to most important conclusions as to the origin of the vertebrates, for it
shows clearly that whereas, as pointed out in this and subsequent chapters,
their ancestors possessed distinct arachnid characteristics, yet that they
cannot have been specialized arachnids, such as our present-day forms, but
rather they were of a primitive arachnid type, with distinct crustacean
characteristics: animals that were both crustacean and arachnid, but not
yet specialized in either direction: animals, in fact, of precisely the
kind which swarmed in the seas at the time when the vertebrates first made
their appearance. In the opinion of the present day, the ancestral forms of
the Crustacea, which were directly derived from the Annelida, may be
classed as an hypothetical group the Protostraca, the nearest approach to
which is a primitive Phyllopod.
"Starting from the Protostraca," say Korschelt and Heider, "according to
the present condition of our knowledge, we may, as has been already
remarked, assume three great series of development of the Arthropodan
stock, by the side of which a number of smaller independent branches have
been retained. One of these series leads through the hypothetical primitive
Phyllopod to the Crustacea; the second through the Palæostraca (Trilobita,
Gigantostraca, Xiphosura) to the Arachnida; the third through forms
resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and
the Tardigrada must probably be regarded as smaller independent branches of
the Arthropodan stock."
To these "three great series of development of the Arthropodan stock" the
evidence of Ammocoetes shows that a fourth must be added, which, starting
also from the Protostraca, and closely connected with the second,
palæostracan branch, leads through the Cephalaspidæ to the great kingdom of
the Vertebrata. Such a direct linking of the earliest vertebrates with the
Annelida through the Protostraca is of the utmost importance, as will be
shown later in the explanation of the origin of the vertebrate coelom and
urinary apparatus.
{117}SUMMARY.
The most important discovery of recent years which gives a direct clue to
the ancestry of the vertebrates is undoubtedly the discovery that the
pineal gland is all that remains of a pair of median eyes which must have
been functional in the immediate ancestor of the vertebrate, seeing how
perfect one of them still is in Ammocoetes. The vertebrate ancestor,
then, possessed two pairs of eyes, one pair situated laterally, the other
median. In striking confirmation of the origin of the vertebrate from
Palæostracans it is universally admitted that all the Eurypterids and
such-like forms resembled Limulus in the possession of a pair of median
eyes, as well as of a pair of lateral eyes. Moreover, the ancient mailed
fishes the Ostracodermata, which are the earliest fishes known, are all
said to show the presence of a pair of median eyes as well as of a pair
of lateral eyes. This evidence directly suggests that the structure of
both the median and lateral vertebrate eyes ought to be very similar to
that of the median and lateral arthropod eyes. Such is, indeed, found to
be the case.
The retina of the simplest form of eye is formed from a group of the
superficial epidermal cells, and the rods or rhabdites are formed from
the cuticular covering of these cells; the optic nerve passes from these
cells to the deeper-lying brain. This kind of retina may be called a
simple retina, and characterizes the eyes, both median and lateral, of
the scorpion group.
In other cases a portion of the optic ganglion remains at the surface,
when the brain sinks inwards, in close contiguity to the epidermal
sense-cells which form the retina; a tract of fibres connects this optic
ganglion with the underlying brain, and is known as the optic nerve. Such
a retina may be called a compound retina and characterizes the lateral
eyes of both crustaceans and vertebrates. Also, owing to the method of
formation of the retina by invagination, the cuticular surface of the
retinal sense-cells, from which the rods are formed, may be directed
towards the source of light or away from it. In the first case the retina
may be called upright, in the second inverted.
Such inverted retinas are found in the vertebrate lateral eyes and in the
lateral eyes of the arachnids, but not of the crustaceans.
The evidence shows that all the invertebrate median eyes possess a simple
upright retina, and in structure are remarkably like the right median or
pineal eye of Ammocoetes; while the lateral eyes possess, as in the
crustaceans, an upright compound retina, or, as in many of the arachnids,
a simple inverted retina. The lateral eyes of the vertebrates alone
possess a compound inverted retina.
This retina, however, is extraordinarily similar in its structure to the
compound crustacean retina, and these similarities are more accentuated
in the retina of the lateral eye of Petromyzon than that of the higher
vertebrates.
The evidence afforded by the lateral eye of the vertebrate points
unmistakably to the conclusion that the ancestor of the vertebrate
possessed both crustacean and arachnid characters--belonged, therefore,
to a group of animals which gave rise to both the crustacean and arachnid
groups. This is precisely the position of the Palæostracan group, which
is regarded as the ancestor of both the crustaceans and arachnids.
{118}In two respects the retina of the lateral eyes of vertebrates
differs from that of all arthropods, for it possesses a special
supporting structure, the Müllerian fibres, which do not exist in the
latter, and it is developed in connection with a tube, the optic
diverticulum, which is connected on each side with the main tube of the
central nervous system. These two differences are in reality one and the
same, for the Müllerian fibres are the altered lining cells of the optic
diverticulum, and this tube has the same significance as the rest of the
tube of the nervous system; it is something which has nothing to do with
the nervous portion of the retina but has become closely amalgamated with
it. The explanation is, word for word, the same as for the tubular
nervous system, and shows that the ancestor of the vertebrate possessed
two anterior diverticula of its alimentary canal which were in close
relationship to the optic ganglion and nerve of the lateral eye on each
side. It is again a striking coincidence to find that Artemia, which with
Branchipus represents a group of living crustaceans most nearly allied to
the trilobites, does possess two anterior diverticula of the gut which
are in extraordinarily close relationship with the optic ganglia of the
retina of the lateral eyes on each side.
The evidence of the optic apparatus of the vertebrate points most
remarkably to the derivation of the Vertebrata from the Palæostraca.
{119}CHAPTER III
_THE EVIDENCE OF THE SKELETON_
The bony and cartilaginous skeleton considered, not the
notochord.--Nature of the earliest cartilaginous skeleton.--The
mesosomatic skeleton of Ammocoetes; its topographical arrangement, its
structure, its origin in muco-cartilage.--The prosomatic skeleton of
Ammocoetes; the trabeculæ and parachordals, their structure, their origin
in white fibrous tissue.--The mesosomatic skeleton of Limulus compared
with that of Ammocoetes; similarity of position, of structure, of origin
in muco-cartilage.--The prosomatic skeleton of Limulus; the entosternite
or plastron compared with the trabeculæ of Ammocoetes; similarity of
position, of structure, of origin in fibrous tissue.--Summary.
The explanation of the two optic diverticula given in the last chapter
accounts in the same harmonious manner for every other part of the tube
around which the central nervous system of the vertebrate has been grouped.
The tube conforms in all respects to the simple epithelial tube which
formed the alimentary canal of the ancient type of marine arthropods such
as were dominant in the seas when the vertebrates first appeared. The whole
evidence so far is so uniform and points so strongly in the direction of
the origin of vertebrates from these ancient arthropods, as to make it an
imperative duty to proceed further and to compare one by one the other
parts of the central nervous system, together with their outgoing nerves in
the two groups of animals.
Before proceeding to do this, it is advisable first to consider the
question of the origin of the vertebrate skeletal tissues, for this is the
second of the great difficulties in the way of deriving vertebrates from
arthropods, the one skeleton being an endo-skeleton composed of cartilage
and bone, and the other an exo-skeleton composed of chitin. Here is a
problem of a totally different kind to that we have just been considering,
but of so fundamental a character that it must, if possible, be solved
before passing on to the consideration of the cranial nerves and the organs
they supply.
{120}Is there any evidence which makes it possible to conceive the method
by which the vertebrate skeleton may have arisen from the skeletal tissues
of an arthropod? By the vertebrate skeleton I mean the bony and
cartilaginous structures which form the backbone, the cranio-facial
skeleton, the pectoral and pelvic girdles, and the bones of the limbs. I do
not include the notochord in these skeletal tissues, because there is not
the slightest evidence that the notochord played any part in the formation
of these structures; the notochordal tissue is something _sui generis_, and
never gives rise to cartilage or bone. The notochord happens to lie in the
middle line of the body and is very conspicuous in the lowest vertebrate;
with the development of the backbone the notochord becomes obliterated more
and more, until at last it is visible in the higher vertebrates only in the
embryo; but that obliteration is the result of the encroachment of the
growing bone-masses, not the cause of their growth. Although, then, the
notochord may in a sense be spoken of as the original supporting axial rod
of the vertebrate, it is so different to the rest of the endo-skeleton, has
so little to do with it, that the consideration of its origin is a thing
apart, and must be treated by itself without reference to the origin of the
cartilaginous and bony skeleton.
THE COMMENCEMENT OF THE BONY SKELETON IN THE VERTEBRATE.
What is the teaching of the vertebrate? What evidence is there as to the
origin of the bony skeleton in the vertebrate phylum itself?
The axial bony skeleton of the higher Mammalia consists of two parts, (1)
the vertebral column with its attached bony parts, and (2) the
cranio-facial skeleton. Of these two parts, the bony tissue of the first
arises in the embryo from cartilage, of the second partly from cartilage,
partly from membrane.
In strict accordance with their embryonic origin is their phylogenetic
origin: as we pass from the higher vertebrates to the lower these
structures can be traced back to a cartilaginous and membranous condition,
so that, as Parker has shown, the cranio-facial bony skeleton of the higher
vertebrates can be derived directly from a non-bony cartilaginous skeleton,
such as is seen in Petromyzon and the cartilaginous fishes.
Balfour, in his "Comparative Embryology," states that the {121}primitive
cartilaginous cranium is always composed of the following parts:--
1. A pair of cartilaginous plates on each side of the cephalic section of
the notochord known as the parachordals (_pa.ch._, Fig. 49; _iv._, Fig.
48). These plates, together with the notochord (_ch._) enclosed between
them, form a floor for the hind and mid-brain.
[Illustration: FIG. 48.--EMBRYO PIG, TWO-THIRDS OF AN INCH LONG (from
PARKER), ELEMENTS OF SKULL SEEN FROM BELOW.
_ch._, notochord; _iv._, parachordals; _au._, auditory capsule; _py._,
pituitary body; _tr._, trabecula; _ctr._, trabecular cornu; _pn._,
pre-nasal cartilage; _ppg._, palato-pterygoid tract; _mn._, mandibular
arch; _th.h._, first branchial arch; _VII.-XII._, cranial nerves.]
[Illustration: FIG. 49.--HEAD OF EMBRYO DOG-FISH (from PARKER), BASAL VIEW
OF CRANIUM FROM ABOVE.
_ol._, olfactory sacs; _au._, auditory capsule; _py._, pituitary body;
_pa.ch._, parachordal cartilage; _tr._, trabecula; _inf._, infundibulum;
_pt.s._, pituitary space; _e._, eye.]
2. A pair of bars forming the floor for the fore-brain, known as the
trabeculæ (_tr_). These bars are continued forward from the parachordals.
They meet posteriorly and embrace the front end of the notochord, and after
separating for some distance bend in again in such a way as to enclose a
space--the pituitary space (_pt.s._). In {122}front of this space they
remain in contact, and generally unite. They extend forward into the nasal
region (_pn._).
3. The cartilaginous capsules of the sense organs. Of these the auditory
(_au._) and the olfactory capsules (_ol._) unite more or less intimately
with the cranial walls; while the optic capsules, forming the usually
cartilaginous sclerotics, remain distinct.
The parachordals and notochord form together the basilar plate, which forms
the floor for that section of the brain belonging to the primitive postoral
part of the head, and its extent corresponds roughly to that of the
basioccipital of the adult skull.
The trabeculæ, so far as their mere anatomical relations are concerned,
play the same part in forming the floor for the front cerebral vesicle as
do the parachordals for the mid- and hind-brain. They differ, however, from
the parachordals in one important feature, viz. that except at their hinder
end they do not embrace the notochord. The notochord always terminates at
the infundibulum, and the trabeculæ always enclose a pituitary space, in
which lies the infundibulum (_inf._) and the pituitary body (_py._).
In the majority of the lower forms the trabeculæ arise quite independently
of the parachordals, though the two sets of elements soon unite.
The trabeculæ are usually somewhat lyre-shaped, meeting in front and
behind, and leaving a large pituitary space between their middle parts.
Into this space the whole base of the fore-brain primitively projects, but
the space itself gradually becomes narrowed until it usually contains only
the pituitary body.
