started as an olfactory tube on the ventral side of the animal, which led
directly up to, and probably into, the oesophagus of the original
alimentary canal of the palæostracan ancestor.
Strikingly enough, although in the crustaceans the first pair of antennæ
form the olfactory organs, no such free antennæ are found in the
arachnids, but they have amalgamated to form a tube or olfactory passage,
which leads directly into the mouth and oesophagus of the animal.
This olfactory passage is very conspicuous in all members of the scorpion
group, and, like the olfactory tube of the vertebrate, is innervated by a
pair of nerves, which resemble those supplying the first pair of antennæ
in crustaceans as to their origin from the supra-oesophageal ganglia.
This nasal passage, or tube of the hypophysis, corresponds in structure
and in position most closely with the olfactory tube of the scorpion
group, the only difference being that in the latter case it opens
directly into the oesophagus, while in the former, owing to the closure
of the old mouth, it cannot open into the infundibulum.
The evidence of the olfactory apparatus, combined with that of the optic
apparatus, is most interesting, for, whereas the former points
indubitably to an ancestor having scorpion-like affinities, the structure
of the lateral eyes points distinctly to crustacean, as well as arachnid,
affinities.
Taking the two together the evidence is extraordinarily strong that the
vertebrate arose from a member of the palæostracan group with marked
scorpion-like affinities.
{233}CHAPTER VII
_THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES_
Comparison of the trigeminal with the prosomatic region.--The prosomatic
appendages of the Gigantostraca.--Their number and nature.--Endognaths
and ectognath.--The metastoma.--The coxal glands.--Prosomatic region of
Eurypterus compared with that of Ammocoetes.--Prosomatic segmentation
shown by muscular markings on carapace.--Evidence of coelomic cavities in
Limulus.--Summary.
The derivation of the olfactory organs of the vertebrate from the olfactory
antennæ of the arthropod in the last chapter is confirmatory proof of the
soundness of the proposition put forward in Chapter IV., that the
segmentation in the cranial region of the vertebrate was derived from that
of the prosomatic and mesosomatic regions of the palæostracan ancestor.
Such a segmentation implies a definite series of body-segments,
corresponding to the mesomeric segmentation of the vertebrate, and a
definite series of appendages corresponding to the splanchnic segmentation
of the vertebrate.
Of the foremost segments belonging to the supra-oesophageal region
characterized by the presence of the median eyes, of the lateral eyes, and
of the olfactory organs, a wonderfully exact replica has been shown to
exist in the pineal eyes, the lateral eyes, and the olfactory organ of the
vertebrate, belonging, as they all do, to the supra-infundibular region.
Of the infra-oesophageal segments belonging to the prosoma and mesosoma
respectively, the correspondence between the mesosomatic segments carrying
the branchial appendages and the uterus, with those in the vertebrate
carrying the branchiæ and the thyroid gland respectively, has been fully
proved in previous chapters.
There remain, then, only the segments of the prosomatic region to be
considered, a region which, both in the vertebrate and invertebrate, is
never respiratory in function but always masticatory, such {234}mastication
being performed in Limulus and its allies by the muscles which move the
foot-jaws or gnathites, which are portions of the prosomatic appendages
specially modified for that purpose, and in the vertebrates by the
masticatory muscles, which are always innervated by the trigeminal or Vth
cranial nerve. This comparison implies that the motor part of the
trigeminal nerve originally supplied the prosomatic appendages.
The investigations of van Wijhe and of all observers since the publication
of his paper prove that in this trigeminal region, as in the vagus region,
a double segmentation exists, of which the ventral or splanchnic segments,
corresponding to the appendages in the invertebrate, are supplied by the
trigeminal nerves, while the dorsal or somatic segments, corresponding to
the somatic segments in the invertebrate, are supplied by the IIIrd or
oculomotor and the IVth or trochlear nerves--nerves which supply muscles
moving the lateral eyes.
In accordance, then, with the evidence afforded by the nerves of the
branchial segments, it follows that the muscles supplied by the motor part
of the trigeminal ought originally to have moved the appendages belonging
to a series of prosomatic segments. On the other hand, the eye-muscles
ought to have belonged to the body-part of the prosomatic segments, and
must therefore have been grouped originally in a segmental series
corresponding to the prosomatic appendages.
The evidence for and against this conclusion will be the subject of
consideration in this and the succeeding chapters. At the outset it is
evident that any such comparison necessitates an accurate knowledge of the
number of the prosomatic segments in the Gigantostraca and of the nature of
the corresponding appendages.
In all this group of animals, the evidence as to the number of segments in
either the prosomatic or mesosomatic regions is given by--
1. The number of appendages.
2. The segmental arrangement of the muscles of the prosoma or mesosoma
respectively.
3. The segmental arrangement of the coelomic or head-cavities.
4. The divisions of the central nervous system, or neuromeres, together
with their outgoing segmental nerves.
It follows, therefore, that if from any cause the appendages are not
apparent, as is the case in many fossil remains, or have dwindled {235}away
and become insignificant, we still have the muscular, coelomic, and nervous
arrangements left to us as evidence of segmentation in these animals, just
as in vertebrates.
In this prosomatic region, we find in Limulus the same tripartite division
of the nerves as in the mesosomatic region, so that the nerves to each
segment may be classed as (1) appendage-nerve; (2) sensory or dorsal
somatic nerve, supplying the prosomatic carapace; (3) motor or ventral
somatic nerve, supplying the muscles of the prosoma, and containing
possibly some sensory fibres. The main difference between these two regions
in Limulus consists in the closer aggregation of the prosomatic nerves,
corresponding to the concentration of the separate ganglia of origin in the
prosomatic region of the brain.
The number of prosomatic segments in Limulus is not evident by examination
of the prosomatic carapace, so that the most reliable guide to the
segmentation of this region is given by the appendages, of which one pair
corresponds to each prosomatic segment.
The number of such segments, according to present opinion, is seven,
viz.:--
(1) The foremost segment, which bears the cheliceræ.
(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor
appendages; and
(7) The last segment, to which belongs a small abortive pair of appendages,
known by the name of the chilaria, situated between the last pair of
locomotor appendages and the operculum or first pair of mesosomatic
appendages. These appendages are numbered from 1-7 in the accompanying
drawing (Fig. 103).
Of these seven pairs of appendages, the significance of the first and the
last has been matter of dispute. With respect to the first pair, or the
cheliceræ, the question has arisen whether their nerves belong to the
infra-oesophageal group, or are in reality supra-oesophageal.
It is instructive to observe the nature and the anterior position of this
pair of appendages in the allied sea-scorpions, especially in Pterygotus,
where the only chelate organs are found in these long, antennæ-like
cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to
be represented by the small non-chelate antennæ seen in Fig. 8, B and C (p.
27), taken from Woodward. If such is the case, then these figures show that
a pair of appendages is missing in each {236}of these forms, for they
possess only five free prosomatic appendages instead of six, as in Limulus
and in Pterygotus. Similarly, Woodward only allowed five appendages for
Pterygotus, so that his restorations were throughout consistent. Schmidt,
in _Pterygotus osiliensis_ has shown that the true number was six, not
five, as seen in his restoration given in Fig. 8, A (p. 27).
[Illustration: FIG. 103.--VENTRAL SURFACE OF LIMULUS. (Taken from
KISHINOUYE.)
The gnathic bases of the appendages have been separated from those of the
other side to show the promesosternite or endostoma (_End._).]
With respect to Eurypterus, Schmidt figures an exceedingly minute pair of
antennæ between the coxal joints of the first pair of appendages, thus
making six pairs of appendages. Gerhard Holm, however, in his recent
beautiful preparations from Schmidt's specimens and others collected at
Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus
were of the same kind as those of Limulus. I reproduce his figure (Fig.
104) showing the small chelate cheliceræ (1) overhanging the mouth orifice,
just as in Limulus or in Scorpio.
{237}So, also, since Woodward's monograph, Laurie has discovered in
_Slimonia acuminata_ a small median pair of chelate appendages exactly
corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio.
We may, therefore, take it for granted that such was also the case in
Stylonurus, and that the foremost pair of prosomatic appendages in all
these extinct sea-scorpions were in the same position and of the same
character as the cheliceræ of the scorpions.
[Illustration: FIG. 104.--_Eurypterus Fischeri._ (From HOLM.)]
In the living scorpion and in Limulus the nerves to this pair of appendages
undoubtedly arise from the foremost prosomatic ganglia, and the reason why
they appear to belong to the supra-oesophageal brain-mass has been made
clear by Brauer's investigations on the embryology of Scorpio; for he has
shown that the cheliceral ganglia shift from the ventral to the dorsal side
of the oesophagus during development, thus becoming
pseudo-supra-oesophageal, though in reality belonging to the
infra-oesophageal ganglia. This cheliceral pair of appendages is, in all
probability, homologous with the second pair of antennæ in the crustacea.
{238}I conclude, then, that the cheliceræ must truly be included in the
prosomatic group, but that they stand in a somewhat different category to
the rest of the prosomatic appendages, inasmuch as they take up a very
median anterior and somewhat dorsal position, and their ganglia of origin
are also exceptional in position.
Next for consideration come the chilaria (7 in Fig. 103), which Lankester
did not consider to belong to appendages at all, but to be a peculiar pair
of sternites. Yet their very appearance, with their spinous hairs
corresponding to those of the other gnathites and their separate
nerve-supply, all point distinctly to their being a modified pair of
appendages, and, indeed, the matter has been placed beyond doubt by the
observations of Kishinouye, who has found embryologically that they arise
in the same way as the rest of the prosomatic appendages, and belong to a
distinct prosomatic segment, viz. the seventh segment. In accordance with
this, Brauer has found that in the scorpion there is in the embryo a
segment, whose appendages degenerate, which is situated between the segment
bearing the last pair of thoracic appendages and the genital operculum--a
segment, therefore, comparable in position to the chilarial segment of
Limulus.
Coming now to the five locomotor appendages, we find that they resemble
each other to a considerable extent in most cases, with, however, certain
striking differences. Thus in Limulus they are chelate, with their basal
joints formed as gnathites, except in the case of the fifth appendage, in
which the extremity is modified for the purpose of digging in the sand. In
Pterygotus, Slimonia, Eurypterus, the first four of these appendages are
very similar, and are called by Huxley and Woodward endognaths; in all
cases they possess a basal part or sterno-coxal process, which acts as a
gnathite or foot-jaw, and a non-chelate tactile part, which possesses no
prehensile power, and in most cases could have had no appreciable share in
locomotion, called by Huxley and Woodward the palpus. These small palps
were probably retractile, and capable of being withdrawn entirely under the
hood. The fifth appendage is usually different, being a large swimming
organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is
known as the ectognath.
Finally, in _Drepanopterus Bembycoides_, as stated by Laurie, all five
locomotor appendages are built up after the same fashion, the last one not
being formed as a paddle-shaped organ or elongated as {239}in Stylonurus,
but all five possess no special locomotor or prehensile power. According to
Laurie this is a specially primitive form of the group.
It is significant to notice from this sketch that with the absence of
special prehensile terminations such as chelæ, or the absence of special
locomotor functions such as walking or swimming, these appendages tend to
dwindle and become insignificant, taking up the position of mere feelers
round the mouth, and at the same time are concentrated and pressed closely
together, so that their appendage-nerves must also be close together.
This sketch therefore shows us that--
Of the six foremost prosomatic appendages, the cheliceræ and the four
endognaths were, at the time when the vertebrates first appeared, in very
many cases dwindling away; the latter especially no longer functioned as
locomotor appendages, but were becoming more and more mere palps or
tentacles situated round the mouth, which could by no possibility afford
any help to locomotion.
On the contrary, the sixth pair of appendages--the ectognaths--remained
powerful, being modified in many cases into large oar-like limbs by which
the animal propelled itself through the water.
It is a striking coincidence that those ancient fishes, Pterichthys and
Bothriolepis, should have possessed a pair of large oar-like appendages.
