out by the glands of the branchiæ, along the whole pharyngeal tract,
especially from the cells of the foremost or hyoid gills, as to mix up with
the food as thoroughly as can possibly be needed. Further, too, the
ciliated pharyngeal bands described by Schneider are amply sufficient to
move this mixed mass along in the way required by Dohrn. Finally, the
evidence given by Miss Alcock is absolutely against the view that the
thyroid takes any part in the process of digestion, while, on the other
hand, her evidence directly favours the view that these glandular
_branchial_ mucus-secreting cells play a most important part in the
digestive process.
In Fig. 93, A is a representation of the respiratory tissue of a normal
gill; B is the corresponding portion of the first or hyoid gill, in which,
as is seen, the whole of the respiratory epithelium is converted into
gland-tissue of the nature of mucous cells.
To sum up, the evidence is clear and conclusive that the Ammocoetes
possesses in its pharyngeal chamber mucus-secreting glands, which take an
active part in the digestive process, which do not in the least resemble
either in structure or arrangement the remarkable cells of the thyroid
gland, and that the experimental evidence that the latter cells either
secrete mucus or take any part in digestion is so far absolutely negative.
It is, of course, possible, that they {214}may contain mucin in the younger
developmental stages, and therefore possible that they might at that stage
secrete it; they certainly, however, show no sign of doing so in their more
adult condition, and cannot be compared in the very faintest degree to the
glandular cells of the pharyngeal region. It is also perfectly possible for
gland-cells belonging to a retrograde organ to become mucus-secreting, and
so to give rise to the cells of Amphioxus and the Tunicata.
[Illustration: Fig. 93.--A, PORTION OF A GILL OF AMMOCOETES WITH ORDINARY
RESPIRATORY EPITHELIUM; B, CORRESPONDING PORTION OF THE FIRST OR HYOID
GILL.]
If, then, these cells were not retained for digestive purposes, what was
their function? To answer this question we must first know the function of
the corresponding gland-cells in the uterus of the scorpion, which
undoubtedly secreted into the cavity of the uterus and took some part in
connection with the generative act, and certainly not with digestion. What
the function of these cells is or in what way they act I am unable at
present to say. I can only suppose that the reason why the thyroid gland
has persisted throughout the vertebrate kingdom, after the generative
tissues had found a new outlet for their products in the body-cavity of the
posterior region, is because it possessed some important function in
addition to that connected with the exit of the products of the generative
organs; a function which was essential to the well-being, or even to the
life of the animal. We do not know its function in the scorpion, or the
nature of its secretion in that animal. We know only that physiology at the
present day has demonstrated clearly that the actual external secretion of
a gland may be by no means its most important function; in addition, glands
possess what is called an internal secretion, viz. a {215}secretion into
the blood and lymph, and this latter secretion may be of the most vital
importance. Now, the striking fact forces itself prominently forward, that
the thyroid gland of the higher vertebrates is the most conspicuous example
of the importance of such internal secretion. Here, although ductless, we
have a gland which cannot be removed without fatal consequences. Here, in
the importance of its internal secretion, we have a reason for the
continued existence of this organ; an organ which remains much the same
throughout the Vertebrata down to and including Petromyzon, but, as is seen
at transformation, is all that remains of the more elaborate, more
extensive organ of Ammocoetes. Surely we may argue that it is this second
function which has led to the persistence of the thyroid, and that its
original form, without its original function, is seen in Ammocoetes,
because that is a larval form, and not a fully-developed animal. As soon as
the generative organs of Petromyzon are developed at transformation, all
trace of its connection with a genital duct vanishes, and presumably its
internal secretory function alone remains.
Yet, strange to say, a mysterious connection continues to exist between the
thyroid gland and the generative organs, even up to the highest vertebrate.
That the thyroid gland, situated as it is in the neck, should have any
sympathy with sexual functions if it was originally a gland concerned with
digestion is, to say the least of it, extremely unlikely, but, on the
contrary, likely enough if it originated from a glandular organ in
connection with the sexual organs of the palæostracan ancestor of the
vertebrate.