The trabecular floor of the brain does not long remain simple. Its sides
grow vertically upwards, forming a lateral wall for the brain, in which in
the higher types, two regions may be distinguished, viz. an alisphenoidal
region behind, growing out from what is known as the basisphenoidal region
of the primitive trabeculæ, and an orbito-sphenoidal region in front,
growing out from the presphenoidal region of the trabeculæ. These plates
form at first a continuous lateral wall of the cranium. The cartilaginous
walls which grow up from the trabecular floor of the cranium generally
extend upwards so as to form a roof, though almost always an imperfect
roof, for the cranial cavity.
The basi-cranial cartilaginous skeleton reduces itself always into
trabeculæ and parachordals with olfactory and auditory cartilaginous
capsules.
{123}In addition, a branchial skeleton exists, which consists of a series
of bars known as the branchial bars, so situated as to afford support to
the successive branchial pouches. An anterior arch known as the mandibular
arch (Fig. 50, _Mn._), placed in front of the hyo-mandibular cleft, and a
second arch, known as the hyoid arch (_Hy._), placed in front of the
hyo-branchial cleft, are developed in all types; the succeeding arches are
known as the true branchial arches (_Br._), and are only fully developed in
the Ichthyopsida. In all cases of jaw-bearing (gnathostomatous) vertebrates
the first arch has become a supporting skeleton for the mouth (Fig. 51),
and in the higher vertebrates in combination with the second or hyoid arch
takes part in the formation of the ear-bones.
[Illustration: FIG. 50.--HEAD OF EMBRYO DOG-FISH, ELEVEN LINES LONG. (From
PARKER.)
_Tr._, trabecula; _Mn._, mandibular cartilage; _Hy._, hyoid arch; _Br_1._,
first branchial arch; _Na._, olfactory sac; _E._, eye; _Au._, auditory
capsule; _Hm._, hemisphere; _C_1_, _C_2_, _C_3_, cerebral vesicles.]
[Illustration: FIG. 51.--SKULL OF ADULT DOG-FISH, SIDE VIEW. (From PARKER.)
_cr._, cranium; _Br._, branchial arches; _Mn._ + _Hy._, mandibular and
hyoid arches.]
The true branchial arches persist, to a certain extent, in the Amphibia,
and become still more degenerated in the Amniota (reptiles, birds, and
mammals) in correlation with the total disappearance of a branchial
respiration at all periods of their life. {124}Their remnants become more
or less important parts of the hyoid bone, and are employed solely in
support of the tongue.
In no single animal is there any evidence that the foremost arch, the
mandibular, is a true branchial arch. As low down as the Elasmobranchs it
becomes divided into two elements which form respectively the upper and
lower jaws; the hyoid arch, on the other hand, although it has altered its
form and acquired the secondary function of supporting the mandibular arch,
still retains its respiratory function.
The evidence afforded by the mode of formation of the skeletal tissues of
vertebrates down to the Elasmobranchs indicates that the primitive cranial
skeleton arose from two paired basal cartilages, the parachordals and
trabeculæ, to which were attached respectively cartilaginous cases
enclosing the organs of hearing and smell. In addition, the branchial
portion of the cranial region was provided with cartilaginous bars arranged
serially for the support of the branchiæ, with the exception of the
foremost, the mandibular bar, which formed supporting tissues for the
mouth--the upper and lower jaws.
Just as in past times the spinal nerves and the segments they supplied were
supposed to represent the type on which the original vertebrate was built,
so also the spinal vertebræ afforded the type of the segmented skeleton,
and the anatomists of those days strove hard to resolve the cranio-facial
skeleton into a series of modified vertebræ. Owing especially to the
labours of Huxley, who showed that the segmentation in the head-region was
essentially a segmentation due to the presence of branchial bars, this
conception was finally laid to rest and nowadays it is admitted to be
hopeless to resolve the cranium into vertebral segments. Still, however,
the vertebrate is a segmented animal and its segmented nature is visible in
the cranial region, so far as the skeletal tissues are concerned, in the
shape of the series of branchial and visceral bars.
To this segmentation the name of 'branchiomeric' has been given, while that
due to the presence of vertebræ is called 'mesomeric.'
As we have seen, the internal bony skeleton of the vertebrate commences as
a cartilaginous and membranous skeleton. For this reason the preservation
of such skeletons is impossible in the fossil form, unless the cartilage
has become impregnated with lime salts, so that there is but little hope of
ever obtaining traces of such {125}structures in the fossils of the
Silurian age either among the vertebrate or invertebrate remains.
Fortunately for this investigation there are still living on the earth two
representatives of that age; on the invertebrate side Limulus, and on the
vertebrate side Ammocoetes.
The Elasmobranchs represent the most primitive of the gnathostomatous
vertebrates. Below them come the Agnatha, known as the cyclostomatous
fishes or Marsipobranchii, the lampreys (Petromyzon) and the hag-fishes
(Myxine).
The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial skeleton
composed of a cartilaginous unsegmented cranium, with the basal trabeculæ
and parachordals and a series of branchial and visceral cartilaginous bars
forming the so-called branchial basket-work; to these must be added
auditory and nasal capsules. In contradistinction to this elaborate
cranio-facial skeleton, the spinal vertebral skeleton is represented only
by segmentally arranged small pieces of cartilage formed in the connective
tissue dissepiments between segmented sheets of body-muscles (myotomes).
[Illustration: FIG. 52.--SKELETON OF PETROMYZON. (From PARKER.)
_na._, nasal capsule; _au._, auditory capsule; _nc._, notochord.]
But Petromyzon is derived from Ammocoetes by a remarkable process of
transformation, and a most important part of that transformation is the
formation of new cartilaginous structures. Thus we see that in Ammocoetes
there is no sign of a cartilaginous vertebral column; at transformation the
rudimentary vertebræ of Petromyzon are formed. In Ammocoetes the brain-case
is a simple fibrous membranous covering; at transformation this becomes
cartilaginous. In Ammocoetes there are no cartilaginous structures
corresponding to the sub-ocular arches; these are all formed at
transformation. It follows, that we can trace back the bony skeleton of the
vertebrate head to the skeleton of Ammocoetes, and we may therefore
conclude {126}that the primitive cartilaginous skeleton of the vertebrate
consisted of the following structures (Fig. 53, B), viz. the branchial bars
forming a basket-work, the trabeculæ and parachordals, the auditory and
nasal capsules--a clear proof that the cranial skeleton is older than the
spinal. Of these structures the branchial bars are the only evidently
segmented parts.
[Illustration: FIG. 53.--COMPARISON OF CARTILAGINOUS SKELETON OF LIMULUS
AND AMMOCOETES.
A, Diagram of cartilaginous skeleton of Limulus. _Soft cartilage_,
entapophysial ligaments, deep black; branchial bars simply hatched; _hard
cartilage_, lateral trabeculæ of entosternite, netted; _Ph._, position of
pharynx.
B, Diagram of cartilaginous skeleton of Ammocoetes. _Soft cartilage_,
sub-chordal cartilaginous bands, deep black; branchial basket-work (first
formed part), simply hatched; _hard cartilage_, cranio-facial skeleton,
trabeculæ, parachordals and auditory capsules, netted; _Inf._, position of
tube of infundibulum (old oesophagus).]
THE SOFT CARTILAGE OF THE BRANCHIAL SKELETON OF AMMOCOETES.
The study of Ammocoetes gives yet another clue to the nature of the
earliest skeleton, for these two marked groups of cartilage--the branchial
and basi-cranial--are characterized by a difference in structure as well as
a difference in topographical position. J. Müller was the first to point
out that these two sets of cartilages differ in appearance and
constitution, and he gave to them the name of yellow and grey cartilage.
Parker has described them fully under the terms soft and hard cartilage,
terms which Schaffer has also used, and I shall also make use of them here.
The whole of the branchial cartilaginous skeleton is composed of soft
cartilage, while the basi-cranial skeleton, consisting of trabeculæ,
parachordals, and auditory capsule, is composed {127}of hard cartilage, the
only soft cartilage in this region being that which forms the nasal
capsule, not represented in Fig. 53, B.
These two groups of cartilage arise independently, so that at first the
basi-cranial system is quite separate from the branchial, and only late in
the history of the animal is a junction effected between the branchial
system and the trabeculæ and parachordals, an initial separation which is
especially striking when we consider that in this animal all the
cartilaginous structures of any one system are continuous: there is no sign
of anything in the nature of joints.
Of these two main groups, the branchial cartilages are formed first in the
embryo, a fact which suggests that they are the most primitive of the
vertebrate cartilages, and that, therefore, the first true formation of
cartilage in the invertebrate ancestor may be looked for in the shape of
bars supporting the branchial mechanism. The evidence of the origin of the
cartilaginous structures in Ammocoetes is given by Shipley in the following
words:--
"The branchial bases are the first part of the skeleton to appear. They
arise about the 24th day as straight bars of cartilage, lying external and
slightly posterior to the branchial vessel.
"The first traces of the basi-cranial skeleton appear on the 30th day as
two rods of cartilage--the trabeculæ."
Our attention must, in the first place, be directed to this branchial
basket-work of Ammocoetes.
Underlying the skin of Ammocoetes in the branchial region is situated the
sheet of longitudinal body-muscles, divided into a series of segments or
myotomes, which forms the somatic muscles so characteristic of all fishes.
This muscular sheet is depicted on the left-hand side of Fig. 54. It does
not extend over the lower lip or over that part in the middle line where
the thyroid gland is situated. In these parts a sheet of peculiar tissue
known by the name of muco-cartilage lies immediately under the skin,
covering over the thyroid gland and lower lip. The somatic muscular sheet
with the superjacent skin can be stripped off very easily owing to the
vascularity and looseness of the tissue immediately underlying it. When
this is done the branchial basket-work comes beautifully into view as is
seen on the right-hand side of Fig. 54. It forms a cage within which the
branchiæ and their muscles lie entirely concealed.
This is the great characteristic of this most primitive form of the
branchial cartilaginous bars and distinguishes it from the branchial
{128}bars of other higher fishes, in that it forms a system of cartilages
which lie external to the branchiæ--an extra-branchial system.
This branchial basket-work is simpler in Ammocoetes than in Petromyzon, and
its actual starting-point consists of a main transverse bar corresponding
to each branchial segment; from this transverse bar the system of
longitudinal bars by which the basket-work is formed has sprung. These
transverse bars arise from a cartilaginous longitudinal rod, situated close
against the notochord on each side. These rods may be called the subchordal
cartilaginous bands (Fig. 53), and, according to the observations of
Schneider and others, each subchordal band does not form at first a
continuous cartilaginous rod, but the cartilage is conspicuous only at the
places where the transverse bars arise. In the youngest Ammocoetes examined
by Schaffer, he could find no absolute discontinuity of the cartilage
except between the first two transverse bars, but he says that the thinning
between the transverse bars was so marked as to make it highly probable
that at an earlier stage there was discontinuity. The whole system of
branchial bars and subchordal rods is at first absolutely disconnected from
the cranial system of trabeculæ and parachordals, and only later do the two
systems join.
[Illustration: FIG. 54.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES.
_Th._, thyroid gland; _M._, lower lip, with its muscles.]
These observations on Ammocoetes lead most definitely to the conclusion
that the starting-point of the whole cartilaginous skeleton of the
vertebrate consisted of a series of transverse cartilaginous bars, for the
purpose of supporting branchial segments; these were connected with two
axial longitudinal cartilaginous rods, which at first contained cartilage
only near the places of junction of the branchial {129}bars. This system
may be called the mesosomatic skeleton, as it is entirely confined to the
branchial or mesosomatic region.
In addition to this primitive cartilaginous framework, which was formed for
the support of the mesosomatic or respiratory segments, but at a slightly
later period in the phylogenetic history, a separate cartilaginous system
was formed for the support of the prosomatic segments, viz. the trabeculæ
and parachordals with the auditory capsules: a system which was at first
entirely separated from the mesosomatic, and, as we shall see, is more
advanced in structure than the branchial system. Later still, the story is
completed at the time of transformation to Petromyzon by the formation of
the simple cartilaginous skull and the rudimentary vertebræ, the structure
of which is also of a more advanced type.
THE STRUCTURE OF THE SOFT BRANCHIAL CARTILAGE.
Having considered the topographical position of the primitive branchial
cartilaginous skeleton, we may now inquire, What was its structure and how
was it formed?
In the higher vertebrates various forms of cartilage are described, viz.
hyaline, fibro-cartilage, elastic cartilage, and parenchymatous cartilage.
Of these, the parenchymatous cartilage is looked upon as the most primitive
form, because it preserves without modification the characters of embryonic
cartilage.