At this time, then, in strong contrast to the endognaths, the ectognaths,
or sixth pair of appendages, remained strong and vigorous. What about the
seventh pair, the chilaria of Limulus?
Of all the prosomatic appendages these are the most interesting from the
point of view of my theory, for whereas in the scorpion of the present day
they have dwindled away and left no trace except in the embryo, in the
sea-scorpions of old, far from dwindling, they had developed and become a
much more important organ than the chilaria of Limulus.
In all these animals a peculiarly striking and unique structure is found in
this region known by the name of the metastoma, or lip-plate (Figs. 8 and
104 (7)); it is universally considered to be formed by the fusion of the
two chilarial appendages.
All observers are agreed that this lip-plate was freely movable. Nieskowski
considers that the movement of the metastoma was entirely in a vertical
direction, whereby the cleft which is seen {240}between the basal joints of
all the pairs of locomotor appendages could be closed from behind. Woodward
says it no doubt represents the labium, and served more effectually to
enclose the posterior part of the buccal orifice, being found exteriorly to
the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with
Nieskowski, and looks on the mestasoma as forming a lower lip within which
the bases of the ectognaths worked.
[Illustration: FIG. 105.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH A,
LIMULUS, B, EURYPTERUS.]
Quite recently Gerhard Holm has worked over again the very numerous
specimens of _Eurypterus Fischeri_, which are obtainable at Rootziküll, and
has thrown new light on the relation of the metastoma to the mouth-parts.
His preparations show clearly that the true lower lip of Eurypterus was not
the metastoma, for when the metastoma is removed another plate (_End._,
Fig. 105, B) situated {241}internally to it is disclosed, which, in his
view, corresponds to the sternite between the bases of the pro-somatic
appendages in Limulus, _i.e._ to the sternite called by Lankester, the
pro-mesosternite (_End._, Fig. 103). This inner plate formed with the
metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed
posteriorly, within which the bases of the ectognaths worked. In other
words, the removal of the metastoma discloses in Eurypterus the true
anterior ventral surface of the animal which corresponds to that of
Limulus, or of the scorpion group, with its pro-mesosternite and laterally
attached gnathites or sterno-coxal processes. To this inner plate or
pro-mesosternite Holm gives the name of _endostoma_.
To the anterior edge of the endostoma a thinner membrane is attached which
passes inwards in the direction of the throat, and forms, therefore, the
lower lip (_Hyp._, Fig. 105, B) of the passage of the mouth (_olf. p._).
This membrane bears upon its surface a tuft of hairs, which he thought were
probably olfactory in function. Consequently, in his preliminary
communication, he describes this lower lip as forming, in all probability,
an olfactory organ; in his full communication he repudiates this
suggestion, because he thinks it unlikely that such an organ would be
situated within the mouth. I feel sure that if Holm had referred to
Croneberg's paper, and seen how the true mouth in all the scorpion group is
situated at the base of an olfactory passage, he would have recognized that
his first suggestion is in striking accordance with the nature of the
entrance to the mouth in other scorpions.
That Eurypterus also possessed a camerostome (_cam._) seems to follow of
necessity from its evident affinities both with Limulus and the scorpions.
We see, in fact, that the mouth of these old sea-scorpions was formed after
the fashion of Limulus, surrounded by masticatory organs in the shape of
foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal
sagittal section of one of these old palæostracan forms would be obtained
by the combination of actual sagittal sections through Limulus and a member
of the scorpion group, with, at the same time, a due recognition of Holm's
researches. Such a section is represented in Fig. 105, B, in which I have
drawn the central nervous system and its nerves, the median eyes (_C.E._),
the olfactory organs (_Cam._), the pharynx (_Ph._), oesophagus (_oes._),
and alimentary canal (_Al._), but have not tried to indicate the lateral
eyes. I have represented the prosomatic appendages by numbers (1-7), and
{242}the foremost mesosomatic segments by numbers (8-13). I have placed the
four endognaths and the nerves going to them close together, and made them
small, mere tentacles, in recognition of the character of these appendages
in Eurypterus, and have indicated the position and size of the large
ectognath, with its separate nerve, by (6). If among the ancient
Eurypterus-like forms, which were living at the time when vertebrates first
appeared, there were some in which the ectognaths also had dwindled to a
pair of tentacles, then such animals would possess a prosomatic chamber
formed by a metastoma or accessory lip, within which were situated five
pairs of short tactile appendages or tentacles. If the vertebrate were
derived from such an animal, then the trigeminal nerve, as the
representative of these prosomatic appendage-nerves, ought to be found to
supply the muscles of this accessory lip and of these five pairs of
tentacles in the lowest vertebrate.
This prosomatic or oral chamber, as it might be called, was limited
posteriorly by the fused metastoma (7) and operculum (8), so that if in the
same imaginary animal one imagines that the gill-chambers, instead of being
separate, are united to form one large respiratory chamber, then, in such
an animal, a prosomatic oral chamber, in which the prosomatic appendages
worked, would be separated from a mesosomatic respiratory chamber by a
septum composed of the conjoined basal portions of the mesosomatic
operculum and the prosomatic metastoma, as indicated in the diagram. In
this septum the nerves to the last prosomatic appendage (equivalent to the
last part of the trigeminal in the vertebrate) and to the first mesosomatic
(equivalent to the thyroid part of the facial) would run, as shown in the
figure, close together in the first part of their course, and would
separate when the ventral surface was reached, to pass headwards and
tailwards respectively.
THE COXAL GLANDS.
One more characteristic of these appendages requires mention, and that is
the excretory glands situated at the base of the four endognaths known as
the coxal glands. These glands are the main excretory organs in Limulus and
the scorpions, and extend into the basal segments or coxæ of the four
endognaths, not into those of the ectognaths or the chilaria (or
metastoma). Hence their name, coxal {243}glands; and, seeing the importance
of the excretory function, it is likely enough that they would remain, even
when the appendages themselves had dwindled away. With the concentration
and dwindling of the endognaths these coxal glands would also be
concentrated, so that in the diagram (Fig. 105) they would rightly be
grouped together in the position indicated (_cox. gl._).
Such a diagram indicates the position of all the important organs of the
head-region except the special organs for taste and hearing. These, for the
sake of convenience, I propose to take separately, in order at this stage
of my argument not to overburden the simplicity of the comparison I desire
to make with too much unavoidable detail.
THE PROSOMATIC REGION OF AMMOCOETES.
Let us now compare this diagram with that of the corresponding region in
Ammocoetes and see whether or no any points of similarity exist.
With respect to this region, as in so many other instances already
mentioned, Ammocoetes occupies an almost unique position among vertebrates,
for the region supplied by the trigeminal nerve--the prosomatic
region--consists of a large oral chamber which was separated from the
respiratory chamber in the very young stage by a septum which is
subsequently broken through, and so the two chambers communicate.
This chamber is bounded by the lower lip ventrally, the upper lip and
trabecular region dorsally, and the remains of the septum or velum
laterally and posteriorly. It contains a number of tentacles arranged in
pairs within the chamber so as to form a sieve-like fringe inside the
circular mouth; of these, the ventral pair are large, fused together, and
attached to the lower lip.
All the muscles belonging to this oral chamber are of the visceral type,
and are innervated by the trigeminal nerve. In accordance with the evidence
obtained up to this point this means that such an oral chamber was formed
by the prosomatic appendages of the invertebrate ancestor, similarly to the
oral chamber just figured for Eurypterus.
This chamber in the full-grown Ammocoetes is not only open to the
respiratory chamber, but is bounded by the large upper lip (_U.L._, Fig.
106, D). On the dorsal surface of this region, in front of the {244}pineal
eye (_C.E._), is the most conspicuous opening of the olfactory tube
(_Na._), which olfactory tube passes from the dorsal region to the ventral
side to terminate blindly at the very spot where the infundibulum comes to
the surface of the brain. Here, also, is situated that extraordinary
glandular organ known as the pituitary body (_Pit._). A sagittal section,
then, in diagram form, of the position of parts in the full-grown
Ammocoetes, would be represented as in Fig. 106, D.
But, as argued out in the last chapter, the diagram of the adult Ammocoetes
must be compared with that of a cephalaspidian fish; the diagram of the
palæostracan must be compared with the larval condition of Ammocoetes. In
other words, Fig. 106, B, must be compared with Fig. 106, C, which
represents a section through the larval Ammocoetes as it would appear if it
reached the adult condition without any forward growth of the upper lip or
any breaking through of the septum between the oral and respiratory
chambers. The striking similarity between this diagram and that of
Eurypterus becomes immediately manifest even to the smallest details. The
only difference between the two, except, of course, the notochord, consists
in the closure of the mouth opening (_o_), in Fig. 106, B, by which the
olfactory passage (_olf. p._) of the scorpion becomes converted into the
hypophysial tube (_Hy._), Fig. 106, C, and later into the nasal tube
(_Na._), Fig. 106, D, of the full-grown Ammocoetes. That single closure of
the old mouth is absolutely all that is required to convert the Eurypterus
diagram into the Ammocoetes diagram.
Such a comparison immediately explains in the simplest manner a number of
anatomical peculiarities which have hitherto been among the great mysteries
of the vertebrate organization. For not only do the median eyes (_C.E._)
correspond in position in the two diagrams, and the infundibular tube
(_Inf._) and the ventricles of the brain (_C.C._) correspond to the
oesophagus (_oes._) and the cephalic stomach (_Al._), as already fully
discussed; but even in the very place where the narrow oesophagus opened
into the wider chamber of the pharynx (_Ph._), there, in all the lower
vertebrates, the narrow infundibular tube opens into the wider chamber of
the membranous _saccus vasculosus_ (_sac. vasc._). This is the last portion
of the membranous part of the tube of the central nervous system which has
not received explanation in the previous chapters, and now it is seen how
simple its explanation is, how natural its presence--it represents the old
pharyngeal chamber of the palæostracan ancestor.
{245}[Illustration: FIG. 106.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH
B, EURYPTERUS; C, LARVAL AMMOCOETES; D, FULL-GROWN AMMOCOETES.]
{246}Next among the mysteries requiring explanation is the pituitary body,
that strange glandular organ always found so closely attached to the brain
in the infundibular region that when it is detached in taking out the brain
it leaves the infundibular canal patent right into the IIIrd ventricle. A
comparison of the two diagrams indicates that such a glandular organ
(_Pit._), Fig. 106, C, was there because the coxal excretory glands (_cox.
gl._), Fig. 106, B, were in a similar position in the palæostracan
ancestor--that, indeed, the pituitary body is the descendant of the coxal
glands.
Finally, the diagrams not only indicate how the mesosomatic
appendage-nerves supplying in the one case the operculum and the
respiratory appendages correspond to the respiratory group of nerves, VII.,
IX., X., supplying in the other case the thyroid, hyoid, and branchial
segments, but also that a similar correspondence exists between the
prosomatic appendage-nerves in the one case and the trigeminal nerve in the
other; a correspondence which supplies the reason why in the vertebrate a
septum originally existed between an oral and respiratory chamber.
Such a comparison, then, leads directly to the suggestion that the
trigeminal nerve originally supplied the prosomatic appendages, such
appendages being: 1. The metastoma, which has become in Ammocoetes the
lower lip supplied by the velar or mandibular branch of the trigeminal
nerve (7); 2. The ectognath, which has become the large median ventral
tentacle, called by Rathke the tongue, supplied by the tongue nerve (6); 3.
The endognaths, which have been reduced to tentacles and are supplied by
the tentacular branch of the trigeminal nerve (2, 3, 4, 5).
I have purposely put these two diagrams of the larval Ammocoetes and of
Eurypterus before the minds of my readers at this early stage of my
argument, so as to make what follows more understandable. I propose now to
consider fully each one of these suggestive comparisons, and to see whether
or no they are in accordance with the results of modern research.