Freund has shown, and shown conclusively, that there is an intimate
connection between the condition of the thyroid gland and the state of the
sexual organs, not only in human beings, but also in numerous animals, such
as dogs, sheep, goats, pigs, and deer. He points out that the swelling of
the gland, which occurs in consequence of sexual excitement (a fact
mentioned both in folk-lore tales and in poetical literature), and also the
swelling at the time of puberty, may both lead to a true goitrous
enlargement; that most of the permanent goitres commence during a menstrual
period; that during pregnancy swelling of the thyroid is almost universal,
and may become so extreme as to threaten suffocation, or even cause death;
that the period of puberty and the climacteric period are the two maximal
periods for the onset of goitre, and that exophthalmic goitre especially is
associated with a special disease connected with the uterus.
{216}SUMMARY.
Step by step in the preceding chapters the evidence is accumulating in
favour of the origin of vertebrates from a member of the palæostracan
group. In a continuously complete and harmonious manner the evidence has
throughout been most convincing when the vertebrate chosen for the
purpose of my arguments has been Ammocoetes.
So many fixed points have been firmly established as to enable us to
proceed further with very great confidence, in the full expectation of
being able ultimately to homologize the Vertebrata with the Palæostraca
even to minute details.
Perhaps the most striking and unexpected result of such a comparison is
the discovery that the thyroid gland is derived from the uterus of the
palæostracan ancestor. Yet so clear is the evidence that it is difficult
to see how the homology can be denied.
In the one animal (Palæostraca) the foremost pair of mesosomatic
appendages forms the operculum, which always bears the terminal
generative organs and is fused in the middle line. In many forms,
essentially in Eurypterus and the ancient sea-scorpions, the operculum
was composed of two segments fused together: an anterior one which
carried the uterus, and a posterior one which carried the first pair of
branchiæ.
In the other animal (Ammocoetes) the foremost segments of the mesosomatic
or respiratory region, immediately in front of the glossopharyngeal
segments, are supplied by the facial nerve, and are markedly different
from those supplied by the vagus and glossopharyngeal, for the facial
supplies two segments fused together; the anterior one, the thyroid
segment, carrying the thyroid gland, the posterior one, the hyoid
segment, carrying the first pair of branchiæ.
Just as in Eurypterus the fused segment, carrying the uterus on its
internal surface, forms a long median tongue which separates the most
anterior branchial segments on each side, so also the fused segment
carrying the thyroid forms in Ammocoetes a long median tongue, which
separates the most anterior branchial segments on each side.
Finally, and this is the most conclusive evidence of all, this thyroid
gland of Ammocoetes is totally unlike that of any of the higher
vertebrates, and, indeed, of the adult form Petromyzon itself, but it
forms an elaborate complicated organ, which is directly comparable with
the uterus and genital ducts of animals such as scorpions. Not only is
such a comparison valid with respect to its shape, but also with respect
to its structure, which is absolutely unique among vertebrates, and very
different to that of any other vertebrate gland, but resembles in a
striking manner a glandular structure found in the uterus, both of male
and female scorpions.
The generative glands in Limulus, together with the liver-glands, form a
large glandular mass, situated in the head-region closely surrounding the
central nervous system, so that the genital ducts pass from the
head-region tailwards to the operculum. In the scorpion they lie in the
abdominal region, so that their ducts pass headwards to the operculum.
Probably in the Palæostraca the generative mass was situated in the
cephalic region as in Limulus, and it is probable that the remnant of it
still exists in {217}Ammocoetes in the shape of the peculiar large cells
packed together, with pigment masses in between them, which form such a
characteristic feature of the glandular-looking material, which fills up
the space between the cranial walls and the central nervous system.
Finally, the relationship which has been known from time immemorial to
exist between the sexual organs and the thyroid in man and other animals,
and has hitherto been a mystery without any explanation, may possibly be
the last reminiscence of a time when the thyroid glands were the uterine
glands of the palæostracan ancestor.
The consideration of the facial nerve, and the segments it supplies,
still further points to the origin of the Vertebrata from the
Palæostraca.
{218}CHAPTER VI
_THE EVIDENCE OF THE OLFACTORY APPARATUS_
Fishes divided into Amphirhinæ and Monorhinæ.--Nasal tube of the
lamprey.--Its termination at the infundibulum.--The olfactory organs of
the scorpion group.--The camerostome.--Its formation as a tube.--Its
derivation from a pair of antennæ.--Its termination at the true
mouth.--Comparison with the olfactory tube of Ammocoetes.--Origin of the
nasal tube of Ammocoetes from the tube of the hypophysis.--Direct
comparison of the hypophysial tube with the olfactory tube of the
scorpion group--Summary.