Embryology, then, would lead to the belief that the earliest form of
cartilage in the vertebrate kingdom ought to be of this type, viz. large
cells, each of which is enclosed in a simple capsule, so that the capsules
of the cells form the whole of the matrix, and thus form a simple
homogeneous honeycomb-structure, in the alveoli of which the
cartilage-cells lie singly. If, then, the branchial cartilages of
Ammocoetes are, as has just been argued, the representatives of the
cartilaginous skeleton of the primitive vertebrate, it is reasonable to
suppose that they should resemble in structure this embryonic cartilage.
Such is undoubtedly the case: all observers who have described the
branchial basket-work of Ammocoetes or Petromyzon have been struck with the
extremely primitive character of the cartilage, and the last observer
(Schaffer) describes it as composed of thin walls of homogeneous material,
in which there are no lines of separation, which form a simple
honeycomb-structure, in the alveoli {130}of which the separate cells lie
singly. These branchial cartilages are each surrounded by a layer of
perichondrium, and in Fig. 55, A, I give a picture of a section of a
portion of one of the bars.
[Illustration: FIG. 55.--A, BRANCHIAL CARTILAGE OF AMMOCOETES, STAINED WITH
THIONIN. B, BRANCHIAL CARTILAGE OF LIMULUS, STAINED WITH THIONIN.]
Hence we see that structurally as well as topographically the branchial
bars of Ammocoetes justify their claim to be considered as the origin of
the vertebrate cartilaginous framework.
ON THE STRUCTURE OF THE MUCO-CARTILAGE IN AMMOCOETES.
We can, however, go further than this, and ask how this cartilage itself is
formed in Ammocoetes? The answer is most definite, most instructive and
suggestive, for in all cases this particular kind of cartilage is formed
from, or at all events in, a peculiar fibrous tissue, which was called by
Schneider "Schleim-Knorpel," or muco-cartilage, a tissue which is
distinguishable from other connective tissues, not only by its structural
peculiarities, but also by its strong affinity for all dyes which
differentiate mucoid or chondro-mucoid substances.
This muco-cartilage is thus described by Schneider:--The perichondrium in
Ammocoetes is not confined to the true cartilaginous structures, but
extends itself in the form of thin plates in definite directions. Between
these plates of perichondrium a peculiar tissue (Fig. 56)--the
muco-cartilage--exists, consisting of fibrillæ, whose direction is mainly
at right angles to the planes of the perichondrial plates, with star-shaped
cells in among them, and with the spaces between the fibrillæ filled up
with a semi-fluid mass.
{131}From this tissue all the primitive cartilages which resemble the
branchial bars are formed, either by the invasion of chondroblasts from the
surrounding perichondrium, or by the proliferation and encapsulation of the
cells of the muco-cartilage itself.
[Illustration: FIG. 56.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE
OF AMMOCOETES.]
This very distinctive tissue--the muco-cartilage--is of very great
importance in all questions of the origin of the skeletal tissues. In all
descriptions of the skeletal tissues it has been practically disregarded
until recent years when, besides my own observations, its distribution has
been mapped out by Schaffer. Thus Parker, in his well-known description of
the skeleton of the marsipobranch fishes, does not even mention its
existence. Its importance is shown by its absolute disappearance at
transformation and its non-occurrence in any of the higher vertebrates. It
is entirely confined to the head-region, and its distribution there is most
suggestive, for, as will be described fully later on, it forms a skeleton
which both in structure and position resembles very closely the
head-shields of cephalaspidian fishes. At the present part of my argument
its more immediate interest lies in the method of tracing this tissue. For
this purpose I made use of the micro-chemical reaction of thionin, a dye
which, as shown by Hoyer, stains all mucin-containing substances a bright
purple. Schaffer made use of a corresponding basophil stain, hæmalum. When
stained with thionin, the matrix, or ground-substance of the branchial
cartilages as well as the matrix or semi-fluid substance in which the
fibrils of the muco-cartilaginous cells are embedded take on a deep purple
colour, while the fibrous material of the cranial walls and other
connective tissue strands, such as the perichondrium, are coloured light
blue. Muco-cartilage, then, may be described as a peculiar form of
connective tissue which differs from other connective tissue not only in
its appearance but in {132}its chemical composition, for unlike white
fibrous tissue it contains a large amount of mucin, and this tissue is the
forerunner of the earliest cartilaginous vertebrate skeleton, the branchial
bars of Ammocoetes.
The conclusions to which we are led by the study of the structure,
position, and mode of origin of these primitive cartilages of Ammocoetes
may be thus summed up:--
1. The immediate ancestor of the vertebrate must have possessed a peculiar
fibrous tissue--the ground-substance of which stained deep purple with
thionin--in which cartilage arose.
2. The cartilage so formed was not like hyaline cartilage, but resembled
in a striking manner parenchymatous cartilage.
3. This cartilage was situated partly in two axial longitudinal bands,
partly as transverse bars, which supported the branchial apparatus.
THE PROSOMATIC OR BASI-CRANIAL SKELETON OF AMMOCOETES.
Before searching for any evidence of a similar tissue in any invertebrate
group, it is advisable to consider the other portion of the cartilaginous
skeleton of Ammocoetes, which consists of the trabeculæ, parachordals and
auditory capsules--the basi-cranial skeleton--and is composed of hard, not
soft cartilage.
This basi-cranial skeleton represented in Fig. 53, B, is confined to the
region of the notochord, the cranial walls being composed entirely of a
white fibrous membrane. It is separated at first entirely from the
sub-chordal portion of the branchial basket-work, and is composed of a
foremost part, the trabeculæ (_Tr._), and of a hindermost part, the
parachordals (_Pr.ch._), which are characterized by the attachment on each
side of the large auditory capsule (_Au._). In Ammocoetes the trabecular
bars are continuous with the parachordals, the junction being marked by a
small lateral projection on each side, which at transformation is seen to
play an important part in the formation of the sub-ocular arch. The
trabecular bar lies close against the notochord on each side up to its
termination; it then bends away from the middle line and curves round until
it meets its fellow on the opposite side, thus forming, as it were, the
head of a racquet of which the notochord forms the splice in the handle.
The strings of the racquet are represented by a thin membrane, in the
centre of which the position of the infundibulum (_Inf._) of the {133}brain
can be clearly seen. In an earlier stage of Ammocoetes the two trabecular
horns do not meet, but are separated by connective tissue, which afterwards
becomes cartilaginous.
As far, then, as the topography of this basi-cranial skeleton is concerned,
the striking points are--the shape of the trabecular portion, diverging as
it does around the infundibulum, and the presence on the parachordal
portion of the two large auditory capsules.
These two points indicate, on the hypothesis that infundibulum and
oesophagus are convertible terms, that two supporting structures of a
cartilaginous nature must have existed in the ancestor of the vertebrate,
the first of which surrounded the oesophagus, and the second was in
connection with its auditory apparatus.
[Illustration: FIG. 57.--A, CARTILAGE OF TRABECULÆ OF AMMOCOETES, STAINED
WITH HÆMATOXYLIN AND PICRIC ACID. B, NESTS OF CARTILAGE CELLS IN
ENTOSTERNITE OF HYPOCTONUS, STAINED WITH HÆMATOXYLIN AND PICRIC ACID.]
STRUCTURE OF THE HARD CARTILAGES.
The structure of this hard cartilage of the trabeculæ and auditory capsules
resembles that of the soft, in so far that it consists of large cells with
a comparatively small amount of intercellular substance. Schaffer, who has
described it lately, considers that it is a nearer approach to hyaline
cartilage than the soft, but yet cannot be called hyaline cartilage in the
usual sense of the term. Its peculiarities and its differences from the
soft are especially well seen by its staining reactions. I have myself been
particularly struck with the effect of picrocarmine or combined hæmatoxylin
and picric acid {134}staining (Fig. 57). In the case of the soft cartilage
the capsular substance stains respectively a brilliant red or blue, while
that of the hard cartilage is coloured a deep yellow, so that the junction
between the parachordals and the branchial cartilages is beautifully marked
out. Then, again, with thionin, which gives so marked a reaction in the
case of the soft cartilage, the hard cartilage of the auditory capsule is
not stained at all, and in the trabeculæ the deep purple colour is confined
to the mucoid cement-substance between the capsules, just as Schaffer has
stated. The same kinds of reactions have been described by Schaffer: thus
by double staining with hæmalum-eosin the hard cartilage stains red, the
soft blue; and he points out that even with over-staining by hæmalum the
auditory capsule remains colourless, just as I have noticed with thionin.
He infers, precisely as I have done from the thionin reaction, that
chondro-mucoid, which is so marked a constituent of the soft cartilage and
of the muco-cartilage, is absent or present in but slight quantities in the
hard cartilage. Similarly, he points out that double staining with
tropoeolin-methyl-violet stains the hard cartilage a bright orange colour,
and the soft cartilage a violet.
The evidence, then, shows clearly that a marked chemical difference exists
between these two cartilages, which may be expressed by saying that the one
contains very largely a basophil substance, which we may speak of as
belonging to the class of chondro-mucoid substances, while the other
contains mainly an oxyphil substance, probably a chondro-gelatine
substance.
We may perhaps go further and attribute this difference of composition to a
difference of origin; for whereas the soft cartilage is invariably formed
in a special tissue, the muco-cartilage, which shows by its reaction how
largely it is composed of a mucoid substance, the hard cartilage is
certainly, in the case of the cartilage of the cranium where its origin has
been clearly made out, formed in the membranous tissue of the cranium of
Ammocoetes--_i.e._ in a tissue which stains light blue with thionin, and
contains a gelatinous rather than a mucoid substratum.
The best opportunity of finding out the mode of origin of the hard
cartilage is afforded at the time of transformation, when so much of this
kind of cartilage is formed anew. Unfortunately, it is very difficult to
obtain the early transformation stages, consequently we cannot be said to
possess any really exhaustive and {135}definite account of how the new
cartilages are formed. Bujor, Kaensche, and Schaffer all profess to give a
more or less definite account of their formation, and the one striking
impression left on the mind of the reader is how their descriptions vary.
In one point only are they agreed, and in that I also agree with them, viz.
the manner in which the new cranial walls are formed. Schaffer describes
the process as the invasion of chondroblasts into the homogeneous fibrous
tissue of the cranial walls. Such chondroblasts not only form the
cartilaginous framework, but also assimilate the fibrous tissue which they
invade, so that finally all that remains of the original fibrous matrix in
which the cartilage was formed are these lines of cement-substance between
the groups of cartilage cells, which, containing some basophil material,
are marked out, as already mentioned (Fig. 57).
We may therefore conclude, from the investigation of Ammocoetes, that the
front part of the basi-cranial skeleton arose as two trabecular bars, to
which muscles were attached, situated bilaterally with respect to the
central nervous system. These bars were composed of tendinous material with
a gelatinous rather than a mucoid substratum, in which nests of
cartilage-cells were formed, the cartilaginous material formed by these
cells being of the hard variety, not staining with thionin, and staining
yellow with picro-carmine, etc. By the increase of such nests and the
assimilation of the intermediate fibrous material, the original
fibro-cartilage was converted into the close-set semi-hyaline cartilage of
the trabeculæ and auditory capsules, in which the fibrous material still
marks out by its staining-reaction the limits of the cell-clusters.
Such I gather to be Schaffer's conclusions, and they are certainly borne
out by my own and Miss Alcock's observations. As far as we have had an
opportunity of observing at present, the first process at transformation
appears to consist of the invasion of the fibrous tissue of the cranial
wall by groups of cells which form nests of cells between the fibrous
strands. These nests of cells form round themselves capsular material, and
thus form cell-territories of cartilage, which squeeze out and assimilate
the surrounding fibrous tissue, until at last all that remains of the
original fibrous matrix is the lines of cement-substance which mark out the
limits of the various cell-groups.
At present I am inclined to think that both soft and hard cartilage
originate in a very similar manner, viz. by the formation of capsular
{136}material around the invading chondroblasts, and that the difference in
the resulting cartilage is mainly due to the difference in chemical
composition of the matrix of the connective tissue which is invaded. Thus
the difference may be formulated as follows:--
The hard cartilage is formed by the invasion of chondroblasts into a
fibrous tissue, which contains a gelatinous rather than a mucoid
substratum, in contradistinction to the soft cartilage which is formed,
probably also by the invasion of chondroblasts, in a tissue--the
muco-cartilage--which contains a specially mucoid substratum.
Such, then, is the very clearly defined starting-point of the vertebrate
skeleton--two distinct formations of different histological and chemical
structure,--the one forming a segmented branchial skeleton, the other a
non-segmented basi-cranial skeleton.
THE CARTILAGINOUS SKELETON OF LIMULUS.