In the first instance, the diagrams suggest that the trigeminal nerve
originally supplied the prosomatic appendages of the palæostracan ancestor,
while the eye-muscle nerves supplied the body-muscles of the prosoma.
{247}As these appendages did not carry any vital organs such as branchiæ,
but were mainly locomotor and masticatory in function, it follows that
their disappearance as such would be much more complete than that of the
mesosomatic branchial appendages. Most probably, then, in the higher
vertebrates no trace of such appendages might be left; consequently the
segmentation due to their presence would be very obscure, so that in this
region the very reverse of what is found in the region of the vagus nerve
would be the rule. There branchiomeric segmentation is especially evident,
owing to the persistence of the branchial part of the branchial appendages;
here, owing to the disappearance of the appendages, the segmentation is no
longer branchiomeric, but essentially mesomeric in consequence of the
persistence of the somatic eye-muscles.
In addition to the evidence of the appendages themselves, the number of
prosomatic segments is well marked out in all the members of the scorpion
group by the divisions of the central nervous system into well-defined
neuromeres in accordance with the appendages, a segmentation the
reminiscence of which may still persist after the appendages themselves
have dwindled or disappeared. In accordance with this possibility we see
that one of the most recent discoveries in favour of a number of segments
in the head-region of the vertebrate is the discovery in the early embryo
of a number of partial divisions in the brain-mass, forming a system of
cephalic neuromeres which may well be the rudiments of the well-defined
cephalic neuromeres of animals such as the scorpion.
THE EVIDENCE OF THE PROSOMATIC MUSCULATURE.
Even if the appendages as such become obscure, yet their muscles might
remain and show evidence of their presence. The most persistent of all the
appendage-muscles are the basal muscles which pass from coxa to carapace
and are known by the name of tergo-coxal muscles. They are large, well
marked, segmentally arranged muscles, dorso-ventral in direction, and,
owing to their connecting the limb with the carapace, are likely to be
retained even if the appendage dwindles away.
The muscular system of Limulus and Scorpio has been investigated by Benham
and Miss Beck under Lankester's direction, and the conclusions to which
Lankester comes are these--
{248}The simple musculature of the primitive animal from which both Limulus
and the scorpions arose consisted of--
1. A series of paired longitudinal dorsal muscles passing from tergite to
tergite of each successive segment.
2. A similar series of paired longitudinal ventral muscles.
3. A pair of dorso-ventral muscles passing from tergite to sternite in
each segment.
4. A set of dorso-ventral muscles moving the coxa of each limb in its
socket.
5. A pair of veno-pericardial muscles in each segment.
Of these groups of muscles, any one of which would indicate the number of
segments, Groups 1 and 2 do not extend into the prosomatic region, and
Group 5 extends only as far as the heart extends in the case of both
Limulus and the Scorpion group; so that we may safely conclude that in the
Palæostraca the evidence of somatic segmentation in the prosomatic region
would be given, as far as the musculature is concerned, by the
dorso-ventral somatic muscles (Group 3), and of segmentation due to the
appendages by the dorso-ventral appendage musculature (Group 4).
Therefore, if, as the evidence so far indicates, the vertebrate has arisen
from a palæostracan stock, we should expect to find that the musculature of
the somatic segments in the region of the trigeminal nerve did not resemble
the segmental muscles of the spinal region, was not, therefore, the
continuation of the longitudinal musculature of the body, but was
dorso-ventral in position, and that the musculature of the splanchic
segments resembled that of the vagus region, where, as pointed out in
Chapter IV., the respiratory muscles arose from the dorso-ventral muscles
of the mesosomatic appendages. This is, of course, exactly what is found
for the muscles which move the lateral eyes of the vertebrate; these
muscles, innervated by the IIIrd, IVth, and VIth nerves, afford one of the
main evidences of segmentation in this region, are always grouped in line
with the somatic muscles of spinal segments, and yet cannot be classed as
longitudinal muscles. They are dorso-ventral in direction, and yet belong
to the somatic system; they are exactly what one ought to find if they
represent Group 3--the dorso-ventral body-muscles of the prosomatic
segments of the invertebrate ancestor.
The interpretation of these muscles will be given immediately; at present I
want to pass in review all the different kinds of evidence {249}of
segmentation in this region afforded by the examination of the
invertebrate, whether living or fossil, so as to see what clues are left if
the evidence of appendages fails us. I will take in the first instance the
evidence of segmentation afforded by the presence of the musculature of
Group 4, even when, as in the case of many fossils, no appendages have yet
been found. In such animals as Mygale and Phrynus the prosomatic carapace
is seen to be marked out into a series of elevations and depressions, and
upon removing the carapace we see that these elevations correspond with and
are due to the large tergo-coxal muscles of the appendages; so that if such
carapace alone were found fossilized we could say with certainty: this
animal possessed prosomatic appendages the number of which can be guessed
with more or less certainty by these indications of segments on the
carapace.
In those forms, then, which are only known to us in the fossil condition,
in which no prosomatic appendages have been found, but which possess, more
or less clearly, radial markings on the prosomatic carapace resembling
those of Phrynus or Mygale, such radial markings may be interpreted as due
to the presence of prosomatic appendages, which are either entirely
concealed by the prosomatic carapace or dorsal head-plate, or were of such
a nature as not to have been capable of fossilization.
The group of animals in question forms the great group of animals, chiefly
extinct, classified by H. Woodward under the order of Merostomata. They are
divided by him into the sub-order of Eurypteridæ, which includes--(1)
Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, (5)
Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9)
Exapinurus, (10) Pseudoniscus; and the sub-order Xiphosura, which
includes--(1) Belinurus, (2) Prestwichia, (3) Limulus.
{250}[Illustration: FIG. 107.--_Phrynus Margine-Maculata._
_Ce._, median eyes; _le._, lateral eyes; _glab._, median plate over brain;
_Fo._, fovea.]
[Illustration: FIG. 108.--_Phrynus sp._ (?). CARAPACE REMOVED.
_cam._, camerostome; _pl._, plastron.]
{251}The evidence of the Xiphosura and of the Hemiaspidæ conclusively
shows, in Woodward's opinion, that the Merostomata are closely related to
the Trilobita, and the Hemiaspidæ especially are supposed to be
intermediate between the trilobites and the king-crabs. They are
characterized, as also Belinurus and Prestwichia, by the absence of any
prosomatic appendages, so that in these cases, as is seen in Fig. 12 (p.
30), representing _Bunodes lunula_, found in the Eurypterus layer at
Rootziküll, we have an animal somewhat resembling Limulus in which the
prosomatic appendages have either dwindled away and are completely hidden
by the prosomatic carapace, or became so soft as not to be preserved in the
fossilized condition. The appearance of the prosomatic carapace is, to my
mind, suggestive of the presence of such appendages, for it is marked out
radially, as is seen in the figure, in a manner resembling somewhat the
markings on the prosomatic carapace of Mygale or Phrynus; the latter
markings, as already mentioned, are due to the aponeuroses between the
tergo-coxal muscles of the prosomatic appendages which lie underneath and
are attached to the carapace.
A very similar radial marking is shown by Woodward in his picture of
_Hemiaspis limuloides_, reproduced in Fig. 109, found in the Lower Ludlow
beds at Leintwardine. This species has yielded the most perfect specimens
of the genus Hemiaspis, which is recognized as differing from Bunodes by
the possession of a telson.
It is striking to find that similar indications of segments have been found
on the dorsal surface of the head-region in many of the most ancient
extinct fishes, as will be fully discussed later on.
[Illustration: FIG.109.--_Hemiaspis limuloides._ (From WOODWARD.)
_gl._, glabellum.]
THE EVIDENCE OF COELOMIC CAVITIES.
In the head-region of the vertebrate, morphologists depend largely upon the
embryonic divisions of the mesoderm for the estimation of the number of
segments, and, therefore, upon the number of coelomic cavities in this
region, the walls of which give origin to the striated muscles of the head,
so that the question of the number of segments depends very largely upon
the origin of the muscles from the walls of these head-cavities. It is
therefore interesting to examine whether a similar criterion of
segmentation holds good in such a segmented {252}animal as Limulus, or in
the members of the scorpion group, in which the number of segments are
known definitely by the presence of the appendages. In Limulus we know,
from the observations of Kishinouye, that a series of coelomic cavities are
formed embryologically in the various segments of the mesosoma and prosoma,
in a manner exceedingly similar to their mode of formation in the
head-region of the vertebrate, and he has shown that in the mesosoma a
separate coelomic cavity exists for each segment, so that just as the
dorso-ventral somatic muscles are regularly segmentally arranged in this
region, so are the coelomic cavities, and we should be right in our
estimation of the number of segments in this region by the consideration of
the numerical correspondence of these cavities with the mesomatic
appendages. Similarly, in the vertebrate, we find every reason to believe
that a single, separate head-cavity corresponds to each of the branchial
segments in the opisthotic region, and therefore we should estimate rightly
the number of segments by the division of the mesoderm in this region.
In the prosomatic region of Limulus, the dorso-ventral muscles are not
arranged with such absolute segmental regularity as in the mesosomatic
region, and Kishinouye's observations show that the coelomic cavities in
this region do not correspond absolutely with the number of prosomatic
appendages. His words are:--
A pair of coelomic cavities appears in every segment except the segments of
the 2nd, 3rd, and 4th appendages, in which coelomic cavities do not appear
at all. At least eleven pairs of these cavities are produced. The eleventh
pair belongs to the seventh abdominal segment.
The first pair of coelomic cavities is common to the cephalic lobe and the
segment of the first appendage (_i.e._ the cheliceræ).
The second coelomic cavity belongs to the segment of the fifth appendage.
It is well developed.
The ventral portion of the second coelomic cavity remains as the coxal
gland.
* * * * * *
Consequently, if we were to estimate the number of segments in this region
by the number of coelomic cavities we should not judge rightly, for we
should find only four cavities and seven appendages, as is seen in the
following table:--
Key: {253}
A Prosomatic.
B Mesosomatic.
C.c. Coelomic cavities.
---------------------------------------------------------+---------------
LIMULUS. | VERTEBRATE.
---------+----------------+-----------------------+------+---------------
Segments.| Appendages. | Eurypterid appendages.| C.c. | Coelomic
| | | | cavities.
---+-----+----------------+-----------------------+------+---------------
| 1 | Cheliceræ or | Cheliceræ | 1 | Anterior
| | 1st locomotor.| | |
| 2 | 2nd locomotor |} | |
A | 3 | 3rd " |} Endognaths | 2 | Premandibular
| 4 | 4th " |} | |
| 5 | 5th " |} | |
| 6 | 6th " | Ectognath | 3 |} Mandibular
| 7 | Chilaria | Metastoma | 4 |}
---+-----+----------------+-----------------------+------+---------------
| 8 | Operculum |} Operculum (Genital) | 5 |} Hyoid
| 9 | 1st branchial |} " (1st branchial) | 6 |}
| 10 | 2nd " | 2nd branchial | 7 | 1st branchial
B | 11 | 3rd " | 3rd " | 8 | 2nd "
| 12 | 4th " | 4th " | 9 | 3rd "
| 13 | 5th " | 5th " | 10 | 4th "
| 14 | 6th " | | 11 |
---+-----+----------------+-----------------------+------+---------------
The second cavity would in reality represent four segments belonging to the
2nd, 3rd, 4th, 5th locomotor appendages, _i.e._ the very four segments
which in the Eurypteridæ are concentrated together to form the endognaths,
and we should be justified in putting this interpretation on it, because,
according to Kishinouye, its ventral portion forms the coxal gland, and,
according to Lankester, the coxal gland sends prolongations into the coxa
of the 2nd, 3rd, 4th, 5th locomotor appendages. Similarly in the
vertebrate, we find three head-cavities in the region which corresponds, on
my theory, to the prosomatic region of Limulus, (1) the anterior cavity
discovered by Miss Platt, (2) the premandibular cavity, and (3) the
mandibular cavity, which, if they corresponded with the prosomatic coelomic
cavities of Limulus, would represent not three segments but seven segments,
as follows:--the anterior cavity would correspond to the first coelomic
cavity, _i.e._ the cavity of the cheliceral segments in both Limulus and
the Eurypteridæ; the premandibular, to the second coelomic cavity,
representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in
Limulus and the endognathal segments in the Eurypteridæ; and the mandibular
to the 3rd and 4th coelomic cavities, representing the last locomotor and
chilarial segments in Limulus, _i.e._ the ectognathal and metastomal
segments in the Eurypteridæ.