In the last chapter I finished the evidence given by the consideration of
the mesosomatic or opisthotic nerves, and the segments they supplied. The
evidence is strongly in accordance with that of previous chapters, and not
only confirms the conclusion that vertebrates arose from some member of the
Palæostraca, but helps still further to delimit the nature of that member.
It is almost startling to see how the hypothesis put forward in the second
chapter, suggested by the consideration of the nature of the vertebrate
central nervous system and of the geological record, has received stronger
and stronger confirmation from the consideration of the vertebrate optic
apparatus, the vertebrate skeleton, the respiratory apparatus, and,
finally, the thyroid gland. All fit naturally into a harmonious whole, and
give a feeling of confidence that a similar harmony will be found upon
consideration of the rest of the vertebrate organs.
Following naturally upon the segments supplied by the opisthotic
(mesosomatic) cranial nerves, we ought to consider now the segments
supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments
belonging to the trigeminal nerve-group in the vertebrate, and in the
invertebrate the segments of the prosoma with their characteristic
appendages. There are, however, in all vertebrates in this foremost cranial
region, in addition to the optic nerves, two other well-marked nerves of
special sense, the olfactory and the auditory. Of these, the former are in
the same class as the optic nerves, for they arise {219}in the vertebrate
from the supra-infundibular nerve-mass, and in the invertebrate from the
supra-oesophageal ganglia. The latter arise in the vertebrate from the
infra-infundibular nerve-mass, and, as the name implies, are situated in
the region where the pro-otic nerves are contiguous to the opisthotic,
_i.e._ at the junction of the prosomatic and mesosomatic nerve-regions.
The chapter dealing with the evidence given by the olfactory nerves and the
olfactory apparatus ought logically to have followed immediately upon the
one dealing with the optic apparatus, seeing that both these special
sense-nerves belong to the supra-infundibular segments in the vertebrate
and to the supra-oesophageal in the invertebrate.
I did not deal with them in that logical sequence because it was necessary
for their understanding to introduce first the conception of modified
appendages as important factors in any consideration of vertebrate
segments; a conception which followed naturally after the evidence afforded
by the skeleton in Chapter III., and by the branchial segments in Chapter
IV. So, too, now, although the discussion of the prosomatic segmentation
ought logically to follow immediately on that of the mesosomatic
segmentation, I have determined to devote this chapter to the evidence of
the olfactory organs, because the arguments as to the segments belonging to
the trigeminal nerve-group are so much easier to understand if the position
of the olfactory apparatus is first made clear.
In all vertebrates the nose is double and opens into the pharynx, until we
descend to the fishes, where the whole group Pisces has been divided into
two subsidiary groups, Monorhinæ and Amphirhinæ, according as they possess
a median unpaired olfactory opening, or a paired opening. The Monorhinæ
include only the Cyclostomata--the lampreys and hag-fishes.
In the lampreys the single olfactory tube ends blindly, while in the
hag-fishes it opens into the pharynx. In the lamprey, both in Petromyzon
and Ammocoetes, the opening of this nasal tube is a conspicuous object on
the dorsal surface of the head in front of the transparent spot which
indicates the position of the right median eye. It is especially
significant, as showing the primitive nature of this median olfactory
passage, that a perfectly similar opening in the {220}same position is
always found in the dorsal head-shields of all the Cephalaspidæ and
Tremataspidæ, as will be explained more fully in Chapter X.
All the evidence points to the conclusion that the olfactory apparatus of
the vertebrate originated as a single median tube, containing the special
olfactory sense-epithelium, which, although median and single, was
innervated by the olfactory nerve of each side. The external opening of
this tube in the lamprey is dorsal. How does it terminate ventrally?
The ventral termination of this tube is most instructive and suggestive. It
terminates blindly at the very spot where the infundibular tube terminates
blindly and the notochord ends. After transformation, when the Ammocoete
becomes the Petromyzon, the tube still ends blindly, and does not open into
the pharynx as in Myxine; it, however, no longer terminates at the
infundibulum, but extends beyond it towards the pharynx.