Among the whole of the invertebrates at present living on the earth, is
there any sign of an internal cartilaginous skeleton that will give a
direct clue to the origin of the primitive vertebrate skeleton? The answer
to this question is most significant: only one animal among all those at
present known possesses a cartilaginous skeleton, which is directly
comparable with that of Ammocoetes, and here the comparison is very
close--only one animal among the thousands of living invertebrate forms,
and that animal is the only representative still surviving of the
palæostracan group, which was the dominant race when the vertebrate first
made its appearance. The Limulus, or king-crab, possesses a segmented
branchial internal cartilaginous skeleton (Fig. 53, A), made up of the same
kind of cartilage as the branchial skeleton of Ammocoetes, confined to the
mesosomatic or branchial region, just as in Ammocoetes, forming, as in
Ammocoetes, cartilaginous bars supporting the branchiæ, and these bars are
situated externally to the branchiæ, as in Ammocoetes. In addition this
animal possesses a basi-cranial internal semi-cartilaginous unsegmented
plate known as the entosternite or plastron situated, with respect to the
oesophagus, similarly to the position of the trabeculæ with respect to the
infundibulum in Ammocoetes. Moreover, the cartilaginous cells in this
tissue differ from those in the branchial region, in precisely the same
manner as the hard cartilage differs from the soft in Ammocoetes.
{137}This plastron, it is true, is found in other animals, all of which are
members of the scorpion tribe, except in one instance, and this, strikingly
enough, is the crustacean Apus--a strange primitive form, which is
acknowledged to be the nearest representative of the Trilobita still living
on the earth. None of these forms, however, possess any sign of an internal
cartilaginous branchial skeleton, such as is possessed by Limulus.
Scorpions, Apus, Limulus, are all surviving types of the stage of
organization which had been reached in the animal world when the vertebrate
first appeared.
THE MESOSOMATIC OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED OF SOFT
CARTILAGE.
Searching through the literature of the histology of the cartilaginous
tissues in invertebrate animals, to see whether any cartilage had been
described similar to that seen in the branchial cartilages of Ammocoetes,
and whether such cartilage, if found, arose in a fibrous tissue resembling
muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's
article on the tropho-skeletal tissues of Limulus, a picture of the
cartilage of Limulus, which would have passed muster for a drawing of the
branchial cartilage of Ammocoetes. This clue I followed out in the manner
described in my former paper in the _Journal of Anatomy and Physiology_,
and mapped out the topography of this remarkable tissue.
Limulus, like other water-dwelling arthropods, breathes by means of gills
attached to its appendages. These gill-bearing appendages are confined to
the mesosomatic region, as is seen in Fig. 59; and these appendages are
very different to the ordinary locomotor appendages, which are confined to
the prosomatic region. Each appendage, as is seen in Fig. 58, consists
mainly of a broad, basal part, which carries the gill-book on its under
surface; the distal parts of the appendage have dwindled to mere rudiments
and still exist, not for locomotor purposes, but because they carry on each
segment organs of special importance to the animal (see Chapter XI.). As is
seen in Fig. 58, the basal parts of each pair of appendages form a broad,
flattened paddle, by means of which the animal is able to swim in a clumsy
fashion. Very striking and suggestive is the difference between these
gill-bearing mesosomatic appendages and the non-gill-bearing locomotor
appendages of the prosoma.
{138}[Illustration: FIG. 58.--TRANSVERSE SECTION THROUGH THE MESOSOMA OF
LIMULUS, TO SHOW THE ANTERIOR (A) AND THE POSTERIOR (B) SURFACES OF A
MESOSOMATIC OR BRANCHIAL APPENDAGE.
In each figure the branchial cartilaginous bar, _Br.C._, has been exposed
by dissection on one side. _Ent._, entapophysis; _Ent.l._, entapophysial
ligament cut across; _Br.C._, branchial cartilaginous bar, which springs
from the entapophysis; _H._, heart; _P._, pericardium; _Al._, alimentary
canal; _N._, nerve cord; _L.V.S._, longitudinal venous sinus; _Dv._,
dorso-ventral somatic muscle; _Vp._, veno-pericardial muscle.]
At the base of each of these appendages, where it is attached to the body
of the animal, the external chitinous surface is characterized by a
peculiar stumpy, rod-like marking, and upon removing the chitinous
covering, this surface-appearance is seen to correspond to a well-marked
rod of cartilage (_Br.C._), which extends from the body {139}of the animal
well into each appendage. This bar of cartilage arises on each side from
the corresponding entapophysis (_Ent._), which is the name given to a
chitinous spur which projects a short distance (Fig. 58, B) into the animal
from the dorsal side, for the purpose of giving attachment to various
segmental muscles. These entapophyses are formed by an invagination of the
chitinous surface on the dorsal side and are confined to the mesosomatic
region, so that the mesosomatic carapace indicates, by the number of
entapophyses, the number of segments in that region, in contradistinction
to the prosomatic carapace, which gives no indication on its surface of the
number of its components.
Each entapophysis is hollow and its walls are composed of chitin; but from
the apex of each spur there stretches from spur to spur a band of tissue,
called by Lankester the entapophysial ligament (_Ent.l._) (Fig. 58), and in
this tissue cartilage is formed. Isolated cartilaginous cells, or rather
groups of cells, are found here and there, but a concentration of such
groups always takes place at each entapophysis, forming here a solid mass
of cartilage, from which the massive cartilaginous bar of each branchial
appendage arises.
Further, not only is this cartilage exactly similar to parenchymatous
cartilage, as it occurs in the branchial cartilages of Ammocoetes, but also
its matrix stains a brilliant purple with thionin in striking contrast to
the exceedingly slight light-blue colour of the surrounding perichondrium.
In its chemical composition it shows, as might be expected, that it is a
cartilage containing a very large amount of some mucin-body.
THE MUCO-CARTILAGE OF LIMULUS.
The resemblance between this structure and that of the branchial bars of
Ammocoetes does not end even here, for, as already mentioned, the cartilage
originates in a peculiar connective tissue band, the entapophysial
ligament, and this tissue bears the same relation in its chemical reactions
to the ordinary connective tissue of Limulus, as muco-cartilage does to the
white fibrous tissue of Ammocoetes. The white connective tissue of Limulus,
as already stated, resembles that of the vertebrate more than does the
connective tissue of any other invertebrate, and, similarly to that of
Ammocoetes, does not stain, or gives only a light-blue tinge with thionin.
The tissue of {140}the entapophysial ligament, on the contrary, just like
muco-cartilage, takes on an intense purple colour when stained with
thionin. It possesses a mucoid substratum, just as does muco-cartilage, and
in both cases a perfectly similar soft cartilage is born from it.
[Illustration: FIG. 59.--DIAGRAM OF LIMULUS, TO SHOW THE NERVES TO THE
APPENDAGES (1-13) AND THE BRANCHIAL CARTILAGES.
The branchial cartilages and the entapophysial ligaments are coloured blue,
the branchiæ red. _gl._, generative and hepatic glands surrounding the
central nervous system and passing into the base of the flabellum (_fl._).]
One difference, however, exists between the branchial cartilages of these
two animals; the innermost axial layer of the branchial bar of Limulus is
very apt to contain a specially hard substance, apparently chalky in
nature, so that it breaks up in sections, and gives the appearance of a
broken-down spongy mass; if, however, the tissue is first placed in a
solution of hydrochloric acid, it then cuts easily, and the whole tissue is
seen to be of the same structure throughout, the main difference being that
the capsular spaces in the axial region are much larger and much more free
from cell-protoplasm than are those of the smaller younger cells near the
periphery.
{141}I have attempted in Fig. 53 to represent this close resemblance
between the segmented branchial skeleton of Limulus and of Ammocoetes, a
resemblance so close as to reach even to minute details, such as the
thinning out of the cartilage in the subchordal bands and entapophysial
ligaments respectively between the places where the branchial bars come
off.
[Illustration: FIG. 60.--DIAGRAM OF AMMOCOETES CUT OPEN TO SHOW THE LATERAL
SYSTEM OF CRANIAL NERVES _V., VII., IX., X._, AND THE BRANCHIAL CARTILAGES.
The branchial cartilages and sub-chordal ligaments are coloured blue, the
branchiæ red. _gl._, glandular substance surrounding the central nervous
system and passing into the auditory capsule with the auditory nerve
(_VIII._).]
In Fig. 59 I have shown the prosoma and mesosoma of Limulus, and indicated
the nerves to the appendages together with the mesosomatic cartilaginous
skeleton.
In Fig. 60 I have drawn a corresponding picture of the prosomatic and
mesosomatic region of Ammocoetes with the corresponding nerves {142}and
cartilages. In this figure the animal is supposed to be slit open along the
ventral mid-line and the central nervous system exposed.
THE PROSOMATIC SKELETON OF LIMULUS, COMPOSED OF HARD CARTILAGE.
The rest of the primitive vertebrate skeleton arose in the prosomatic
region, and formed a support for the base of the brain. This skeleton was
composed of hard cartilage, and arose in white fibrous tissue containing
gelatin rather than mucin.
Is there, then, any peculiar tissue of a cartilaginous nature in Limulus
and its allies, situated in the prosomatic region, which is entirely
separate from the branchial cartilaginous skeleton, which acts as a
supporting internal framework, and contains a gelatinous rather than a
mucoid substratum?
It is a striking fact, common to the whole of the group of animals to which
our inquiries, deduced from the consideration of the structure of
Ammocoetes, have, in every case, led us in our search for the vertebrate
ancestor, that they do possess a remarkable internal semi-cartilaginous
skeleton in the prosomatic region, called the entosternite or plastron,
which gives support to a large number of the muscles of that region; which
is entirely independent of the branchial skeleton, and differs markedly in
its chemical reactions from that cartilage, in that it contains a
gelatinous rather than a mucoid substratum.
In Limulus it is a large, tough, median plate, fibrous in character, in
which are situated rows and nests of cartilage-cells. The same structure is
seen in the plastron of Hypoctonus, of Thelyphonus, and to a certainty in
all the members of the scorpion group. Very different is the behaviour of
this tissue to staining from that of the branchial region. No part of the
plastron stains purple with thionin; it hardly stains at all, or gives only
a very slight blue colour. In its chemical composition there is a marked
preponderance of gelatin with only a slight amount of a mucin-body. In some
cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules of the
cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while
the fibres between the cell-nests stain a blue-brown colour, partly from
the hæmatoxylin, partly from the picric acid.
All the evidence points to the plastron as resembling the basi-cranial
skeleton of Ammocoetes in its composition and in the origin {143}of its
cells in a white fibrous tissue. What, then, is its topographical position?
It is in all cases a median structure lying between the cephalic stomach
and the infra-oesophageal portion of the central nervous system, and in all
cases it possesses two anterior horns which pass around the oesophagus and
the nerve-masses which immediately enclose the oesophagus (Fig. 61, A).
These lateral horns, then, which lie laterally and slightly ventral to the
central nervous system, and are called by Ray Lankester and Benham the
sub-neural portion of the entosternite, are very nearly in exactly the
position of the racquet-shaped head of the trabeculæ in Ammocoetes. It is
easy to see that, with a more extensive growth of the nervous material
dorsally, such lateral horns might be caused to take up a still more
ventral position. Now, these two lateral horns of the plastron of Limulus
are continued along its whole length so as to form two thickened lateral
ridges, which are conspicuous on the flat surface of the rest of this
median plate. In other cases, as in the Thelyphonidæ, the plastron consists
mainly of these two lateral ridges or trabeculæ, as they might be called,
and Schimkéwitsch, who more than any one else has made a comparative study
of the entosternite, describes it as composed in these animals of two
lateral trabeculæ crossed by three transverse trabeculæ. I myself can
confirm his description, and give in Fig. 61, B, the appearance of the
entosternite of Thelyphonus or of Hypoctonus. The supra-oesophageal ganglia
and part of the infra-oesophageal ganglia fill up the space _Ph._;
stretching over the rest of the infra-oesophageal mass is a transverse
trabecula, which is very thin; then comes a space in which is seen the rest
of the infra-oesophageal mass, and then the posterior part of the plastron,
ventrally to which lies the commencement of the ventral nerve-cord.
[Illustration: FIG. 61.--A, ENTOSTERNITE OF LIMULUS; B, ENTOSTERNITE OF
THELYPHONUS.
_Ph._, position of pharynx.]