{254}It is worthy of note that, in respect to their coelomic cavities, as
in the position and origin of their nerves in the central nervous system,
the first pair of appendages, the cheliceræ, retain a unique position,
differing from the rest of the prosomatic appendages.
In the table I have shown how the vertebrate coelomic cavities may be
compared with those of Limulus. The next question to consider is the
evidence obtained by morphologists and anatomists as to the number of
segments supplied by the trigeminal nerve-group; this question will be
considered in the next chapter.
SUMMARY.
In Chapters IV. and V. I have dealt with the opisthotic segments of the
vertebrate, including therein the segments supplied by the facial nerve,
and shown that they correspond to the mesosomatic segments of the
palæostracan; consequently the facial (VII.), glossopharyngeal (IX.), and
vagus (X.) nerves originally supplied the branchial and opercular
appendages.
In this chapter the consideration of the pro-otic segments is commenced,
that is, the segments supplied by the trigeminal (V.) and the eye-muscle
nerves (III., IV., VI.). I have considered the VIth nerve with the rest
of the eye-muscle nerves for convenience' sake, though in reality it
belongs to the same segment as the facial. Of these, that part of the
trigeminal which innervates the muscles of mastication corresponds to the
splanchnic segments, while the eye-muscle nerves belong to the
corresponding somatic segments; but the pro-otic segments of the
vertebrate ought to correspond to the prosomatic segments of the
invertebrate, just as the opisthotic correspond to the mesosomatic.
Therefore the motor part of the trigeminal ought to supply muscles which
originally moved the prosomatic appendages, while the eye-muscles ought
to have belonged to the somatic part of the same segments.
The first question considered is the number of segments which ought to be
found in this region. In Limulus, the Eurypteridæ, and the scorpions
there are seven prosomatic segments which carry (1) the cheliceræ, (2, 3,
4, 5) the four first locomotor appendages--the endognaths, (6) the large
special appendage--the ectognath--and (7) the appendages, which in
Limulus are known as the chilaria, and are small and insignificant, but
in Eurypterus and other forms grow forwards, fuse together, and form a
single median lip to an accessory oral chamber, which lip is known as the
metastoma. Of these appendages the cheliceræ and endognaths tend to
dwindle away and become mere tentacles, while the large swimming
ectognath and metastoma remain strong and vigorous.
In this, the prosomatic region, the somatic segmentation is not
characterized by the presence of the longitudinal muscle segments, for
they do not extend into this head-region, but only by the presence of the
segmental somatic {255}ventro-dorsal muscles. Among the muscles of the
appendages the system of large tergo-coxal muscles is especially
apparent.
From these considerations it follows that the number of segments in this
region in the vertebrate ought to be seven; that the musculature supplied
by the trigeminal nerve ought to represent seven ventral or splanchnic
segments, of which only the last two are likely to be conspicuous; and
that the musculature supplied by the eye-muscle nerves ought to be
dorso-ventral in direction, which it is, and represent seven dorsal or
somatic segments.
A further peculiarity of this region, both in Limulus and the scorpions,
is found in the excretory organs which are known by the name of coxal
glands, because they extend into the basal joint, or coxa, of certain of
the prosomatic limbs. The appendages so characterized are always the four
endognaths, and it follows that if these four endognaths lose their
locomotor power, become reduced in size, and concentrated together to
form mere tentacles, then of necessity the coxal glands will be
concentrated together, and tend to form a glandular mass in the region of
the mouth; in fact, take up a position corresponding to that of the
pituitary body in vertebrates.
Taking all these facts into consideration, it is possible to construct a
drawing of a sagittal section through the head-region of Eurypterus,
which will represent, with considerable probability, the arrangement of
parts in that animal. This can be compared with the corresponding section
through the head of Ammocoetes.
Now, as pointed out in the last chapter, the early stage of Ammocoetes is
remarkably different from the more advanced stage; at that time the
septum between the oral and respiratory chambers has not yet broken
through, and the olfactory or nasal tube, known at this stage as the tube
of the hypophysis, is directed ventrally, not dorsally.
The comparison of the diagram of Eurypterus with that of the early stage
of Ammocoetes is remarkably close, and immediately suggests not only that
the single nose of the former is derived from the corresponding organ in
the palæostracan, but that the pituitary body is derived from the
concentrated coxal glands, and the lower lip from the metastoma. The
further working out of these homologies will be discussed in the next
chapter.
In addition to the evidence of segmentation afforded by the appendages,
there are in this region, in Limulus and the scorpion group, three other
criteria of segmentation available to us, if from any cause the evidence
of appendages fails us. These are--
1. The number of neuromeres are marked out in this region of the brain
more or less plainly, especially in the young animal, just as they are
also in the embryo of the vertebrate.
2. The segmentation is represented here, just as in the mesosomatic
region, by two sets of muscle-segments; the one _somatic_, consisting of
the segmentally arranged dorso-ventral muscles, the continuation of the
group already discussed in connection with the mesosomatic segmentation,
and the other _appendicular_ characterized by the tergo-coxal muscles.
These latter segmental muscles are especially valuable, for in such forms
as Mygale, Phrynus, etc., their presence is indicated externally by
markings on the prosomatic carapace, and thus corresponding markings
found on fossil carapaces or on dorsal head-shields can be
{256}interpreted. These two sets of muscle-segments correspond in the
vertebrate to the somatic and splanchnic segmentations.
3. In the vertebrate the segmentation in this region is indicated by the
coelomic or head-cavities, which are cavities formed in the mesoderm of
the embryo, the walls of which give origin to the striated muscles of the
head. In Limulus corresponding coelomic cavities are found, which are
directly comparable with those found in the vertebrate.
{257}CHAPTER VIII
_THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP_
The prosomatic segments of the vertebrate.--Number of segments belonging
to the trigeminal nerve-group.--History of cranial segments.--Eye-muscles
and their nerves.--Comparison with the dorso-ventral somatic muscles of
the scorpion.--Explanation of the oculomotor nerve and its group of
muscles.--Explanation of the trochlearis nerve and its dorsal
crossing.--Explanation of the abducens nerve.--Number of segments
supplied by the trigeminal nerves.--Evidence of their motor
nuclei.--Evidence of their sensory ganglia.--Summary.
From the evidence given in the last chapter, combined with that given in
Chapter IV., the probability of the theory that the trigeminal group of
nerves of the vertebrate have been derived from the prosomatic group of
nerves of the invertebrate can be put to the test by the answers to the
following morphological and anatomical questions:--
1. Do we find in the vertebrate two segmentations in this region
corresponding to the two segmentations in the branchial region, _i.e._ a
somatic or dorsal series of segments, and a splanchnic or ventral series of
segments? The latter would not be branchial, but rather of the nature of
free tactile appendages; so that it is useless to look for or talk about
gill-slits, although such appendages, being serially homologous with the
branchial mesosomatic appendages, would readily give rise to the conception
of branchial segments.
2. Is there morphological evidence that the trigeminal nerve is not the
nerve belonging to a single segment, or even to two segments, but is really
a concentration of at least six, probably seven, segmental nerves?
3. Is there morphological evidence that the oculomotor and trochlear
nerves, which on all sides are regarded as belonging to the trigeminal
segments, are not single nerves corresponding each {258}to a single
segment, but are the somatic motor roots belonging to the same segments as
those to which the trigeminal supplies the splanchnic roots?
4. Do the mesoderm segments, which give origin to the eye-muscles, and
therefore do the head-cavities of this region, correspond with the
trigeminal segments? Considering the concentration of parts in this region
and the difficulty already presented by the want of numerical agreement
between the prosomatic appendages and the prosomatic coelomic cavities in
Limulus, it may very probably be difficult to determine the actual number
of the mesoderm segments.
5. Is there anatomical evidence that the ganglion of origin of the motor
part of the trigeminal nerve is not a single ganglion, but a representative
of many, probably seven?
6. Is there anatomical evidence that the ganglia of origin of the
oculomotor and trochlear nerves represent many ganglia?
7. Is there any evidence that the organs originally supplied by the motor
part of the trigeminal nerve are directly comparable with prosomatic
appendages?
It is agreed on all sides that in this region of the head there is distinct
evidence of double segmentation, the dorsal mesoderm segments giving origin
to the eye-muscles, and the ventral segments to the musculature innervated
by the trigeminal nerve. Originally, according to the scheme of van Wijhe,
two segments only were recognized, the dorsal parts of which were
innervated by the IIIrd and IVth nerves respectively. Since his paper, the
tendency has been to increase the number of segments in this region, as is
seen in the following sketch, taken from Rabl, of the history of cranial
segmentation.
HISTORY OF CRANIAL SEGMENTATION.
The first attempt to deal with this question was made by Goethe and Oken.
They considered that the cranial skeleton was composed of a series of
vertebræ, but as early as 1842 Vogt pointed out that only the occipital
segments could be reduced to vertebræ. In 1869, Huxley showed that vertebræ
were insufficient to explain the cranial segmentation, and that the nerves
must be specially considered. The olfactory and optic nerves he regarded as
parts of the brain, not true segmental nerves; the rest of the cranial
nerves {259}were segmental, with special reference to branchial arches and
clefts, the facial, glossopharyngeal, and separate vagus branches supplying
the walls of the various branchial pouches. In a similar manner, the supra-
and infra-maxillary branches of the trigeminal were arranged on each side
of the mouth, and the inner and outer twigs of the first (ophthalmic)
branch of the trigeminal on each side of the orbito-nasal cleft, the
trabecular and the supra-maxillary arches being those on each side of this
cleft. Thus Huxley considered that there was evidence of a series of pairs
of ventral arches belonging to the skull, viz. the trabecular and maxillary
in front of the mouth, the mandibular, hyoid, and branchial arches behind,
and that the Vth, VIIth, IXth, and Xth nerves were segmental in relation to
these arches and clefts. Gegenbaur, in 1871 and 1872, considered that the
branchial arches represented the lower arches of cranial vertebræ, and
therefore corresponded to lower arches in the spinal region, _i.e._ the
skull was composed of as many vertebræ as there are branchial arches. These
vertebræ were confined to the notochordal part of the skull, the prechordal
part having arisen secondarily from the vertebral part, while the number of
vertebræ are at least nine, possibly more. The nerves which could be
homologized with spinal nerves were, he thought, divisible into two great
groups--(1) the trigeminal group, which included the eye-muscle nerves, the
facial, and its dorsal branch, the auditory; (2) the vagus group, which
included the glossopharyngeal and vagus.
Such was the outcome of the purely comparative anatomical work of Huxley
and Gegenbaur--work that has profoundly influenced all the views of
segmentation up to the present day.
Now came the investigations of the embryologists, of whom I will take, in
the first instance, Balfour, whose observations on the embryology of the
Selachians led him to the conclusion that besides the evidence of
segmentation to be found in the cranial nerves and in the branchial clefts,
further evidence was afforded by the existence of head-cavities, the walls
of which formed muscles just as they do in the spinal region. He came to
the conclusion that the first head-cavity belonged to one or more pre-oral
segments, of which the nerves were the oculomotor, trochlearis, and
possibly abducens; while there were seven post-oral segments, each with its
head-cavity and its visceral arch, of which the trigeminal, facial,
glossopharyngeal, and the four parts of the vagus were the respective
nerves.
{260}Marshall, in 1882, considered that the cranial segments were all
originally respiratory, and that all the segmental nerves are arranged
uniformly with respect to a series of gill-clefts which have become
modified anteriorly and have been lost, to a certain extent, posteriorly.