This position of the nasal tube suggests that it may originally have opened
into the tube of the central nervous system by way of the infundibular
tube. This suggestion is greatly enhanced in value by the fact that in the
larval Amphioxus the tube of the central nervous system is open to the
exterior, its opening being known as the anterior neuropore, and this
anterior neuropore is situated at the base of a pit, known as the olfactory
pit because it is supposed to represent the olfactory organ of other
fishes.
Following the same lines of argument as in previous chapters, this
suggestion indicates that the special olfactory organs of the invertebrate
ancestor of the vertebrates consisted of a single median olfactory tube or
passage, which led directly into the oesophagus and was innervated, though
single and median, by a pair of olfactory nerves which arose from the
supra-oesophageal ganglia. Let us see what is the nature of the olfactory
organs among arthropods, and whether such a suggestion possesses any
probability.
THE OLFACTORY ORGANS OF THE SCORPION GROUP.
At first sight the answer appears to be distinctly adverse, for it is well
known that in all the Insecta, Crustacea, and the large majority of
Arthropoda, the first pair of antennæ, often called the antennules, are
olfactory in function, and these are free-moving, bilaterally
{221}situated, independent appendages. Still, even here there is the
striking fact that the nerves of these olfactory organs always arise from
the supra-oesophageal ganglia, although those to the second pair of antennæ
arise from the infra-oesophageal ganglia, just as the olfactory nerves of
the vertebrate arise from the supra-infundibular brain-mass. Not only is
there this similarity of position, but also a similarity of structure in
the olfactive lobes of the brain itself of so striking a character as to
cause Bellonci to sum up his investigations as follows:--
"The structure and connections of the olfactive lobes present the same
fundamental plan in the higher arthropods and in the vertebrates. In the
one, as in the other, the olfactory fibres form, with the connecting fibres
of the olfactory lobes, a fine meshwork, which, consisting as it does of
separate groups, may each one be called an olfactory glomerulus."
He attributes this remarkable resemblance to a physiological necessity that
similarity of function necessitates similarity of structure, for he
considers it out of the question to suppose any near relationship between
arthropods and vertebrates.
Truly an interesting remark, with the one fallacy that relationship is out
of the question.
The evidence so far has consistently pointed to some member of the
palæostracan group as the ancestor of the vertebrates--a group which had
affinities both to the crustaceans and the arachnids; indeed, many of its
members resembled scorpions much more than they resemble crustaceans. The
olfactory organs of the scorpions and their allies are, therefore, more
likely than any others to give a clue to the position of the desired
olfactory organs. In these animals and their allies paired olfactory
antennæ are not present, either in the living land-forms or the extinct
sea-scorpions, for all the antennæ-like, frequently chelate, appendages
seen in Pterygotus, etc. (Fig. 8), represent the cheliceræ, and correspond,
therefore, to the second pair of antennæ in the crustaceans.
What, then, represents the olfactory antennæ in the scorpions? The answer
to this question has been given by Croneberg, and very striking it is. The
two olfactory antennæ of the crustacean have combined together to form a
hollow tube at the base of which the mouth of the animal is situated, so
that the food passes along this olfactory passage before it reaches the
mouth. This organ is often called after Latreille, the camerostome,
sometimes the rostrum; it is naturally median in position and appears,
therefore, to be an unpaired organ; its paired {222}character is, of
course, evident enough, for it is innervated by a pair of nerves, and these
nerves, as ought to be the case, arise from the supra-oesophageal ganglia.
In Galeodes it is a conspicuously paired antennæ-like organ (Fig. 94).
Croneberg has also shown that this rostrum, or camerostome, arises
embryologically as a pair of appendages similar to the other appendages.
This last observation of Croneberg has been confirmed by Brauer in 1894,
who describes the origin of the upper lip, as he calls it, in very similar
terms, without, however, referring to Croneberg's paper. Croneberg further
shows that this foremost pair of antennæ not only forms the so-called upper
lip or camerostome, but also a lower lip, for from the basal part of the
camerostome there projects on each side of the pharynx a dependent
accessory portion, which in some cases fuses in the middle line, and forms,
as it were, a lower lip. The entosclerite belonging to this dependent
portion is apparently the post-oral entosclerite of Lankester and Miss
Beck.