{144}In these forms, in which the central nervous system is more
concentrated towards the cephalic end than in Limulus, the whole of the
concentrated brain-mass is separated from the gut only by this thin
transverse band of tissue. Judging, then, from the entosternite of
Thelyphonus, it is not difficult to suppose that a continuation of the same
growth of the brain-region of the central nervous system would cause the
entosternite to be separated into two lateral trabeculæ, which would then
take up the ventro-lateral position of the two trabeculæ of Ammocoetes.
On the other hand, it might be that two lateral trabeculæ, similar to those
of Thelyphonus and situated on each side of the central nervous system,
were the original form from which, by the addition of transverse fibres
running between the gut and nervous system, the entosternite of Thelyphonus
and of the scorpions, etc., was formed. From an extensive consideration of
the entosternite in different animals, Schimkéwitsch has come to the
conclusion that this latter explanation is the true one. He points out that
the lateral trabeculæ can be distinguished from the transverse by their
structure, being much more cellular and less fibrous, and the cell-cavities
more rounded, or, as I should express it, the two lateral trabeculæ are
more cartilaginous, while the transverse are more fibrous. Schimkéwitsch,
from observations of structure and from embryological investigations, comes
to the conclusion that the entosternite was originally composed of two
parts--
1. A transverse muscle corresponding to the adductor muscle of the shell of
certain crustaceans, such as Nebalia.
2. A pair of longitudinal mesodermic tendons, which may have been formed
originally out of a number of segmentally arranged mesodermic tendons, and
are crossed by the fibrils of the transverse muscular bundles.
These paired tendons of the entosternite he considers to correspond to the
intermuscular tendons, situated lengthways, which are found in the ventral
longitudinal muscles of most arthropods.
It is clear from these observations of Schimkéwitsch, that the essential
part of the entosternite consists of two lateral trabeculæ, which were
originally tendinous in nature and have become of the nature of
cartilaginous tissue by the increase of cellular elements in the matrix of
the tissue: these two trabeculæ function as supports for the attachment of
muscles, which are specially attached at certain places. At these places
transverse fibres belonging to some {145}of the muscular attachments cross
between the two longitudinal trabeculæ, and so form the transverse
trabeculæ.
I entirely agree with Schimkéwitsch that the nests of cartilage-cells are
much more extensive in, and indeed nearly entirely confined to, these two
lateral trabeculæ in the entosternite of Hypoctonus. Ray Lankester
describes in the entosternite of Mygale peculiar cell-nests strongly
resembling those of Hypoctonus, and he also states that they are confined
to the lateral portions of the entosternite.
From this evidence it is easy to see that that portion of the basi-cranial
skeleton known as the trabeculæ may have originated from the formation of
cartilage in the plastron or entosternite of a palæostracan animal. Such an
hypothesis immediately suggests valuable clues as to the origin of the
cranium and of the rest of the basi-cranial skeleton--the parachordals and
the auditory capsules. The former would naturally be a dorsal extension of
the more membranous portion of the plastron, in which, equally naturally,
cartilaginous tissue would subsequently develop; and the reason why it is
impossible to reduce the cranium into a series of segments would be
self-evident, for even though, as Schimkéwitsch thinks, the plastron may
have been originally segmented, it has long lost all sign of segmentation.
The latter would be derived from a second entosternite of the same nature
as the plastron, but especially connected with the auditory apparatus of
the invertebrate ancestor. The following out of these two clues will be the
subject of a future chapter.
In our search, then, for a clue to the origin of the skeletal tissues of
the vertebrate we see again that we are led directly to the palæostracan
stock on the invertebrate side and to the Cyclostomata on that of the
vertebrate; for in Limulus, the only living representative of the
Palæostraca, and in Limulus alone, we find a skeleton marvellously similar
to the earliest vertebrate skeleton--that found in Ammocoetes. Later on I
shall give reasons for the belief that the earliest fishes so far found,
the Cephalaspidæ, etc., were built up on the same plan as Ammocoetes, so
that, in my opinion, in Limulus and in Ammocoetes we actually possess
living examples allied to the ancient fauna of the Silurian times.
{146}SUMMARY.
The skeleton considered in this chapter is not the notochord, but that
composed of cartilage. The tracing downwards of the vertebrate bony and
cartilaginous skeleton to its earliest beginnings leads straight to the
skeleton of the larval lamprey (Ammocoetes), in which vertebræ are not
yet formed, but the cranial and branchial skeleton is well marked.
The embryological and phylogenetic histories are in complete unison to
show that the cranial skeleton is older than the spinal, and this
primitive branchial skeleton is also in harmony with the laws of
evolution, in that its structure, even in the adult lamprey (Petromyzon),
never gets beyond the stage characteristic of embryonic cartilage in the
higher vertebrates.
The simplest and most primitive skeleton is that found in Ammocoetes and
consists of two parts: (1) a prosomatic, (2) a mesosomatic skeleton.
The prosomatic skeleton forms a non-segmented basi-cranial skeleton of
the simplest kind--the trabeculæ and the parachordals with their attached
auditory capsules, just as the embryology of the higher vertebrates
teaches us must be the case. There in the free-living, still-existent
Ammocoetes we find the manifest natural outcome of the embryological
history in the shape of simple trabeculæ and parachordals, from which the
whole complicated basi-cranial skeleton of the higher vertebrates arose.
The mesosomatic skeleton, which is formed before the prosomatic,
consisted, in the first instance, of simple branchial bars segmentally
arranged, which were connected together by a longitudinal subchordal bar,
situated laterally on each side of the notochord. These simple branchial
bars later on form the branchial basket-work, which forms an open-work
cage within which the branchiæ are situated.
The cartilages which compose these two skeletons respectively are
markedly different in chemical constitution, in that the first (hard
cartilage) is mainly composed of chondro-gelatin, the second (soft
cartilage) of chondro-mucoid material.
The same kind of difference is seen in the two kinds of connective tissue
which are the forerunners of these two kinds of cartilage. Thus, the
cranial walls in Ammocoetes are formed of white fibrous tissue, an
essentially gelatin-containing tissue; at transformation these are
invaded by chondro-blasts and the cartilaginous cranium, formed of hard
cartilage, results. On the other hand, the forerunner of the branchial
soft cartilage is a very striking and peculiar kind of connective tissue
loaded with mucoid material, to which the name muco-cartilage has been
given.
The enormous interest of this muco-cartilage consists in the fact that it
forms very well-defined plates of tissue, entirely confined to the
head-region, which are not found in any higher vertebrate, not even in
the adult form Petromyzon, for every scrap of the tissue as such
disappears at transformation.
It is this evidence of primitive non-vertebrate tissues, which occur in
the larval but not in the adult form, which makes Ammocoetes so valuable
for the investigation of the origin of vertebrates.
The evidence, then, is extraordinarily clear as to the beginnings of the
vertebrate skeletal tissues.
{147}In the invertebrate kingdom true cartilage occurs but scantily.
There is a cartilaginous covering of the brain of cephalopods. It is
never found in crabs, lobsters, bees, wasps, centipedes, butterflies,
flies, or any of the great group of Arthropoda, except, to a slight
extent, in some members of the scorpion group, and more fully in one
single animal, the King-crab or Limulus: a fact significant of itself,
but still more so when the nature of the cartilage and its position in
the animal is taken into consideration, for the identity both in
structure and position of this internal cartilaginous skeleton with that
of Ammocoetes is extraordinarily great.
Here, in Limulus, just as in Ammocoetes, an internal cartilaginous
skeleton is found, composed of two distinct parts: (1) prosomatic, (2)
mesosomatic. As in Ammocoetes, the latter consists of simple branchial
bars, segmentally arranged, which are connected together on each side by
a longitudinal ligament containing cartilage--the entapophysial ligament.
This cartilage is identical in structure and in chemical composition with
the soft cartilage of Ammocoetes, and, as in the latter case, arises in a
markedly mucoid connective tissue. The former, as in Ammocoetes, consists
of a non-segmental skeleton, the plastron, composed of a white fibrous
connective tissue matrix, an essentially gelatin-containing tissue, in
which are found nests of cartilage cells of the hard cartilage variety.
This remarkable discovery of the branchial cartilaginous bars of Limulus,
together with that of the internal prosomatic plastron, causes the
original difficulty of deriving an animal such as the vertebrate from an
animal resembling an arthropod to vanish into thin air, for it shows that
in the past ages when the vertebrates first appeared on the earth, the
dominant arthropod race at that time, the members of which resembled
Limulus, had solved the question; for, in addition to their external
chitinous covering, they had manufactured an internal cartilaginous
skeleton. Not only so, but that skeleton had arrived, both in structure
and position, exactly at the stage at which the vertebrate skeleton
starts.
What the precise steps are by which chitin-formation gives place to
chondrin-formation are not yet fully known, but Schmiedeberg has shown
that a substance, glycosamine, is derivable from both these skeletal
tissues, and he concludes his observations in the following words: "Thus,
by means of glycosamine, the bridge is formed which connects together the
chitin of the lower animals with the cartilage of the more highly
organized creations."
The evidence of the origin of the cartilaginous skeleton of the
vertebrate points directly to the origin of the vertebrate from the
Palæostraca, and is of so strong a character that, taken alone, it may
almost be considered as proof of such origin.
{148}CHAPTER IV
_THE EVIDENCE OF THE RESPIRATORY APPARATUS_
Branchiæ considered as internal branchial appendages.--Innervation of
branchial segments.--Cranial region older than spinal.--Three-root system
of cranial nerves, dorsal, lateral, ventral.--Explanation of van Wijhe's
segments.--Lateral mixed root is appendage-nerve of invertebrate.--The
branchial chamber of Ammocoetes.--The branchial unit, not a pouch but an
appendage.--The origin of the branchial musculature.--The branchial
circulation.--The branchial heart of the vertebrate.--Not homologous with
the systemic heart of the arthropod.--Its formation from two longitudinal
venous sinuses.--Summary.
The respiratory apparatus in all the terrestrial vertebrates is of the same
kind--one single pair of lungs. These lungs originate as a diverticulum of
the alimentary canal. On the other hand, the aquatic vertebrates breathe by
means of a series of branchiæ, or gills, which are arranged segmentally,
being supported by the segmental branchial cartilaginous bars, as already
mentioned in the last chapter.
The transition from the gill-bearing to the lung-bearing vertebrates is
most interesting, for it has been proved that the lungs are formed by the
modification of the swim-bladder of fishes; and in a group of fishes, the
Dipnoi, or lung-fishes, of which three representatives still exist on the
earth, the mode of transition from the fish to the amphibian is plainly
visible, for they possess both lungs and gills, and yet are not amphibians,
but true fishes. But for the fortunate existence of Ceratodus in Australia,
Lepidosiren in South America, and Protopterus in Africa, it would have been
impossible from the fossil remains to have asserted that any fish had ever
existed which possessed at the same moment of time the two kinds of
respiratory organs, although from our knowledge of the development of the
amphibian we might have felt sure that such a transitional stage must have
existed. Unfortunately, there is at present no likelihood of any
corresponding transitional stage being discovered {149}living on the earth
in which both the dorsal arthropod alimentary canal and the ventral
vertebrate one should simultaneously exist in a functional condition; still
it seems to me that even if Ceratodus, Lepidosiren, and Protopterus had
ceased to exist on the earth, yet the facts of comparative anatomy,
together with our conception of evolution as portrayed in the theory of
natural selection, would have forced us to conclude rightly that the
amphibian stage in the evolution of the vertebrate phylum was preceded by
fishes which possessed simultaneously lungs and gills.
In the preceding chapter the primitive cartilaginous vertebrate skeleton,
as found in Ammocoetes, was shown to correspond in a marvellous manner to
the cartilaginous skeleton of Limulus. In a later chapter I will deal with
the formation of the cranium from the prosomatic skeleton; in this chapter
it is the mesosomatic skeleton which is of interest, and the consideration
of the necessary consequences which logically follow upon the supposition
that the branchial cartilaginous bars of Limulus are homologous with the
branchial basket-work of Ammocoetes.
INTERNAL BRANCHIAL APPENDAGES.
Seeing that in both cases the cartilaginous bars of Limulus and Ammocoetes
are confined to the branchial region, their homology of necessity implies
an homology of the two branchial regions, and leads directly to the
conclusion that the branchiæ of the vertebrate were derived from the
branchiæ of the arthropod, a conclusion which, according to the generally
accepted view of the origin of the respiratory region in the vertebrate, is
extremely difficult to accept; for the branchiæ of Limulus and of the
Arthropoda in general are part of the mesosomatic appendages, while the
branchiæ of vertebrates are derived from the anterior part of the
alimentary canal. This conclusion, therefore, implies that the vertebrate
has utilized in the formation of the anterior portion of its new alimentary
canal the branchial appendages of the palæostracan ancestor.