He included the olfactory nerves among the segmental nerves, and looked
upon the olfactory pit, the orbito-nasal lacrymal duct, the mouth, and the
spiracle as all modified gill-slits, so that he reckoned three pre-oral and
oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and eight
post-oral segments belonging respectively to the VIIth and VIth nerves, and
to the IXth nerve, and six segments belonging to the Xth nerve. He pointed
out that muscles supplied by the oculomotor nerve develop from the outer
wall of the first head-cavity; not, however, the _obliquus superior_ and
_rectus externus_, the latter originating probably from the walls of the
third cavity.
In the same year, 1882, came van Wijhe's well-known paper, in which he
showed that the mesoderm of the head in the selachian divided into two sets
of segments, dorsal and ventral; that the dorsal segments were continuous
with the body-somites, and that the ventral segments formed the lateral
plates of mesoblast between each of the visceral and branchial pouches. He
concluded that the dorsal somites were originally nine in number, that each
was supplied with a ventral nerve-root, in the same way as the somites in
the trunk, and that to each one a visceral pouch corresponded, whose walls
were supplied by the corresponding dorsal nerve-root; of these nine
segments, the ventral nerve-roots of the first three segments were
respectively the oculomotor, trochlearis, and abducens nerves. The next
three segments possessed no definable ventral root or muscles, and the
seventh, eighth, and ninth segments possessed as ventral roots the
hypoglossal nerve, with its muscular supply. The corresponding dorsal
nerve-roots were the trigeminal, facial, auditory, glossopharyngeal and
vagus nerves, the difference between cranial and spinal dorsal roots being
that the former contain motor fibres.
Ahlborn, in 1884, drew a sharp distinction between the segments of the
mesoderm and those of the endoderm. The former segmentation he called
mesomeric, the latter branchiomeric. He considered the two segmentations to
be independent, and concluded that the branchiomeric was secondary to the
mesomeric, and therefore not of {261}segmental value. As to the segments of
the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ
remain permanently, and correspond to the three last segments in the
selachian head. Of the anterior mesoderm segments, he considered that there
were originally six, and that there are six typical eye-muscles in all
Craniota, which have been compressed into three segments, as in Selachia.
Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal
nerve belongs to three proto-vertebræ posterior to the vagus region, which
were true spinal segments. He therefore modified Gegenbaur's conceptions to
this extent: that portion of the skull designated by Gegenbaur as vertebral
must be divided into two parts--a hind or occipital region, which is
clearly composed of modified vertebræ and is the region of the hypoglossal
nerves, and a front region, extending from the oculomotor to the
accessorius nerves, which is characterized segmentally by the formation of
branchial arches, but in which there is no evidence that proto-vertebræ
were ever formed. He therefore divides the head-skeleton into three parts--
1. Gegenbaur's evertebral part--the region of the olfactory and optic
nerves--which cannot be referred to any metameric segmentation.
2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be
segmented from the consideration of the nerves and branchial arches, but
not referable to proto-vertebræ--the region of the trigeminal and vagus
nerves.
3. The vertebral spinal part--the region of the hypoglossal nerves.
He further showed that the ganglia of the specially branchial nerves, the
facial, glossopharyngeal, and vagus, are at one stage in connection with
the epidermis, so that these parts of the epidermis represent sense-organs
which do not develop; these organs probably belonged to the lateral line
system. As the connection takes place at the dorsal edge of the gill-slits,
they may also be called rudimentary branchial sense-organs.
Since this paper of Froriep's, it has been generally recognized, and
Gegenbaur has accepted Froriep's view, that the three hindmost metameres,
which distinctly show the characteristics of vertebræ, belong to the spinal
and not to the cranial region, so that the metameric segmentation of the
cranial region proper has become {262}more and more associated with the
branchial segmentation. Froriep's discovery of the rudimentary branchial
sense-organs as a factor in the segmentation question has led Beard to the
conclusion that the olfactory and auditory organs represent in a permanent
form two of these rudimentary branchial sense-organs. He therefore includes
both the olfactory and auditory nerves in his list of cranial segmental
nerves, and makes eleven cranial branchial segments in front of the spinal
segments represented by the hypoglossal.
A still larger number of cranial segments is supposed to exist, according
to the researches of Dohrn and Killian, in the embryos of _Torpedo
ocellata_. The former, holding to the view that vertebrates arose from
annelids, considered that the head was formed of a series of metameres, to
each one of which a mesoderm-segment, a gill-arch, a gill-cleft, a
segmental nerve and vessel belonged. He found in the front head-region of a
Torpedo embryo, corresponding to van Wijhe's first four somites, no less
than twelve to fifteen mesoderm segments, and concluded, therefore, that
the eye-muscle nerves, especially the oculomotor, represented many
segmental nerves, and were not the nerves of single segments; so, also,
that the inferior maxillary part of the trigeminal and the hyoid nerve of
the facial are probably not single nerves, but a fusion of several. Killian
comes to much the same conclusion as Dohrn, for he finds seventeen to
eighteen separate mesoderm segments in the head, of which twelve belong to
the trigeminal and facial region.
Since Rabl's paper, a number of papers have appeared, especially from
America, dealing with yet another criterion of the original segmentation of
the head, viz. a series of divisions of the central nervous system itself,
which are seen at a very early stage of development, and are called
neuromeres; the divisions in the cranial region being known as
encephalomeres, and those of the spinal region as myomeres. Locy's paper
has especially brought these divisions into prominence as a factor in the
question of segmentation. They are essentially segments of the epiblast and
not of the mesoblast; they are conspicuous in very early stages, and appear
to be in relation with the cranial nerves, according to Locy. He recognizes
in _Squalus acanthias_, in front of the spino-occipital region, fourteen
pairs of such encephalomeres and a median unsegmented termination, which
may represent one more pair fused in the middle line, making at least
fifteen. He distributes these fifteen segments as follows: {263}fore-brain
three and unsegmented termination, mid-brain two, and hind-brain nine.
Again, Kupffer, in his recent papers on the embryology of Ammocoetes,
asserts that especial information as to the number of primitive segments is
afforded by the appearance in the early stages of a series of epibranchial
ganglia in connection with the cranial nerves, which remain permanently in
the case of the vagus nerves, but disappear in the case of pro-otic nerves.
He considers that the evidence points to the number of segments in the mid-
and hind-brain region as being primitively fifteen, viz. six segments
belonging to the trigeminal and abducens group, three segments belonging
respectively to the facial, auditory, and glossopharyngeal, and six to the
vagus.
From this sketch we see that the modern tendency is to make six segments at
least out of the region of the trigeminal nerves rather than two. In this
region, as already mentioned, the evidence of segmentation is based more
clearly on the somatic than on the splanchnic segments. We ought,
therefore, in the first place, to consider the teaching of the eye-muscles
and their nerves and the coelomic cavities in connection with them, and see
whether the hypothesis that such muscles represent the original
dorso-ventral somatic muscles of the palæostracan ancestor is in harmony
with and explains the facts of modern research.
EYE-MUSCLES AND THEIR NERVES.
The only universally recognized somatic nerves belonging to these segments
which exist in the adult are the nerves to the eye-muscles, of which,
according to van Wijhe, the oculomotor is the nerve of the 1st segment, the
trochlearis of the 2nd, and the abducens of the 3rd; while the nerves and
muscles belonging to the 4th and 5th segments, _i.e._ the 2nd facial and
glossopharyngeal segments respectively, show only the merest rudiments, and
do not exist in the adult. One significant fact appears in this statement
of van Wijhe, and is accepted by all those who follow him, viz. that the
oculomotor nerve has equal segmental value with the trochlearis and the
abducens, although it supplies a number of muscles, each of which, on the
face of it, has the same anatomical value as the superior oblique or
external rectus. Dohrn alone, as far as I know, as already pointed out,
insists upon the multiple character of the oculomotor nerve.
{264}As far as the anatomist is concerned, the evidence is becoming clearer
and clearer that the nucleus of the IIIrd nerve is a composite ganglion
composed of a number of nuclei, each similar to that of the trochlearis, so
that if the trochlearis nucleus is a segmental motor nucleus, then the
oculomotor nucleus is a combined nucleus belonging to at least four
segmental nerves, each of which has the same value as that of the
trochlearis.
The investigations of a number of anatomists, among whom may be mentioned
Gudden, Obersteiner, Edinger, Kölliker, Gehuchten, all lead directly to the
conclusion that this oculomotor nucleus is composed of a number of separate
nuclei, of which the most anterior as also the Edinger-Westphal nucleus
contains small cells, while the others contain large cells. Thus Edinger
divides the origin of the oculomotor nerve into a small-celled anterior
part and a larger posterior part, of which the cells are larger and
distinctly arranged in three groups--(1) dorsal, (2) ventral, and (3)
median. Between the anterior and posterior groups lies the Edinger-Westphal
nucleus, which is small-celled; naturally, the large-celled group is that
which gives origin to the motor nerves of the eye-muscles, the small-celled
being possibly concerned with the motor nerves of the pupillary and ciliary
muscles. I may mention that Kölliker considers that the anterior lateral
nucleus has nothing to do with the oculomotor nerve, but is a group of
cells in which the fibres of the posterior longitudinal bundle and of the
deep part of the posterior commissure terminate.
These conclusions of Edinger are the outcome of work done in his laboratory
by Perlia, who says that in new-born animals the nucleus of origin of the
oculomotor nerve is made up of a number of groups quite distinct from each
other, each group being of the same character as that of the trochlearis.
He finds the same arrangement in various mammals and birds. Further, he
finds that some of the fibres arise from the nucleus of the opposite side,
thus crossing, as in the trochlearis; these crossing fibres belong to the
most posterior of the dorsal group of nuclei, _i.e._ to the nerve to the
inferior oblique muscle.
The evidence, therefore, points to the conclusion that the oculomotor
nucleus is a multiple nucleus, each part of which gives origin to one of
the nerves of one of the eye-muscles.
Edinger says that such an array of clinical observations exists, {265}and
of facts derived from post-mortem dissections, that one may venture to
designate the portion of the nucleus from which the innervation of each
individual ocular muscle comes. He gives Starr's table, the latest of these
numerous attempts, begun by Pick. According to Starr, the nuclei of the
nerves to the individual muscles are arranged from before backward, thus--
_m. sphincter iridis._ _m. ciliaris._
_m. levator palpebræ._ _m. rectus internus._
_m. rectus superior._ _m. rectus inferior._
_m. obliquus inferior._
Further, the evidence of the well-known physiological experiments of Hensen
and Völckers that the terminal branches of the oculomotor nerve arise from
a series of segments of the nucleus, arranged more or less one behind the
other in a longitudinal row, leads them to the conclusion that the nuclei
of origin are arranged as follows, proceeding from head to tail:--
Nearest brain. 1. _m. ciliaris._
2. _m. sphincter iridis._
3. _m. rectus internus._
4. _m. rectus superior._
5. _m. levator palpebræ._
6. _m. rectus inferior._
Most posterior. 7. _m. obliquus inferior._
It is instructive to compare this arrangement of Hensen and Völckers with
the arrangement of the origin of these muscles from the premandibular
cavity as given by Miss Platt.
Thus she states that the most posterior part of the premandibular cavity is
cut off so as to form a separate cavity, resembling, except in position,
the anterior cavity; this separate, most posterior part gives origin to the
inferior oblique muscle. She then goes on to describe how the dorsal wall
of the remainder of the premandibular cavity becomes thickened, to form
posteriorly the rudiment of the inferior rectus and anteriorly the
rudiments of the superior and internal recti, a slight depression in the
wall of the cavity separating these rudiments. The internal rectus is the
more median of the two anterior muscles. In other words, her evidence
points not only to a fusion of somites to form the premandibular cavity,
but also to the arrangement of these somites as follows, from head to tail:
(1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior
{266}oblique--an order precisely the same as that of Hensen and Völckers,
and of Starr.