[Illustration: FIG. 94.--DORSAL VIEW OF BRAIN AND CAMEROSTOME OF GALEODES.
_cam._, camerostome; _pr. ent._, pre-oral entosclerite; _l.l._, dependent
portion of camerostome; _ph._, pharynx; _al._, alimentary canal; _n. op._,
median optic nerves; _pl._, plastron; _v.c._, ventral nerve chain; 2, 3,
second and third appendages.]
At the base of the tubular passage formed by this modified first pair of
antennæ the true mouth is found opening directly into the dilated pharynx,
the muscles of which enable the act of suction to be carried out. The
narrow oesophagus leads out from the pharynx and is completely surrounded
by the supra- and infra-oesophageal nerve masses.
Huxley also describes the mouth of the scorpion in precisely the same
position (_cf. o_, Fig. 96).
{223}In order to convey to my readers the antennæ-like character of the
camerostome in Galeodes (Fig. 101), and its position, I give a figure (Fig.
94) of the organ from its dorsal aspect, after removal of the cheliceræ and
their muscles. A side view of the same organ is given in Fig. 95 to show
the feathered termination of the camerostome, and the position of the
dependent accessory portion (_l.l._) (Croneberg's 'untere Anhang') with its
single long antenna-like feather. In both figures the alimentary canal
(_al._) is seen issuing from the conjoined supra- and infra-oesophageal
mass.
As is seen in the figures, the bilateral character of the rostrum, as
Croneberg calls it, is apparent not only in its feathered extremity but
also in its chitinous covering, the softer median dorsal part (left white
in figure) being bounded by two lateral plates of hard chitin, which meet
in the middle line near the extremity of the organ. In all the members of
the scorpion group, as is clearly shown in Croneberg's figures, the rostrum
or camerostome is built up on the same plan as in Galeodes, though the
antenna-like character may not be so evident.
[Illustration: FIG. 95.--LATERAL VIEW OF BRAIN AND CAMEROSTOME OF GALEODES.
_gl. supr. oes._, supra-oesophageal ganglion; _gl. infr. oes._,
infra-oesophageal ganglion. The rest of the lettering same as in Fig. 94.]
When we consider that the first pair of antennæ in the crustaceans are
olfactory in function, Croneberg's observations amount to this--
In the arachnids and their allies the first pair of antennæ form a pre-oral
passage or tube, olfactory in function; the small mouth, which opens
directly into the pharynx, being situated at the end of this olfactory
passage.
{224}Croneberg's observations and conclusions are distinctly of very great
importance in bringing the arachnids into line with the crustaceans, and it
is therefore most surprising that they are absolutely ignored by Lankester
and Miss Beck in their paper published in 1883, in which Latreille only is
mentioned with respect to this organ, and his term "camerostome," or upper
lip, is used throughout, in accordance with the terminology in Lankester's
previous paper. That this organ is not only a movable lip or tongue, but
essentially a sense-organ, almost certainly of smell and taste, as follows
from Croneberg's conclusions, is shown by the series of sections which I
have made through a number of young Thelyphonus (Fig. 102).
[Illustration: FIG. 96.--MEDIAN SAGITTAL SECTION THROUGH A YOUNG
THELYPHONUS.]
I give in Fig. 96 a sagittal median section through the head-end of the
animal, which shows clearly the nature of Croneberg's conception. At the
front end of the body is seen the median eye (_ce._), _o_ is the mouth,
_Ph._ the pharynx, _oes._ the narrow oesophagus, compressed between the
supra-oesophageal (_supr. oes._) and infra-oesophageal (_infr. oes._) brain
mass, which opens into the large alimentary canal (_Al._); _Olf. pass._ is
the olfactory passage to the mouth, lined with thick-set, very fine hairs,
which spring from the hypostome (_Hyp._) as well as from the large
conspicuous camerostome (_Cam._), which limits this tube anteriorly. The
space between the camerostome and the median eye is filled up by the
massive cheliceræ, which are not shown in this section, as they begin to
appear in the {225}sections on each side of the median one. The muscles of
the pharynx and the muscles of the camerostome are attached to the pre-oral
entosclerite (_pr. ent._). The post-oral entosclerite is shown in section
as _post. ent._ The dorsal blood-vessel, or heart, is indicated at _H._
In Fig. 97 I give a transverse section through another specimen of the same
litter, to show the nature of this olfactory tube when cut across. Both
sections show most clearly that we are dealing here with an elaborate
sense-organ, the surface of which is partly covered with very fine long
hairs, partly, as is seen in the figure, is composed of long, separate,
closely-set sense-rods (_bat._), well protected by the long hairs which
project on every side in front of them, which recall to mind Bellonci's
figure of the 'batonnets olfactives' on the antennæ of Sphæroma. Finally,
we have the observation of Blanchard quoted by Huxley, to the effect that
this camerostome is innervated by nerves from the supra-oesophageal ganglia
which are clearly bilateral, seeing that they arise from the ganglion on
each side and then unite to pass into the camerostome; in other words,
paired olfactory nerves from the supra-oesophageal ganglia.