{150}[Illustration: FIG. 62.--_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.]
Let us consider dispassionately whether such a suggestion is _a priori_ so
impossible as it at first appears. One of the principles of evolution is
that any change which is supposed to have taken place in the process of
formation of one animal or group of animals from a lower group must be in
harmony with changes which are known to have occurred in that lower group.
On the assumption, therefore, that the vertebrate branchiæ represent the
branchial portion of the arthropod mesosomatic appendages which have sunk
in and so become internal, we ought to find that in members of this very
group such inclusion of branchial appendages has taken place. This, indeed,
is exactly what we do find, for in all the scorpion tribe, which is
acknowledged to be closely related to Limulus, there are no external
mesosomatic appendages, but in all cases these appendages have sunk into
the body, have disappeared as such, and retained only the vital part of
them--the branchiæ. In this way the so-called lung-books of the scorpion
are formed, which are in all respects homologous with the branchiæ or
gill-books of Limulus. Now, as already mentioned, the lords of creation in
the palæostracan times were the sea-scorpions, which, as is seen in Fig.
62, resembled the land-scorpions of the present day in the entire absence
of any external appendages on the segments of the mesosomatic region. As
they lived in the sea, they must have breathed with gills, and those
branchial appendages must have been internal, just as in the land-scorpions
of the present time. Indeed, markings have been found on the internal side
of the segments 1-5, Fig. 62, which are supposed to indicate branchiæ, and
these segments are therefore supposed to have borne the branchiæ. Up to the
present time no indication of gill-slits has been found, and we cannot say
with certainty how these animals breathed. Further, in the Upper Silurian
of Lesmahago, Lanarkshire, a scorpion (_Palæophonus Hunteri_), closely
resembling the modern scorpion, has been found, which, as Lankester states,
was in all probability aquatic, and not terrestrial in its habits. How it
{151}breathed is unknown; it shows no signs of stigmata, such as exist in
the scorpion of to-day.
Although we possess as yet no certain knowledge of the position of the
gill-openings in these ancient scorpion-like forms, what we can say with
certainty--and that is the important fact--is, that at the time when the
vertebrates appeared, a very large number of the dominant arthropod race
possessed internally-situated branchiæ, which had been directly derived
from the branchiæ-bearing appendages of their Limulus-like kinsfolk.
This abolition of the branchiæ-bearing appendages as external organs of
locomotion, with the retention of the important branchial portion of the
appendage as internal branchiæ, is a very important suggestion in any
discussion of the way vertebrates have arisen from arthropods; for, if the
same principle is of universal application, it leads directly to the
conclusion that whenever an appendage possesses an organ of vital
importance to the animal, that organ will remain, even though the appendage
as such completely vanishes. Thus, as will be shown later, special
sense-organs such as the olfactory remain, though the animal no longer
possesses antennæ; the important excretory organs, the coxal glands, and
important respiratory organs, the branchiæ, are still present in the
vertebrate, although the appendages to which they originally belonged have
dwindled away, or, at all events, are no longer recognizable as arthropod
appendages.
INNERVATION OF BRANCHIAL SEGMENTS.
Passing from _a priori_ considerations to actual facts, it is advisable to
commence with the innervation of the branchial segments; for, seeing that
the foundation of the whole of this comparative study of the vertebrate and
the arthropod is based upon the similarity of the two central nervous
systems, it follows that we must look in the first instance to the
innervation of any organ or group of organs in order to find out their
relationship in the two groups of animals.
The great characteristic of the vertebrate branchial organs is their
segmental arrangement and their innervation by the vagus group of nerves,
_i.e._ by the hindermost group of the cranial segmental nerves. These
cranial nerves are divided by Gegenbaur into two great groups--an anterior
group, the trigeminal, which supplies the muscles of mastication, and a
posterior group, the vagus, which is essentially {152}respiratory in
function. Of these two groups, I will consider the latter group first.
In Limulus the great characteristic of the branchial region is its
pronounced segmental arrangement, each pair of branchial appendages
belonging to a separate segment. This group of segments forms the mesosoma,
and these branchial appendages are the mesosomatic appendages. Anterior to
them are the segments of the prosoma, which bear the prosomatic or
locomotor appendages. The latter are provided at their base with gnathites
or masticating apparatus, so that the prosomatic group of nerves, like the
trigeminal group in the vertebrate, comprises essentially the nerves
subserving the important function of mastication. As already pointed out,
the brain-region of the vertebrate is comparable to the supra-oesophageal
and infra-oesophageal ganglia of the invertebrate, and it has been shown
(p. 54) how, by a process of concentration and cephalization, the foremost
region of the infra-oesophageal ganglia becomes the prosomatic region, and
is directly comparable to the trigeminal region in the vertebrate; while
the hindermost region is formed from the concentration of the mesosomatic
ganglia, and is directly comparable to the medulla oblongata, _i.e._ to the
vagus region of the vertebrate brain.
As far, then, as concerns the centres of origin of these two groups of
nerves and their exits from the central nervous system, they are markedly
homologous in the two groups of animals.
COMPARISON OF THE CRANIAL AND SPINAL SEGMENTAL NERVES.
It has often been held that the arrangements of the vertebrate nervous
system differ from those of other segmented animals in one important
particular. The characteristic of the vertebrate is the origin of every
segmental nerve from two roots, of which one contains the efferent fibres,
while the other possesses a sensory ganglion, and contains only afferent
fibres. This arrangement, which is found along the whole spinal cord of all
vertebrates, is not found in the segmental nerves of the invertebrates; and
as it is supposed that the simpler arrangement of the spinal cord was the
primitive arrangement from which the vertebrate central nervous system was
built up, it is often concluded that the animal from which the vertebrate
arose must have possessed a series of nerve-segments, from each of which
there arose bilaterally ventral (efferent) and dorsal (afferent) roots.
{153}Now, the striking fact of the vertebrate segmental nerves consists in
this, that, as far as their structure and the tissues which they innervate
are concerned, the cranial segmental nerves are built up on the same plan
as the spinal; but as far as concerns their exit from the central nervous
system they are markedly different. A large amount of ingenuity, it is
true, has been spent in the endeavour to force the cranial nerves into a
series of segmental nerves, which arise in the same way as the spinal by
two roots, of which the ventral series ought to be efferent and the dorsal
series afferent, but without success. We must, therefore, consider the
arrangement of the cranial segmental nerves by itself, separately from that
of the spinal nerves, and the problem of the origin of the vertebrate
segmental nerves admits of two solutions--either the cranial arrangement
has arisen from a modification of the spinal, or the spinal from a
simplification of the cranial. The first solution implies that the spinal
cord arrangement is older than the cranial, the second that the cranial is
the oldest.
In my opinion, the evidence of the greater antiquity of the cranial region
is overwhelming.
The evidence of embryology points directly to the greater phylogenetic
antiquity of the cranial region, for we see how, quite early in the
development, the head is folded off, and the organs in that region thereby
completed at a time when the spinal region is only at an early stage of
development. We see how the first of the trunk somites is formed just
posteriorly to the head region, and then more and more somites are formed
by the addition of fresh segments posteriorly to the one first formed. We
see how, in Ammocoetes, the first formed parts of the skeleton are the
branchial bars and the basi-cranial system, while the rudiments of the
vertebræ do not appear until the Petromyzon stage. We see how, with the
elongation of the animal by the later addition of more and more spinal
segments, organs, such as the heart, which were originally in the head,
travel down, and the vagus and lateral-line nerves reach their ultimate
destination. Again, we see that, whereas the cranial nerves, viz. the
ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and vagus
nerves, are wonderfully fixed and constant in all vertebrates, the only
shifting being in the spino-occipital region, in fact, at the junction of
the cranial and spinal region, the spinal nerves, on the other hand, are
not only remarkably variable in number in different {154}groups of animals,
but that even in the same animal great variations are found, especially in
the manner of formation of the limb-plexuses. Such marked meristic
variation in the spinal nerves, in contrast to the fixed character of the
cranial nerves, certainly points to a more recent formation of the former
nerves.
Also the observations of Assheton on the primitive streak of the rabbit,
and on the growth in length of the frog embryo, have led him to the
conclusion that, as in the rabbit so in the frog, there is evidence to show
that the embryo is derived from two definite centres of growth: the first,
phylogenetically the oldest, being a protoplasmic activity, which gives
rise to the anterior end of the embryo; the second, one which gives rise to
the growth in length of the embryo. This secondary area of proliferation
coincides with the area of the primitive streak, and he has shown, in a
subsequent paper, by means of the insertion of sable hairs into the
unincubated blastoderm of the chick, that a hair inserted into the centre
of the blastoderm appears at the anterior end of the primitive streak, and
subsequently is found at the level of the most anterior pair of somites.
He then goes on to say--
"From these specimens it seems clear that all those parts in front of the
first pair of mesoblastic somites--that is to say, the heart, the brain and
medulla oblongata, the olfactory, optic, auditory organs and foregut--are
developed from that portion of the unincubated blastoderm which lies
anterior to the centre of the blastoderm, and that all the rest of the
embryo is formed by the activity of the primitive streak area."
In other words, the secondary area of growth, _i.e._ the primitive streak
area, includes the whole of the spinal cord region, while the older primary
centre of growth is coincident with the cranial region.
In searching, then, for the origin of the segmental nerves, we must
consider the type on which the cranial nerves are arranged rather than that
of the spinal nerves.
The first striking fact occurs at the spino-occipital region, where the
spinal cord merges into the medulla oblongata, for here in the cervical
region we find each spinal segment gives origin to three distinct roots,
not two--a dorsal root, a ventral root, and a lateral root. This third root
gives origin to the spinal accessory nerve, and in the region of the
medulla oblongata these lateral roots merge directly into the roots of the
vagus nerve; more anteriorly the same system {155}continues as the roots of
the glossopharyngeal nerve, as the roots of the facial nerve, and as a
portion, especially the motor portion, of the trigeminal nerve. Now, all
these nerves belong to a well-defined system of nerves, as Charles Bell[1]
pointed out in 1830, a system of nerves concerned with respiration and
allied mechanisms, such as laughing, sneezing, mastication, deglutition,
etc., nerves innervating a set of muscles of very different kind from the
ordinary body-muscles concerned with locomotion and equilibration. Also the
centres from which these motor nerves arise are well defined, and form
cell-masses in the central nervous system, quite separate from those which
give origin to somatic muscles.
This original idea of Charles Bell, after having been ignored for so long a
time, is now seen to be a very right one, and it is an extraordinary thing
that his enunciation of the dual nature of the spinal roots, which was, to
his mind, of subordinate importance, should so entirely have overshadowed
his suggestion, that in addition to the dorsal and ventral roots, a lateral
system of nerves existed, which were not exclusively sensory or exclusively
motor, but formed a separate system of respiratory nerves.
Further, anatomists divide the striated muscles of the body into two great
natural groups, characterized by a difference of origin and largely by a
difference of appearance. The one set is concerned with the movements of
internal organs, and is called visceral, the other is derived from the
longitudinal sheet of musculature which forms the myotomes of the fish, and
has been called parietal or somatic. The motor nerves of these two sets of
muscles correspond with the lateral or respiratory and ventral roots
respectively.
Finally, it has been shown that the segments of which a vertebrate is
composed are recognizable in the embryo by the segmented manner in which
the musculature is laid down, and van Wijhe has shown that in the cranial
region two sets of muscles are laid down segmentally, thus forming a dorsal
and ventral series of commencing muscular segments. Of these the anterior
segments of the dorsal series give origin to the striated muscles of the
eye which are innervated by the IIIrd (oculomotor), IVth (trochlearis), and
VIth (abducens) nerves, while the posterior segments give origin to the
{156}muscles from the cranium to the shoulder-girdle, innervated by the
XIIth (hypoglossal) nerve. The ventral series of segments give origin to
the musculature supplied by the trigeminal, facial, glossopharyngeal, and
vagus nerves.
Also, the afferent or sensory nerves of the skin over the whole of this
head-region are supplied by the trigeminal nerve, while the afferent nerves
to the visceral surfaces are supplied by the vagus, glossopharyngeal and
facial nerves.
In van Wijhe's original paper he arranged the segments belonging to the
cranial nerves in the following table:--
---------+---------------------------------
Segments.| Ventral nerve-roots and muscles
| derived from myotomes.