I conclude, from the agreement between the anatomical, physiological, and
morphological evidence, that the IIIrd and IVth nerves contain the motor
somatic nerves belonging to the same segments as the motor trigeminal, in
other words, to the prosomatic segments, so that the eye-muscles,
innervated by III. and IV., represent segmental muscles belonging to the
prosoma. Further, I conclude that originally there were seven prosomatic
segments, the first of which is represented by the anterior cavity
described by Miss Platt, and does not form any permanent muscles; that the
next four belong to the premandibular cavity, and the muscles formed are
the superior rectus, internal rectus, inferior rectus, and inferior
oblique; and that the last two belong to the mandibular cavity, the muscles
formed being Miss Platt's mandibular muscle and the superior oblique. It
is, to say the least of it, a striking coincidence that such an arrangement
of the coelomic cavities as here given should be so closely mimicked by the
arrangement in the prosomatic region of Limulus as already mentioned; it
suggests inevitably that the head-cavities of the vertebrate are nothing
more than the prosomatic and mesosomatic segmental coelomic cavities, as
found in animals such as Limulus. In the table on p. 253, I have inserted
the segments in the vertebrate for comparison with those of Limulus.
Before we can come to any conclusion as to the original position of these
eye-muscles, it is necessary to consider the VIth nerve and the external
rectus muscle. This nerve and this muscle belong to van Wijhe's 4th
segment. The muscle is, therefore, the somatic segmental muscle belonging
to the same segment as the facial and is, in fact, a segmental muscle
belonging not to the prosoma, but to the mesosoma. Neal comes to the
conclusion that the existing abducens is the only root which remains
permanent among a whole series of corresponding ventral roots belonging to
the opisthotic segments, and further points out that the external rectus
was originally an opisthotic muscle which has taken up a pro-otic position,
or, translating this statement into the language of Limulus, etc., it is a
mesosomatic muscle which has taken up a prosomatic position.
There is, however, another muscle--the _Retractor oculi_--belonging to the
same group which is innervated by the VIth nerve. Quite recently Edgeworth
has shown that in birds and reptiles this muscle {267}belongs to the hyoid
segment; so that in this respect also the hyoid segment proclaims its
double nature.
With respect to the external rectus muscle, Miss Platt has shown that the
mandibular muscle is formed close alongside the external rectus, so that
the two are in close relationship as long as the former exists.
Further, as already mentioned, the eye-muscles in Ammocoetes must be
considered by themselves; they do not belong in structure or position to
the longitudinal somatic muscles innervated by the spinal nerves; their
structure is not the same as that of the tubular constrictor or branchial
muscles, but resembles that structure somewhat; their position is
dorso-ventral rather than longitudinal; they may be looked upon as a
primitive type of somatic muscles segmentally arranged, the direction of
which was dorso-ventral.
Anderson also has shown that the time of medullation of the nerves
supplying these muscles is much earlier than that of the nerves belonging
to the somatic trunk-muscles, their medullation taking place at the same
time as that of the motor nerves supplying the striated visceral muscles;
and Sherrington has observed that these muscles do not possess
muscle-spindles, while all somatic trunk-muscles do. Both these
observations are strong confirmation of the view that the eye-muscles must
be classified in a different category to the ordinary somatic trunk muscle
group.
What, then, is the interpretation of these various embryological and
anatomical facts?
Remembering the tripartite division of each segmental nerve-group in
Limulus into (1) dorsal or sensory somatic nerve, (2) appendage-nerve, and
(3) ventral somatic nerve, I venture to suggest that the three nerves--the
_oculomotorius_, the _trochlearis_, and the _abducens_--represent the
ventral somatic nerves of the prosoma, and partly also of the mesosoma;
that they are nerves, therefore, which may have originally contained
sensory fibres, and which still contain the sensory fibres of the
eye-muscles themselves, as stated by Sherrington. According to this
suggestion, the eye-muscles are the sole survivors of the segmental
dorso-ventral somatic muscles, so characteristic of the group from which I
imagine the vertebrates to have sprung. In the mesosomatic region the
dorso-ventral muscles which were retained were those of the appendages and
not of the mesosoma itself, because the presumed ancestor breathed after
the fashion of the water-breathing Limulus, by means of the dorso-ventral
muscles of its {268}branchial appendages, and not after the fashion of the
air-breathing scorpion, by means of the dorso-ventral muscles of the
mesosoma. The only mesosomatic dorso-ventral muscles which were retained
were those of the foremost mesosomatic segments, _i.e._ those supplied by
the VIth nerve, which were preserved owing to their having taken on a
prosomatic position and become utilized to assist in the movements of the
lateral eyes.
Let us turn now to the consideration of the corresponding musculature in
Limulus and in the scorpion group. These muscles constitute the markedly
segmental muscles to which I have given the name 'dorso-ventral somatic
muscles.' They are most markedly segmental in the mesosomatic region, both
in Limulus and in Scorpio, each mesosomatic segment possessing a single
pair of these vertical mesosomatic muscles, as Benham calls them (_cf._
Fig. 58 (_Dv._)). In the prosomatic region the corresponding muscles are
not so clearly defined in Limulus; they are apparently attached to the
plastron forming the group of plastro-tergal muscles. From Benham's
description it is sufficiently evident that they formed originally a single
pair to each prosomatic segment.
In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles
are situated near the middle line on each side and form the following
well-marked series of pairs of muscles, shown in Fig. 110, A, taken from
her paper, and thus described by her:--
1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the
dorso-ventral muscles. It is very small, and is attached to the carapace
near the median line anteriorly to the central eyes.
2. The median dorso-preoral-entosclerite muscle (62) is a large muscle,
between which and its fellow of the opposite side the eyes are situated. It
is attached dorsally to the carapace and ventrally to the pre-oral
entosclerite.
3. The anterior dorso-plastron muscle (63) is attached dorsally to the
carapace in the middle line, being joined to its fellow of the opposite
side. They separate, and are attached ventrally to the plastron. Through
the arch thus formed the alimentary canal and the dorsal vessel pass.
4. The median dorso-plastron muscle (64) is attached dorsally to the
posterior part of the carapace. It runs forward on the anterior surface of
the posterior flap of the plastron to the body of the plastron, to which it
is attached.
{269}[Illustration] A.
DORSO-VENTRAL MUSCLES ON CARAPACE OF SCORPION. (From MISS BECK.)
[Illustration] B.
SIMILAR MUSCLES ON CARAPACE OF EURYPTERUS.
[Illustration] C.
SIMILAR MUSCLES ON HEAD-SHIELD OF A CEPHALASPID.
_l.e._, lateral eyes; _c.e._, central eyes; _Fro._, narial opening.
62-65 refer to Miss Beck's catalogue of the scorpion muscles.
FIG. 110.
{270}To these may be added, owing to its attachment to the plastron,
5. The posterior dorso-plastron muscle (65). This is the first of the
dorso-ventral muscles attached to the mesosomatic tergites, being attached
to the tergite of the first segment of the mesosoma.
This muscle is of interest, in connection with the prosomatic dorso-ventral
muscles, because it is attached to the plastron, and runs a course in close
contact with the muscle (64), the two muscles being attached dorsally close
together, on each side of the middle line, the one at the very posterior
edge of the prosomatic carapace, and the other at the very anterior edge of
the mesosomatic carapace.
Taking these muscles separately into consideration, it may be remarked with
respect to (61) that the cheliceral segment in its paired dorso-ventral
muscles, as in its tergo-coxal muscles, takes up a separate position
isolated from the rest of the prosomatic segments.
Next comes (62) the median dorso-preoral-entosclerite muscle, which is
strikingly different from all the other dorso-ventral muscles in its large
size and the extent of its attachment to the dorsal carapace, according to
Miss Beck's figures. The reason of its large size is clearly seen upon
dissection of the muscles in _Buthus_, for I find that, strictly speaking,
it is not a single muscle, but is composed of a series of muscle-bundles,
separated from each other by connective tissue. There are certainly three
separate muscles included in this large muscle, which are attached in a
distinct series along the pre-oral entosclerite, and present the appearance
given in Fig. 110, A, at their attachment to the prosomatic carapace. Of
this muscle-group the most anterior and the most posterior bundle are
distinctly separate muscles; I am not, however, clear whether the middle
bundle represents one or two muscles.
This division of Miss Beck's muscle (62) into three or four muscles brings
the prosomatic region of the scorpion into line with the mesosomatic, and
enables us to feel sure that a single pair of dorso-ventral somatic muscles
belongs to each prosomatic segment just as to each mesosomatic, and,
conversely, that each such single pair of muscles possesses segmental value
in this region as much as in the mesosomatic.
It is very striking to see how in all the Scorpionidæ, in which the two
median eyes are the principal eyes, this muscle group (62) on the two sides
closely surrounds these two eyes, so that with a fixed {271}pre-oral
entosclerite, a slight movement of the eyes, laterally or anteriorly, owing
to the flexibility of the carapace, might result as the consequence of
their contraction. But this cannot be the main object of these muscles. The
pre-oral entosclerite is firmly fixed to the camerostome, as is seen in
Fig. 94, _pr. ent._, so that the main object of these muscles is, as Huxley
has pointed out, the movement of this organ.
In order to avoid repetition of the long name given to this muscle group
(62) by Miss Beck, because of their position, and for other reasons which
will appear in the sequel, I will call this group of muscles the group of
recti muscles. These recti muscles belong clearly to the segments posterior
to the first prosomatic or cheliceral segment, and represent certainly
three, probably four, of these segments, _i.e._ belong to the segments
corresponding to the second, third, fourth, and fifth prosomatic locomotor
appendages--the endognaths of the old Eurypterids.
The next pair of muscles is the pair of anterior dorso-plastron muscles
(63). This muscle-pair evidently belongs to a segment posterior to the
segments represented by the group already discussed, and belongs,
therefore, in all probability to the same segment as the sixth pair of
prosomatic appendages--the ectognaths of the old Eurypterids. This can be
settled by considering either the nerve-supply or the embryological
development. In the Eurypteridæ it seems most highly probable that the
dorso-ventral muscles of each half of the segments belonging to the
endognaths should be compressed together and separate from the
dorso-ventral muscle belonging to the ectognathal segment, on account of
the evident concentration and small size of the endognathal segments in
contradistinction to the separateness and large size of the ectognathal
segment.
The striking peculiarity of this muscle-pair, which distinguishes it from
all other muscles in the scorpion, is the common attachment of the muscles
of the two sides in the mid-dorsal line, so that the pair of muscles forms
an arch through which the alimentary canal and dorsal blood-vessel pass.
The same dorso-ventral muscles are present in _Phrynus_, and in this animal
the fibres of this pair of muscles (63) actually interlace before the
attachment to the prosomatic carapace, so that the attachment of the muscle
on each side overpasses the mid-dorsal line, and a true crossing occurs. In
Fig. 108 the position of this pair of {272}muscles is shown just
posteriorly to the brain-mass. This muscle I will call the oblique muscle.
Finally we come to the muscles (64) and (65), the median and posterior
dorso-plastron muscles, which run close together. Both muscles are attached
to the plastron, and, therefore, to that extent belong to the prosomatic
region; they are attached dorsally close to the junction of the prosoma and
mesosoma. This position of the first mesosomatic dorso-ventral muscle
belonging to the opercular segment may be compared with the position of the
first mesosomatic dorso-ventral muscle in Limulus which has become attached
to the prosomatic carapace; in both cases we see an indication that the
foremost pair of mesosomatic dorso-ventral somatic muscles tend to take up
a prosomatic position.
As to the pair of small muscles (64), I believe that they represent the
dorso-ventral muscles of the seventh prosomatic segment (if the pair of
muscles (63) belongs to the segment of the sixth locomotor prosomatic
appendages), _i.e._ they belong to the chilarial segment or metastoma.