These facts demonstrate with wonderful clearness that in one group of the
Arthropoda the olfactory antennæ have been so modified as to form an
olfactory tube or passage, which leads directly into the mouth and so to
the oesophagus of the animal, and, strikingly enough, this group, the
Arachnida, is the very one to which the scorpions belong.
If for any cause the mouth _o_ in Fig. 96 were to be closed, then the
olfactory tube (_olf. pass._) might still remain, owing to its importance
as the organ of smell, and the olfactory tube would terminate blindly at
the very spot where the corresponding tube does terminate in the
vertebrate, according to the theory put forward in this book.
THE OLFACTORY TUBE OF AMMOCOETES.
In all cases where there is similarity of topographical position in the
organs of the vertebrate and arthropod we may expect also to find
similarity of structure. At first sight it would appear as though such
similarity fails us here, for a cross-section of the olfactory tube in
Petromyzon represents an elaborate organ such as is shown in Fig. 98, very
different in appearance to the section across the olfactory passage of a
young Thelyphonus given in Fig. 97.
{226}[Illustration: FIG. 97.--TRANSVERSE SECTION THROUGH THE OLFACTORY
PASSAGE OF A YOUNG THELYPHONUS.
1 and 2, sections of first and second appendages.]
[Illustration: FIG. 98.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE
OF PETROMYZON.
_cart._, nasal cartilage.]
{227}As is seen, it is difficult to see any connection between these folds
of olfactory epithelium and the simple tube of the scorpion. But in the
nose, as in all other parts of the head-region of the lamprey, remarkable
changes take place at transformation, and examination of the same tube in
Ammocoetes demonstrates that the elaborate structure of the adult olfactory
organ is actually derived from a much simpler form of organ, represented in
Fig. 99. Here, in Ammocoetes, the section is no longer strikingly different
from that of the Thelyphonus organ, but, instead, most strikingly similar
to it. Thus, again, it is shown that this larval form of the lamprey gives
more valuable information as to vertebrate ancestry than all the rest of
the vertebrates put together.
[Illustration: FIG. 99.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE
OF AMMOCOETES.
_cart._, nasal cartilage.]
Still, even now the similarity between the two organs is not complete, for
the tube in the lamprey opens on to the exterior on the dorsal surface of
the head, while in the scorpion tribe it is situated ventrally, being the
passage to the mouth and alimentary canal. In accordance with this there is
no sign of any opening on the dorsal carapace of any of the extinct
sea-scorpions or of the living land-scorpions, such as is so universally
found in the cephalaspids, tremataspids, and lampreys. Here is a
discrepancy of an apparently serious character, yet so wonderfully does the
development of the individual recapitulate the development of the race,
that this very discrepancy becomes converted into a triumphant vindication
of the {228}correctness of the theory advocated in this book, as soon as we
turn our attention to the development of this nasal tube in the lamprey.
We must always remember not only the great importance of a larval stage for
the unriddling of problems of ancestry, but also the great advantage of
being able to follow more favourably any clues as to past history afforded
by the development of the larva itself, owing to the greater slowness in
the development of the larva than of the embryo. Such a clue is especially
well marked in the course of development of Ammocoetes according to
Kupffer's researches, for he finds that when the young Ammocoetes is from 5
to 7 mm. in length, some time after it has left the egg, when it is living
a free larval life, a remarkable series of changes takes place with
considerable rapidity, so that we may regard the transformation which takes
place at this stage, as in some degree comparable with the great
transformation which occurs when the Ammocoetes becomes a Petromyzon.