---------+---------+-----------------------
1 | III. |M. rectus superior,
| | m. rectus
| | internus, m.
| | rectus inferior,
| | m. obliquus inferior
2 | IV. |M. obliquus
| | superior
3 | VI. |M. rectus externus
4 | -- | --
5 | -- | --
6 | -- | --
7 | XII. |} Muscles from
8 | XII. |} cranium to
9 | XII. |}shoulder-girdle
---------+---------+-----------------------
---------+------------------+--------------------------------
Segments.| Visceral clefts.| Dorsal nerve-roots and muscles.
| |
---------+------------------+--------------+-----------------
1 | | V. N. op- |
| | thalmicus |
| | profundus |
2 | | V. | Masticating
| 1st Mandibular | | muscles.
3 | | VII._1 |{Facial muscles
| | |{(VIII. is dorsal
| 2nd {Hyoid_1 | VII._2 |{branch of VII.)
4 | {Hyoid_2 | |
5 |3rd 1st Branchial | IX. |
6 |4th 2nd " | X._1 |
7 |{5th 3rd " | X._2 |Branchial and
8 |{6th 4th " | X._3} |visceral muscles.
9 |{7th 5th " | X._4 |
---------+------------------+--------------+-----------------
As is seen in the table, van Wijhe attempts to arrange the cranial
segmental nerves into dorsal and ventral roots, in accordance with the
arrangement in the spinal region. In order to do this he calls the Vth,
VIIth, IXth, and Xth nerves dorsal roots, although they are not purely
sensory nerves, but contain motor fibres as well.
It is not accidental that he should have picked out for his dorsal roots
the very nerves which form Charles Bell's lateral series of roots, inasmuch
as this system of lateral roots, apart from dorsal and ventral roots,
really is, as Charles Bell thought, an important separate system, dependent
upon a separate segmentation in the embryo of the musculature supplied by
these roots. This segmentation may receive the name of _visceral_ or
_splanchnic_ in contradistinction to _somatic_, since all the muscles
without exception belong to the visceral group of striated muscles.
{157}These observations of van Wijhe lead directly to the following
conclusion. In the cranial region there is evidence of a double set of
segments, which may be called somatic and splanchnic. The somatic segments,
consisting of the outer skin and the body musculature, are _doubly_
innervated as are those of the spinal cord by a series of ventral motor
roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the
abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series
of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve.
But the splanchnic segments are innervated by _single_ roots, the vagus or
Xth nerve, glossopharyngeal or IXth nerve, facial or VIIth nerve, and
trigeminal or Vth nerve, which are mixed, containing both sensory and motor
fibres, thus differing markedly from the arrangement of the spinal nerves.
From this sketch it follows that the arrangement seen in the spinal cord,
would result from the cranial arrangement if this third system of lateral
roots were left out. Further, since the cranial system is the oldest, we
must search in the invertebrate ancestor for a tripartite rather than a
dual system of nerve-roots for each segment; a system composed of a dorsal
root supplying only the sensory nerves of the skin-surfaces, a lateral
mixed root supplying the system connected with respiration with both
sensory and motor fibres, and a ventral root supplying the motor nerves to
the body-musculature.
COMPARISON OF THE APPENDAGE NERVES OF LIMULUS AND BRANCHIPUS TO THE LATERAL
ROOT SYSTEM OF THE VERTEBRATE.
If the argument used so far is correct, and this tripartite system of
nerve-roots, as seen in the cranial nerves of the vertebrate, really
represents the original scheme of innervation in the palæostracan ancestor,
then it follows that each segment of Limulus ought to be supplied by three
nerves--(1), a sensory nerve supplying its own portion of the skin-surface
of the prosomatic and mesosomatic carapaces; (2), a lateral mixed nerve
supplying exclusively the appendage of the segment, for the appendages
carry the respiratory organs; and (3), a motor nerve supplying the
body-muscles of the segment.
It is a striking fact that Milne-Edwards describes the nerve-roots in
exactly this manner. The great characteristic of the nerve-roots {158}in
Limulus as in other arthropods is the large appendage-nerve, which is
always a mixed nerve; in addition, there is a system of sensory nerves to
the prosomatic and mesosomatic carapaces, called by him the epimeral
nerves, which are purely sensory, and a third set of roots which are motor
to the body-muscles, and possibly also sensory to the ventral surface
between the appendages.
Moreover, just as in the vertebrate central nervous system the centres of
origin of the motor nerves of the branchial segmentation are distinct from
those of the somatic segmentation, so we find, from the researches of
Hardy, that a similar well-marked separation exists between the centres of
origin of the motor nerves of the appendages and those of the somatic
muscles in the central nervous system of Branchipus and Astacus.
In the first place, he points out that the nervous system of Branchipus is
of a very primitive arthropod type; that it is, in fact, as good an example
of an ancient type as we are likely to find in the present day; a matter of
some importance in connection with my argument, since the arthropod
ancestor of the vertebrate, such as I am deducing from the study of
Ammocoetes, must undoubtedly have been of an ancient type, more nearly
connected with the strange forms of the trilobite era than with the crabs
and spiders of the present day.
His conclusions with respect to Branchipus may be tabulated as follows:--
1. Each ganglion of the ventral chain is formed mainly for the innervation
of the appendages.
2. Each ganglion is divided into an anterior and posterior division, which
are connected respectively with the motor and sensory nerves of the
appendages.
3. The motor nerves of the appendages arise as well-defined axis-cylinder
processes of nerve-cells, which are arranged in well-defined groups in the
anterior division of the ganglion.
4. A separate innervation exists for the muscles and sensory surfaces of
the trunk. The trunk-muscles consist of long bundles, from which slips pass
off to the skin in each segment; they are thus imperfectly segmented. In
accordance with this, a diffuse system of nerve-fibres passes to them from
certain cells on the dorsal surface of each lateral half of the ganglion.
These cell-groups are therefore very distinct from those which give origin
to the motor {159}appendage-nerves, and, moreover, are not confined to the
ganglion, but extend for some distance into the interganglionic region of
the nerve-cords which connect together the ganglia of the ventral chain.
Hardy's observations, therefore, combined with those of Milne-Edwards, lead
to the conclusion that in such a primitive arthropod type as my theory
postulates, each segment was supplied with separate sensory and motor
somatic nerves, and with a pair of nerves of mixed function, devoted
entirely to the innervation of the pair of appendages; that also, in the
central nervous system, the motor nerve-centres were arranged in accordance
with a double set of segmented muscles in two separate groups of
nerve-cells. These nerve-cells in the one case were aggregated into
well-defined groups, which formed the centres for the motor nerves of the
markedly segmented muscles of the appendages, and in the other case formed
a system of more diffused cells, less markedly aggregated into distinct
groups, which formed the centres for the imperfectly segmented somatic
muscles.
Such an arrangement suggests that in the ancient arthropod type a double
segmentation existed, viz. a segmentation of the body, and a segmentation
due to the appendages. Undoubtedly, the segments originally corresponded
absolutely as in Branchipus, and every appendage was attached to a
well-defined separate body-segment. In, however, such an ancient type as
Limulus, though the segmentation may be spoken of as twofold, yet the
number of segments in the prosomatic and mesosomatic regions are much more
clearly marked out by the appendages than by the divisions of the soma;
for, in the prosomatic region such a fusion of somatic segments to form the
tergal prosomatic carapace has taken place that the segments of which it is
composed are visible only in the young condition, while in the mesosomatic
region the separate somatic segments, though fused to form the mesosomatic
carapace, are still indicated by the entapophysial indentations.
Clearly, then, if the mesosomatic branchial appendages of forms related to
Limulus were reduced to the branchial portion of the appendage, and that
branchial portion became internal, just as is known to be the case in the
scorpion group, we should obtain an animal in which the _mesosomatic
region_ would be characterized by a segmentation predominantly branchial,
which might be termed, as in vertebrates, the _branchiomeric segmentation_,
but yet would show {160}indications of a corresponding somatic or
_mesomeric segmentation_. The nerve supply to these segments would consist
of--
1. The epimeral purely sensory nerves to the somatic surface, equivalent
in the vertebrate to the ascending root of the trigeminal.
2. The mixed nerves to the internal branchial segments, equivalent in the
vertebrate to the vagus, glossopharyngeal, and facial.
3. The motor nerves to the somatic muscles, equivalent in the vertebrate
to the original nerve-supply to the somatic muscles belonging to these
segments, _i.e._ to the muscles derived from van Wijhe's 4th, 5th, and 6th
somites.
Further, the centres of origin of these appendage-nerves would form centres
in the central nervous system separate from the centres of the motor nerves
to the somatic muscles, just as the centres of origin of the motor parts of
the facial, vagus, and glossopharyngeal nerves form groups of cells quite
distinct from the centres for the hypoglossal, abducens, trochlear, and
oculomotor nerves.
In fact, if the vertebrate branchial nerves are looked upon as the
descendants of nerves which originally supplied branchial appendages, then
every question connected with the branchial segmentation, with the origin
and distribution of these nerves, receives a simple and adequate
solution--a solution in exact agreement with the conclusion that the
vertebrate arose from a palæostracan ancestor.
It would, therefore, be natural to expect that the earliest fishes breathed
by means of branchial appendages situated internally, and that the evidence
for such appendages would be much stronger in them than in more recent
fishes.
Although we know nothing of the nature of the respiratory apparatus in the
extinct fishes of Silurian times, we have still living, in the shape of
Ammocoetes, a possible representative of such types. If, then, we find, as
is the case, that the respiratory apparatus of Ammocoetes differs markedly
from that of the rest of the fishes, and, indeed, from that of the adult
form or Petromyzon, and that that very difference consists in a greater
resemblance to internal branchial appendages in the case of Ammocoetes,
then we may feel that the proof of the origin of the branchial apparatus of
the vertebrate from the internal branchial appendages of the invertebrate
has gained enormously.
{161}THE RESPIRATORY CHAMBER OF AMMOCOETES.
In order to make clear the nature of the branchial segments in Ammocoetes,
I have divided the head-part of the animal by means of a longitudinal
horizontal section into halves--ventral and dorsal--as shown in Figs. 63
and 64. These figures are each a combination of a section and a solid
drawing. The animal was slit open by a longitudinal section in the
neighbourhood of the gill-slits, and each half was slightly flattened out,
so as to expose the ventral and dorsal internal surfaces respectively. The
structures in the cut surface were drawn from one of a series of horizontal
longitudinal sections taken through the head of the animal. These figures
show that the head-region of Ammocoetes consists of two chambers, the
contents of which are different. In front, an oral or stomodæal chamber,
which contains the velum and tentacles, is enclosed by the upper and lower
lips, and was originally separated by a septum from the larger respiratory
chamber, which contains the separate pairs of branchiæ. A glance at the two
drawings shows clearly that Rathke's original description of this chamber
is the natural one, for he at that time, looking upon _Ammocoetes
branchialis_ as a separate species, described the branchial chamber as
containing a series of paired gills, with the gill-openings between
consecutive gills. His branchial unit or gill, therefore, was represented
by each of the so-called diaphragms, which, as seen in Figs. 63, 64, are
all exactly alike, except the first and the last. Any one of these is
represented in section in Fig. 65, and represents a branchial unit in
Rathke's view and in mine. Clearly, it may be described as a branchial
appendage which projects into an open pharyngeal chamber, so that the
series of such appendages divides the chamber into a series of
compartments, each of which communicates with the exterior by means of a
gill-slit, and with each other by means of the open space between opposing
appendages.
Each of these appendages possesses its own cartilaginous bar (_Br. cart._),
as explained in Chapter III.; each possesses its own branchial or visceral
muscles (coloured blue in Figs. 63 and 64), separated absolutely from the
longitudinal somatic muscles (coloured dark red in Figs. 63 and 64) by a
space (_Sp._) containing blood and peculiar fat-cells, etc. Each possesses
its own afferent branchial blood-vessel from the ventral aorta, and its own
efferent vessel to the dorsal aorta (Fig. 65, _a. br._ and _v. br._). Each
possesses its own segmental nerve, which supplies its own branchial muscles
and no others with motor fibres, and sends sensory fibres to the general
surface of each appendage, as also to the special sense-organs in the shape
of the epithelial pits (_S._, Fig. 65) arranged along the free edges of the
diaphragms; each of these nerves possesses its own ganglion--the
epibranchial ganglion.
{162}[Illustration: FIG. 63.--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.]
{163}[Illustration: FIG. 64.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.
_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._,
median serrated flange of velar folds.]
{164}[Illustration: FIG. 65.--SECTION THROUGH BRANCHIAL APPENDAGE OF
AMMOCOETES.