I desire to draw especial attention to the fact that the dorso-ventral
muscle (64), which represents the seventh segment, always runs close
alongside the dorso-ventral muscle (65), which represents the first
mesosomatic or opercular segment.
The comparison, then, of these two sets of facts leads to the following
conclusions:--
The foremost prosomatic or trigeminal segment stood separate and apart,
being situated most anteriorly; the musculature of this segment does not
develop, so that the only evidence of its presence is given by the anterior
coelomic cavity. This corresponds, according to my scheme, with the first
or anterior coelomic cavity of Limulus, and therefore represents, as far as
the prosomatic appendages are concerned, the first prosomatic
appendage-pair, or the cheliceræ; the appendage-muscles being the muscles
of the cheliceræ, and the dorso-ventral somatic muscles the pair of
dorso-cheliceral sternal muscles (61) in the scorpion. Both these sets of
muscles, therefore, dwindle and disappear in the vertebrate.
Then came four segments fused together to form the premandibular segment,
the characteristic of which is the apparent non-formation of any permanent
musculature from the ventral mesoderm-segments, and the formation of the
eye-muscles innervated by the {273}oculomotor nerve from the dorsal
mesoderm segments. These four segments have been so fused together that van
Wijhe looked upon them as a single segment, and the premandibular cavity as
the cavity of a single segment. They represent, according to my scheme, the
segments belonging to the endognaths, _i.e._ the second, third, fourth,
fifth pairs of prosomatic appendages; the premandibular cavity, therefore,
represents the second coelomic cavity in Limulus, which, according to
Kishinouye, is the sole representative of the coelomic cavities of the
second, third, fourth, fifth prosomatic segments. The muscles derived from
the ventral mesoderm-segments represent the muscles of these appendages,
which therefore dwindle and disappear in the vertebrate, with the possible
exception of the muscles innervated by the descending root of the
trigeminal. The muscles derived from the dorsal mesoderm-segments, _i.e._
the eye-muscles supplied by the oculomotor nerve, represent the
dorso-ventral somatic muscles of these four segments, muscles which are
represented in the scorpion by the recti group of muscles, _i.e._ the
median dorso-preoral-entosclerite muscles (62).
Then came two segments, the mandibular, in which muscles are formed both
from the ventral and from the dorsal mesoderm-segments. From the former
arose the main mass of muscles innervated by the motor root of the
trigeminal, from the latter the superior oblique muscle and the mandibular
muscle of Miss Platt, of which the former alone survives in the adult
condition. These two segments are looked upon as a single segment by van
Wijhe, of which the mandibular cavity is the coelomic cavity. They
represent, according to my scheme, the segments belonging to the sixth pair
of prosomatic appendages or ectognaths, and the seventh pair, _i.e._ the
chilaria or metastoma.
The first part, then, of the mandibular cavity represents the third
coelomic cavity in Limulus and the muscles derived from the ventral
mesoderm, in all probability the muscles of the tongue in the lamprey
(_cf._ Chap. IX.), which represents the ectognaths or sixth pair of
prosomatic appendages, while the muscles derived from the dorsal mesoderm,
_i.e._ the superior oblique muscles, represent the dorso-ventral somatic
muscles of this segment, muscles which are represented in the scorpion
group by the pair of anterior dorso-plastron or oblique muscles (63).
The second part of the mandibular cavity represents the 4th {274}coelomic
cavity in Limulus and the muscles derived from the ventral mesoderm, in all
probability the muscles of the lower lip in the lamprey (_cf._ Chap. IX.),
which represents the metastoma; while the muscles derived from the dorsal
mesoderm, _i.e._ Miss Platt's pair of mandibular muscles, represent the
dorso-ventral somatic muscles of this segment, muscles which are
represented in the scorpion group by the pair of median dorso-plastron
muscles (64).
In connection with this last pair of muscles we find that the external
rectus in the vertebrate represents the first dorso-ventral mesosomatic
muscle in the scorpion, _i.e._ the posterior dorso-plastron muscle (65),
and, as already mentioned (p. 267), that it always lies closely alongside
the mandibular muscle, just as in the scorpion group muscle (65) always
lies alongside muscle (64).
In the invertebrate as well as in the vertebrate this muscle is a
mesosomatic muscle which has taken up a prosomatic position.
The question naturally arises, what explanation can be given of the fact
that these dorso-ventral muscles attached on each side of the mid-dorsal
line to the prosomatic carapace became converted into the muscles moving
the eyeballs of the two lateral eyes? An explanation which must take into
account not only the isolated position of the abducens nerve, but also the
extraordinary course of the trochlearis. The natural and straightforward
answer to this question appears to me quite satisfactory, and I therefore
venture to commend it to my readers.
I have argued the case out to myself as follows: The lateral eyes must have
been originally situated externally to the group of muscles innervated by
the oculomotor nerve, for a sheet of muscle representing the superior
_internal_ and inferior rectus muscles could only wrap round the internal
surface of each lateral eye; _i.e._ the arrangement of the muscle-sheet, as
in the scorpion, about two median eyes, is in the wrong position, for if
those two eyes, which are the main eyes in the scorpion, were to move
outwards to become two lateral eyes, then such a muscle-group would form a
superior _external_ and inferior rectus group. The evidence, however, of
Eurypterus and similar forms is to the effect that the lateral eyes became
big and the median eyes insignificant and degenerate. If, then, with the
degeneration of the one and the increasing importance of the other, these
lateral eyes came near the middle line, then the muscular group (62), which
I have called the recti group, would naturally be pressed into their
{275}service, and would form an internal and not an external group of
eye-muscles.
In Fig. 110, A, taken from Miss Beck's paper, I have shown the relative
position of the eyes and the segmental dorso-ventral prosomatic muscles on
the carapace of the scorpion. In Fig. 110, B, I have drawn the prosomatic
carapace of _Eurypterus Scouleri_, taken from Woodward's paper, with the
eyes as represented there; in this I have inserted the segmental
dorso-ventral muscles as met with in the scorpion, thereby demonstrating
how, with the degeneration of the median eyes and the large size of the
lateral eyes, the recti muscles of the scorpion would approach the position
of an internal recti group to the lateral eyes, and so give origin to the
group of muscles innervated by the oculomotor nerve. In the Eurypterus
these large eyes are large single eyes, not separate ocelli, as in the
scorpion.
All, then, that is required is that in the first formed fishes, which still
possessed the dorso-ventral muscles of their Eurypterid ancestors, the
lateral eyes should be the important organs of sight, large and near the
mid-dorsal line. Such, indeed, is found to be the case. In amongst the
masses of Eurypterids found in the upper Silurian deposits at Oesel, as
described by Rohon, numbers of the most ancient forms of fish are found
belonging to the genera Thyestes and Tremataspis. The nature of the dorsal
head-shields of these fishes is shown in Fig. 14, which represents the
dorsal head-shield of _Thyestes verrucosus_, and Fig. 111 that of
_Tremataspis Mickwitzi_. They show how the two lateral eyes were situated
close on each side of the mid-dorsal line in these Eurypterus-like fishes,
in the very position where they must have been if the eye-muscles were
derived from the dorso-ventral somatic muscles of a Eurypterid ancestor.
[Illustration: FIG. 111.--DORSAL HEAD-SHIELD OF _Tremataspis Mickwitzi_.
(From ROHON.)
_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum plate over
brain; _Occ._, occipital spine.]
In Lankester's words, one of the characteristics of the Osteostraci
(Cephalaspis, Auchenaspis, etc.), as distinguished from the Heterostraci
(Pteraspis), are the large orbits placed near the centre of the shield. The
apparent exception of Thyestes mentioned by him is no {276}exception, for
orbits of the same character have since been discovered, as is seen in
Rohon's figure (Fig. 14). In Fig. 110, C, I give an outline of the frontal
part of the head-shield of a Cephalaspid, in which I have drawn the
eye-muscles as in the other two figures.
Although all the members of the Osteostraci possess large lateral eyes
towards the centre of the head-shield, the other group of ancient fishes,
the Heterostraci, are characterized by the presence of lateral eyes far
apart, situated on the margin of the head-shield on each side (_cf._ Fig.
142, _o_, p. 350).
So, also, on the invertebrate side, the lateral eyes of Pterygotus and
Slimonia are situated on the margin of the prosomatic carapace, while those
of Eurypterus and Stylonurus are situated much nearer the middle line of
the prosomatic carapace.
Next comes the question of the superior oblique muscle and the trochlearis
nerve. Why does this nerve (_n.IV._ in Fig. 106, C and D) alone of all the
nerves in the body take the peculiar position it always does take? The only
suggestion that I know of which sounds reasonable and worth consideration
is that put forward by Fürbringer, which is an elaboration of the original
suggestion of Hoffmann. Hoffmann suggested in 1889 that the trochlearis
nerve represented originally a nerve for a protecting organ of the pineal
eye, which became secondarily a motor nerve for the lateral eye as the
pineal eye degenerated. Fürbringer differs from Hoffmann in that he
considers that the nerve was originally a motor nerve, and was not
transformed from sensory to motor, yet thinks Hoffmann's suggestion is in
the right direction.
He points out that the crossing of the trochlearis is not a crossing of
fibres between two centres in the central nervous system, but may be
explained by the shifting of the peripheral organ, _i.e._ the muscle, from
one side to the other, and the nerve following this shift. Consequently,
says Fürbringer, the course of the nerve indicates the original position of
the muscle, and therefore he imagines that the ancestor of the superior
oblique muscle was a muscle the fibres of which were attached in the
mid-dorsal line, and interlaced with those of the other side, the two
muscles thus forming an arch through which the nervous system with its
central canal passed. Then, for the sake of getting a more efficient pull,
the crossing muscle-fibres became more definitely attached to the opposite
side of the middle line, and finally obtained a new attachment on the
opposite side, with the {277}obliteration of the muscular arch; the nerve
on each side, following the shifts of the muscle, naturally took up the
position of the original muscular arch, and so formed the trochlear nerve,
with its dorsal crossing. This explanation of Fürbringer's was associated
by him with movements of the median pineal eyes, the length of their nerve,
according to him, even yet indicating their previous mobility. This
assumption is not, it seems to me, necessary. The length of the nerve is
certainly no indication of mobility, for in Limulus and the scorpion group
the nerve to each median eye is remarkably long, yet these eyes are
immovably fixed in the carapace. All that is required is a pair of
dorso-ventral muscles belonging to the segment immediately following the
group of segments represented by the oculomotor nerves, the fibres of which
should cross the mid-dorsal line at their attachment; for, seeing that the
lateral eyes were originally so near this position, it follows that such
muscles might form part of the muscular group belonging to the lateral eye
without having previously moved the pineal eyes. In fact, Fürbringer's
explanation requires as starting-point that the pair of muscles which
ultimately become the superior oblique should have the exact position of
the pair of dorso-ventral muscles in the scorpion, called by Miss Beck the
anterior dorso-plastron muscles (63), which I have named the oblique
muscles. Here, and here only, do we find an interlacement, across the
mid-dorsal line, of the fibres of attachment of the muscles on the two
sides, in consequence of which this pair of muscles is described by her as
forming an arch encircling the alimentary canal and dorsal vessel. If,
then, as I have previously argued, the primitive plastron formed a pair of
trabeculæ, and the nervous system grew round the alimentary canal, such an
arch would encircle the tubular central nervous system of the vertebrate.
Still more striking is this pair of muscles (63) in Phrynus (Fig. 108),
where we see how the arch formed by them almost touches the posterior
extremity of the supra-oesophageal brain-mass, crossing, therefore, over
the beginning of the stomach region of the animal. The angle formed by the
arch is much more obtuse than that formed in Scorpio, so that an actual
crossing of the muscle-fibres has taken place at the point of attachment to
the carapace. Also, only the part nearest the carapace is muscular, the
rest forming a long tendinous prolongation of the plastron wall (the
primordial cranium), as seen in the figure.