All the evidence emphasizes the fact that the latter transformation
indicates the passage from a lower into a higher form of vertebrate, and is
to be interpreted phylogenetically as an indication of the passage from the
Cephalaspidian towards the Dipnoan style of fish. If, then, the former
transformation is of the same character, it would indicate the passage from
the Palæostracan to the Cephalaspid.
What is the nature of this transformation process as described by Kupffer?
It is characterized by two most important events. In the first place, up to
this time the oral chamber has been cut off from the respiratory chamber by
a septum--the velum--so that no food could pass from the mouth to the
alimentary canal. At this stage the septum is broken through, the oral
chamber communicates with the respiratory chamber, and the velar folds of
the more adult Ammocoetes are left as the remains of the original septum.
The other striking change is the growth of the upper lip, by which the
orifice of the nasal tube is transferred from a ventral to a dorsal
position. Fig. 100, taken from Kupffer's paper, represents a sagittal
section through an Ammocoetes 4 mm. long; _l.l._ is the lower lip, _u.l._
the upper lip, and, as is seen, the short oral chamber is closed by the
septum, _vel._ Opening ventrally is a tube called the tube of the
hypophysis, _Hy._, which extends close up to the termination of the
infundibulum. On the anterior surface of this tube is the projection called
by Kupffer the olfactory plakode. At this stage the upper lip grows with
great {229}rapidity and thickens considerably, thus forcing the opening of
the hypophysial tube more and more dorsalwards, until at last, in the
full-grown Ammocoetes, it becomes the dorsal opening of the nasal tube, as
already described. Here, then, in the hypophysial tube we have the original
position of the olfactory tube of the vertebrate ancestor, and it is
significant, as showing the importance of this organ, to find that such a
hypophysial tube is characteristic of the embryological development of
every vertebrate, whatever may be the ultimate form of the external nasal
orifices.
The single median position of the olfactory organ in the Cyclostomata, in
contradistinction to its paired character in the rest of the vertebrates,
has always been a stumbling-block in the way of those who desired to
consider the Cyclostomata as degenerated Selachians, for the origin of the
olfactory protuberance, as a single median plakode, seemed to indicate that
the nose arose as a single organ and not as a paired organ.
[Illustration: FIG. 100.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4
MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)
_A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._,
notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper
lip; _l.l._, lower lip; _vel._, septum between oral and respiratory
cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four
epibranchial ganglia.]
On the other hand, the two olfactory nerves of Ammocoetes compare
absolutely with the olfactory nerves of other vertebrates, and force one to
the conclusion that this median organ of Ammocoetes arose from a pair of
bilateral organs, which have fused in the middle line.
{230}[Illustration: FIG. 101.--_Galeodes._ (From the Royal Natural
History.)]
{231}The comparison of this olfactory organ with the camerostome gives a
satisfactory reason for its appearance in the lowest vertebrates as an
unpaired median organ; equally so, the history of the camerostome itself
supplies the reason why the olfactory nerves are double, why the organ is
in reality a paired organ and not a single median one. Thus, in a sense,
the grouping of the fishes into Monorhinæ and Amphirhinæ has not much
meaning, seeing that the olfactory organ is in all cases double.
[Illustration: FIG. 102.--_Thelyphonus._ (From the Royal Natural History.)]
The evidence of the olfactory organs in the vertebrate not only confirms,
in a most striking manner, the theory of the origin of the {232}vertebrate
from the Palæostracan, but points indubitably to an origin from a
scorpion-like rather than a crustacean-like stock. To complete the
evidence, it ought to be shown that the ancient sea-scorpions did possess
an olfactory passage similar to the modern land-scorpions. The evidence on
this question will come best in the next chapter, where I propose to deal
with the prosomatic appendages of the Palæostracan group.
SUMMARY.
The vertebrate olfactory apparatus commences as a single median tube
which terminates dorsally in the lamprey, and is supplied by the two
olfactory nerves which arise from the supra-infundibular portion of the
brain. It is a long, tapering tube which passes ventrally and terminates
blindly at the infundibulum in Ammocoetes. The dorsal position of the
nasal opening is not the original one, but is brought about by the growth
of the upper lip. The nasal tube originally opened ventrally, and was at
that period of development known as the tube of the hypophysis.