_br. cart._, branchial cartilage; _v. br._, branchial vein; _a. br._,
branchial artery; _b.s._, blood-spaces; _p._, pigment; _S._, sense-organ;
_c._, ciliated band; _E., I._, external and internal borders; _m. add._,
adductor muscle; _m.c.s._, striated constrictor muscle; _m.c.t._, tubular
constrictor muscle; _m._ and _m.v._, muscles of valve.]
[Illustration: FIG. 66.--SECTION THROUGH BRANCHIAL APPENDAGE OF LIMULUS.
_br. cart._, branchial cartilage; _v.br._, branchial vein; _b.s._,
blood-spaces formed by branchial artery; _P._, pigment; _m_1_, posterior
entapophysio-branchial muscle; _m_2_, anterior entapophysio-branchial
muscle; _m_3_, external branchial muscle.]
The work of Miss Alcock has shown that the segmental branchial nerve
supplies solely and absolutely such an appendage or branchial {165}segment,
and does not supply any portion of the neighbouring branchial segments. The
nerve-supply in Ammocoetes gives no countenance to the view that the
original unit was a branchial pouch, the two sides of which each nerve
supplied, but is strong evidence that the original unit was a branchial
appendage, which was supplied by a _single_ nerve with both motor and
sensory fibres.
Any observer having before him only this picture of the respiratory chamber
of Ammocoetes, upon which to base his view of a vertebrate respiratory
chamber, would naturally look upon the branchial unit of a vertebrate as a
gilled appendage projecting into the open cavity of the anterior part of
the alimentary canal or pharynx. This is not, however, the usual
conception. The branchial unit is ordinarily described as a gill-pouch,
which possesses two openings or slits, an internal one into the lumen of
the alimentary canal, and an external one into the surrounding medium. This
view is based upon embryological evidence of the following character:--
The alimentary canal of all vertebrates forms a tube stretching the whole
length of the animal; the anterior part of this tube becomes pouched on
each side at regular intervals, and the walls of each pouch becoming folded
form the respiratory surfaces or gills. The openings of these separate
pouches into the central lumen of the gut form the internal gill-pouch
openings; the other extremity of the pouch approaches the external surface
of the animal, and finally breaks through to form a series of external
gill-pouch openings.
From the mesoblastic tissue, between each gill-pouch, there is formed a
supporting cartilaginous bar, to which are attached a system of branchial
muscles, with their nerves and blood-vessels. These cartilaginous bars, in
all fishes above the Cyclostomata, form a supporting framework for the
internal gill-slit, so that the gills are situated externally to them; the
more primitive arrangement is, as already mentioned, a system of
cartilaginous bars, extra-branchial in position, so that the gills are
situated internally to them.
From this description of the mode of formation of the respiratory apparatus
in water-breathing vertebrates the conception has arisen of the gill-pouch
as the branchial unit, a conception which is absolutely removed from all
idea of a branchial unit such as is found in an arthropod, viz. an
appendage.
This conception of spaces as units pervades the whole of embryology, and is
the outcome of the gastrula theory--a theory which {166}teaches that all
animals above the Protozoa are derived from a form which by invagination of
its external surface formed an internal cavity or primitive gut. From
pouches of this gut other cavities were said to be formed, called coelomic
cavities, and thus arose the group of coelomatous animals. To speak of the
developmental history of animals in terms of spaces; to speak of the
atrophy of a cavity as though such a thing were possible, is, to my mind,
the wrong way of looking at the facts of anatomy. It resembles the
description of a net as a number of holes tied together with string, which
is not usually considered the best method of description.
There are two ways in which a series of pouches can be formed from a simple
tube without folding, either by a thinning at regular intervals of the
original tissue surrounding the tube, or by the ingrowth into the tube of
the surrounding tissue at regular intervals, thus--
[Illustration: FIG. 67.--DIAGRAMS TO SHOW THE TWO METHODS OF
POUCH-FORMATION.
A, by the thinning of the mesoblast at intervals. B, by the ingrowth of
mesoblast at intervals. _Ep._, epiblast; _Mes._, mesoblast; _Hy._,
hypoblast.]
In the first case (A) the formation of a pouch is the significant act, and
therefore the branchial segments might be expressed in terms of pouches. In
the second case (B) the formation of a pouch is {167}brought about in
consequence of the ingrowth of the mesoblastic tissues at intervals; here,
although the end-result is the same as in the first case, the
pouch-formation is only secondary, the true branchial unit is the
mesoblastic ingrowth.
The evidence all points directly to the second method of formation. Thus
Shipley, in his description of the development of the lamprey, says--
"The gill-slits appear to me to be the result of the ventral downgrowth of
mesoblast taking place only at certain places, these forming the gill-bars.
Between each downgrowth the hypoblastic lining of the alimentary canal
remains in contact with the epiblast; here the gill-opening subsequently
appears about the twenty-second day."
Dohrn describes and gives excellent pictures of the growth of the
diaphragms, as the Ammocoetes grows in size, pictures which are distinctly
reminiscent of the corresponding illustrations given by Brauer of the
growth of the internal gills in the scorpion embryo.
Another piece of evidence confirmatory of the view that the branchial
segments are really of the nature of internal appendages, as the result of
which gill-pouches are formed, is given by the presence in each of these
branchial bars or diaphragms of a separate coelomic cavity. From the walls
of this cavity the branchial muscles and cartilaginous bar are formed.
Now, from an embryological point of view, the vertebrate shows that it is a
segmented animal by the formation of somites, which consist of a series of
divisions of the coelom, of which the walls form a series of muscular and
skeletal segments. In the head-region, as already mentioned, such coelomic
divisions form two rows--a dorsal and a ventral set. From the walls of the
dorsal set the somatic musculature is formed. From those of the ventral set
the branchial musculature. From the latter also the branchial cartilaginous
bars are formed. Thus Shipley, in his description of the development of the
lamprey, says: "The mesoblast between the gills arranges itself into
head-cavities, and the walls of these cavities ultimately form the skeleton
of the gill-arches."
Similarly, in the arthropod, the segments in the embryo are marked out by a
series of coelomic cavities and Kishinouye has described in Limulus a
separate coelomic cavity for every one of the mesosomatic or branchial
segments, and he states that in Arachnida {168}the segmental coelomic
cavities extend into the limbs. These cavities both in the vertebrate and
in the arthropod disappear before the adult condition is reached.
The whole evidence thus points strongly to the conclusion that the true
branchial segmental units are the branchial bars or diaphragms, not the
pouches between them.
It is possible to understand why such prominence has been given to the
conception of the branchial unit as a gill-pouch rather than as a
gill-appendage, when the extraordinary change of appearance in the
respiratory chamber of the lamprey which occurs at transformation, is taken
into consideration. This change is of a very far-reaching character, and
consists essentially of the formation of a new alimentary canal in this
region, whereby the pharyngeal chamber of Ammocoetes is cut off posteriorly
from the alimentary canal, and is confined entirely to respiratory
purposes, its original lumen now forming a tube called the bronchus, which
opens into the mouth and into a series of branchial pouches.
In Fig. 68 I give diagrammatic illustrations taken from Nestler's paper to
show the striking change which takes place at transformation, (A)
representing three branchial segments of Ammocoetes, and (B) the
corresponding three segments of Petromyzon. The corresponding parts in the
two diagrams are shown by the cartilages (_br. cart._), the sense-organs
(S), and the branchial veins (_V. br._); the corresponding diaphragms are
marked by the figures 1, 2, 3 respectively. As is clearly seen, it is
perfectly possible in the latter case to describe the respiratory chamber,
as Nestler has done, as divided into a series of separate smaller
chambers--the gill-pouches--by means of a series of diaphragms or branchial
bars. The surface of these gill-pouches is in part thrown into folds for
respiratory purposes, and each gill-pouch opens, on the one hand, into the
bronchus (_Bro._), and, on the other, to the exterior by means of the
gill-slit. The branchial unit in Petromyzon is, therefore, according to
Nestler and other morphologists, the folded opposed surfaces of two
contiguous diaphragms, and each one of the diaphragms is intersegmental
between two gill-pouches.
Nestler then goes on to describe the arrangement in Ammocoetes in the same
terms, although there is no bronchus or gill-pouch, but only an open
chamber into which these gill-bearing diaphragms project, which open
chamber serves both for the passage of food and {169}of the water for
respiration. This is manifestly the wrong way to look at the matter: the
adult form is derived from the larval, not _vice versâ_, and the
transformation process shows exactly how the gills, in Rathke's sense, come
together to form the bronchus and so make the gill-pouches of Petromyzon.
When we bear in mind that almost all observers consider that the internal
branchiæ of the scorpion group are directly derived from branchial
appendages of a kind similar to those of Limulus, it is evident that a
branchial appendage such as that of Ammocoetes might also have arisen from
such an appendage, because in various respects it is easier to compare the
branchial appendage of Ammocoetes, than that of the scorpion group, with
that of Limulus.
[Illustration: FIG. 68.--DIAGRAM OF THREE BRANCHIAL SEGMENTS OF AMMOCOETES
(A) COMPARED WITH THREE BRANCHIAL SEGMENTS AFTER TRANSFORMATION (B) TO SHOW
HOW THE BRANCHIAL APPENDAGES OF AMMOCOETES FORM THE BRANCHIAL POUCHES OF
PETROMYZON. (After NESTLER.)
In both figures the branchial cartilages (_br. cart._), the branchial view
(_V. br._), and the sense-organs (_S_), are marked out in order to show
corresponding points. The muscles, blood-spaces, branchial arteries, etc.,
of each branchial segment are not distinguished, being represented a
uniform black colour. _Bro._, the bronchus into which each gill-pouch
opens.]
In the case of the scorpions, various suggestions have been made as to the
manner in which such a conversion may have taken place. The most probable
explanation is that given by Macleod, in which {170}each of the branchiæ of
the scorpion group is directly compared with the branchial part of the
Limulus appendage which has sunk into and amalgamated with the ventral
surface.
According to this view, the modification which has taken place in
transforming the branchial Limulus-appendage into the branchial
scorpion-appendage is a further stage of the process by which the Limulus
branchial appendage itself has been formed, viz. the getting rid of the
free locomotor segments of the original appendage, thus confining the
appendage more and more to the basal branchial portion. So far has this
process been carried in the scorpion that all the free part of the
appendage has disappeared; apparently, also, the intrinsic muscles of the
appendage have vanished, with the possible exception of the post-stigmatic
muscle, so that any direct comparison between the branchial appendages of
Limulus and the scorpions is limited to the comparison of their branchiæ,
their nerves, and their afferent and efferent blood-vessels.
In the case of Ammocoetes the comparison must be made not with
air-breathing but with water-breathing scorpions, such as existed in past
ages in the forms of Eurypterus, Pterygotus, Slimonia, and with the crowd
of trilobite and Limulus-like forms which were in past ages so predominant
in the sea; forms in some of which the branchial appendages had already
become internal, but which, from the very fact of these forms being
water-breathers, probably resembled, in respect of their respiratory
apparatus, Limulus rather than the present-day scorpion.
On the assumption that the branchial appendages of Ammocoetes, like the
branchial appendages of the scorpion group, are to a certain extent
comparable with those of Limulus, it becomes a matter of great interest to
inquire whether the mode in which respiration is effected in Ammocoetes
resembles most that of Limulus or of the scorpion.
THE ORIGIN OF THE BRANCHIAL MUSCULATURE.
The difference between the movements of respiration in Limulus and those of
the scorpions consists in the fact that, although in both cases respiration
is effected mainly by dorso-ventral muscles, these muscles are not
homologous in the two cases: in the former, the dorso-ventral
appendage-muscles are mainly concerned, in the latter, the dorso-ventral
somatic muscles.
{171}The paper by Benham gives a full description of the musculature of
Limulus, and according to his arrangement the muscles are divided into two
sets, longitudinal and dorso-ventral. Of the latter, each mesosomatic
segment possesses a pair of dorso-ventral muscles, attached to the
mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig. 58,
_Dv._). These muscles are called by Benham the vertical mesosomatic
muscles. I shall call them the somatic dorso-ventral muscles, in
contradistinction to the dorso-ventral muscles of the branchial appendages.
Of the latter, the two chief are the external branchial (Fig. 66, _m_3_)
and the posterior entapophysio-branchial (Fig. 66, _m_1_); a third muscle
is the anterior entapophysio-branchial (Fig. 66, _m_2_). Of these muscles,
the posterior entapophysio-branchial (_m_1_) is closely attached along the
branchial cartilaginous bar up to its round-headed termination on the
anterior surface of the appendage. The anterior entapophysio-branchial
muscle (_m_2_) is attached to the branchial cartilage near the
entapophysis.