{278}This muscle-pair is, as it should be, the pair of dorso-ventral
muscles belonging to the segment immediately following on the group of
segments represented by the recti muscles, _i.e._ according to previous
argument, the segment belonging to the sixth pair of locomotor appendages
or ectognaths; a muscle, therefore, which would arise in the vertebrate
from the mandibular, and not from the premandibular cavity. A similar
muscle probably existed in Eurypterus (_M.obl._ in Fig. 106, B), and, as in
the case of the formation of the oculomotor group, derived from the recti
group of the scorpion, would form the commencement of the superior oblique
muscle in Thyestes and Tremataspis.
[Illustration: FIG. 112.--A, DIAGRAM OF POSITION OF OBLIQUE MUSCLE IN
SCORPION; B, DIAGRAM OF TRANSITION STAGE; C, DIAGRAM OF SUPERIOR OBLIQUE
MUSCLE IN VERTEBRATE.
_l.e._, lateral eyes; _c.e._, central eyes; _C.N._, central nervous system;
_Al._, alimentary canal; _c._, _aqueductus Sylvii_.]
It is instructive to notice that the original position of attachment of
this muscle is naturally posterior to that of the oculomotor group of
muscles, and that Fürbringer, in his description of the eye-muscles of
Petromyzon, asserts that this muscle in this primitive vertebrate {279}form
is not attached as in other vertebrates, but is posterior to the other
muscles, so that he calls it the posterior rather than the superior
oblique. The nature of the change by which the muscle known in the scorpion
as the anterior dorso-plastron muscle (63) was probably converted into the
superior oblique muscle of the vertebrate, is represented in the drawings
Fig. 112, in which also are indicated the dwindling of the median eyes, and
the progressive superiority of the lateral eyes, as well as the
transformation of the recti muscle-group of the scorpion into the muscles
supplied by the oculomotor nerve of the vertebrate.
With respect to the external rectus muscle, it follows naturally that if
the muscles (64) and (65) are to follow suit with the rest of the group and
become attached to the lateral eyes, they must take up an external
position. These two muscles, which always run together, as seen in Fig.
110, A, the one belonging to the prosoma and the other to the mesosoma, are
represented by the mandibular muscle of Miss Platt and the external rectus,
the former derived from the walls of the last pro-otic head-cavity, the
latter from the foremost of the opisthotic head-cavities.
Such, then, is the simple explanation of the origin of the eye-muscles
which follows from my theory, and we see that the successive alterations of
the position of the orbit, and, therefore, of the globe of the eye with its
muscles, as we pass from Thyestes to man, is the natural consequence of the
growth of the frontal bone, _i.e._ of the brain.
THE TRIGEMINAL NERVES AND THE MUSCLES SUPPLIED BY THEM.
Turning now to the evidence as to the number of ventral segments, _i.e._
the motor and sensory supply to the prosomatic appendages afforded by the
trigeminal nerve, we must, I think, come to the same conclusion as Dohrn,
viz. that if there were originally seven dorsal or somatic segments in this
region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles
of the premandibular cavity, _sup. rectus_, _inf. rectus_, _int. rectus_,
_inf. oblique_, supplied by IIIrd nerve; 6, 7, muscles of the mandibular
cavity, _sup. oblique_, supplied by IVth nerve and muscle lost, there must
have been also seven corresponding ventral or splanchnic segments supplied
by the trigeminal. At present the evidence for such segments is nothing
like so strong as for the corresponding somatic ones; there are, however,
certain suggestive {280}facts which point distinctly in this direction in
connection with both the motor and sensory parts of the trigeminal. The
origin of the trigeminal motor fibres in the central nervous system is most
striking. We may take it for granted that a nucleus of cells giving origin
to one or more segmental motor nerves will possess a greater or less
longitudinal extension in the central nervous system, according to the
number of fused separate segmental centres it represents. Thus a nucleus
such as that of the IVth nerve or of the facial is small and compact in
comparison to the extensive conjoint nucleus of the vagus and cranial
accessory.
Upon examination of the motor nucleus of the trigeminal, we find a compact
or well-defined nucleus, the _nucl. masticatorius_, the nerves of which
supply the masseter, temporal, and other muscles, so that the anatomical
evidence at first sight appears to bear out van Wijhe's conclusion that the
motor trigeminal supplies at most two segments. Further examination,
however, shows that this is not all, for the extraordinary so-called
descending root of the Vth must be taken into consideration in any question
of the origin of the motor elements, just as the equally striking ascending
root enters into the consideration of the meaning of the sensory elements
of the Vth.
It is not necessary here to discuss the controversy as to whether this
descending root is motor or sensory. It is universally considered at
present to be motor, and is believed to supply, as Kölliker suggested,
among other muscles, the _m. tensor tympani_ and the _m. tensor veli
palati_. It is thus described by Obersteiner--
"From the region of the mid-brain the motor root receives an important
addition of thick fibres, which form the cerebral or descending root. The
large, round vesicular cells from which the fibres of the descending root
arise form no single compact group, but are partly single, partly arranged
like little bunches of grapes, as far as the region of the anterior corpora
quadrigemina. The further we go brainwards, the smaller is the number of
fibres. In the region of the anterior corpora quadrigemina, the few cells
of origin are found more and more median; so that the uppermost trigeminal
fibres descend in curves almost from the mid-line, as is shown by the
exceptional occurrence of one or more of the characteristic cells above the
aqueduct. At the height of the posterior commissure one finds the last of
these trigeminal cells."
{281}The anatomy of the Vth nerve reveals, then, three most striking
facts:--
1. The motor nucleus of the Vth extends from the very commencement of the
infra-infundibular region to nearly the commencement of the nucleus of the
VIIth; in other words, the motor nucleus of the Vth extends through the
whole prosomatic region, just as it must have done originally if its motor
nerves supplied the muscles of the prosomatic appendages. Such an extended
range of origin is indicative of the remains of an equally extended series
of segmental centres or ganglia.
2. Of these centres the caudalmost have alone remained large and vigorous,
constituting the _nucleus masticatorius_, which in the fish is divided into
an anterior and posterior group, thus indicating a double rather than a
single nucleus; while the foremost ones have dwindled away until they are
represented only by the cells of the descending root, the muscles of these
segments being still represented by possibly the _tensor veli palati_ and
the other muscles innervated from these cells.
3. The headmost of these cells takes up actually a position dorso-lateral
to the central canal, so that the groups on each side nearly come together
in the mid-dorsal line; a very unique and extraordinary position for a
motor cell-group, but not improbable when we recall to mind Brauer's
assertion as to the shifting of the foremost prosomatic ganglion-cells of
the scorpion from the ventral to the dorsal side of the alimentary canal.
On the sensory side the evidence is also suggestive, the question here
being not so much the distribution of the sensory nerves as the number of
ganglia belonging to each of the cranial nerves.
With respect to this question, morphologists have come to the conclusion
that there is a marked difference between spinal and cranial nerves, in
that whereas the posterior root-ganglia of the spinal nerves arise from the
central nervous system itself, _i.e._ from the neural crest, the ganglia of
the cranial nerves arise partly from the neural crest, partly from the
proliferation of cells on the surface of the animal; and because of the
situation of these proliferating epidermal patches over the gill-clefts in
the case of the vagus and glossopharyngeal nerves, they have been called by
Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia
into two sets, one connected with the neural ridges, called the neural
ganglia, {282}and the other connected with the surface-cells, which he
calls the lateral ganglia. This second set corresponds to Kupffer's
epibranchial ganglia. Now it is clear that in the case of the vagus nerve,
where, as is well shown in Ammocoetes, the nerve is not a single segmental
nerve, but is in reality made up of a number of nerves going to separate
branchial segments, the indication of such segments is not given by the
main vagus ganglion or neural ganglion, but by the series of lateral
ganglia. So also it is argued in the case of the trigeminal, that if in
addition to the ganglion-cells arising from the neural crest separate
ganglion-masses are found in the course of development, in connection with
proliferating patches of the surface (plakodes, Kupffer calls them), then
such isolated lateral ganglia are indications of separate segments, just as
in the case of the vagus, even though the separate segments do not show
themselves in the adult. So far the argument appears to me just, but the
further conclusion that the presence of such plakodes shows the previous
existence of _branchial_ sense-organs, and, therefore, that such ganglia
are _epibranchial_ ganglia, indicating the position of a lost gill-slit, is
not justified by the premises. If, as I suppose, the trigeminal nerve
supplied a series of non-branchial appendages serially homologous with the
branchial appendages supplied by the vagus, then it is highly probable that
the trigeminal should behave with respect to its sensory ganglia similarly
to the vagus nerve, without having anything to do with branchiæ.
Such plakodal ganglia, then, may give valuable indication of non-branchial
segments as well as of branchial segments. The researches of Kupffer on the
formation of the trigeminal ganglia in Ammocoetes are the chief attempt to
find out from the side of the sensory ganglia the number of segments
originally belonging to the trigeminal. The nature and result of these
researches is described in my previous paper (_Journal of Anatomy and
Physiology_, vol. xxxiv.), and it will suffice here to state that he
himself concludes that the trigeminal originally supplied five at least,
probably six, segments. As I have stated there, the evidence as given by
him seems to me to indicate even as many as seven segments.
In the full-grown Ammocoetes, as is well known, there are two distinct
ganglia belonging to the trigeminal, the one the ganglion of the _ramus
ophthalmicus_, the other the main ganglion.
According to Kupffer the larval Ammocoetes possesses three sets of ganglia,
not two, for between the foremost and hindmost ganglion {283}he describes a
nerve (_x._, Fig. 113), with four epibranchial ganglia, which do not
persist as separate ganglia, but either disappear or are absorbed into the
two main ganglia (Fig. 113). This discovery of Kupffer's is very
suggestive, for, as already stated, a transformation takes place when the
Ammocoetes is 5 mm. long, so that the arrangement of the parts before that
period is distinctly more indicative of the ancestral arrangement than any
later one.
If we use the name plakodal ganglia to represent that part of these ganglia
which was originally connected with the skin, then Kupffer's researches
assert that in the larval Ammocoetes there were seven such plakodal
ganglia, one in front belonging to the foremost trigeminal ganglion, two
behind, parts of the hindmost ganglion, and four in between, which do not
exist later as separate ganglia.
[Illustration: FIG. 113.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4
MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)
_A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._,
notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper
lip; _l.l._ lower lip; _vel._, septum between oral and respiratory
cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four
epibranchial ganglia.]
In accordance with the views put forward in this book, a possible
interpretation of these plakodal ganglia would be given as follows:--
Beard, who, after Froriep, drew attention to this relation of the cranial
ganglia to special skin-patches, has compared them with the parapodial
ganglia of annelids, _i.e_. ganglia in connection with annelidan
appendages; whether we are here obtaining a glimpse of the far-off
annelidan ancestry of both arthropods and vertebrates it would be premature
at present to say. It is natural enough to expect, on my view, to find
evidence of annelidan ancestry in {284}vertebrate embryology (as has been
so often asserted to be the case), seeing that undoubtedly the Arthropoda
are an advanced stage of Annelida; and, indeed, the way is not a long one
when we consider Beecher's evidence that the Trilobita belong to the
Phyllopoda, certainly a primitive crustacean group, which Bernard derives
directly from the annelid group Chætopoda. If, then, these plakodal ganglia
indicate the former presence of appendages, we obtain this result:--The
foremost ganglion on each side possesses one plakodal ganglion, and
therefore indicates an anterior pair of appendages, possibly the cheliceræ.
Then comes the peculiar nerve with four plakodal ganglia indicating on each
side four appendages close together, possibly the endognaths. Then,
finally, on each side, the second large ganglion with two plakodal ganglia,
indicating two pairs of appendages, possibly the ectognaths and the
metastoma.
SUMMARY.
The consideration of the history of the cranial segmentation shows that
whereas, from the commencement of that history, the evidence for two
ventral segments supplied by the trigeminal nerve is clear and
unmistakable, later observers have tended more and more to increase the
number of these segments, until at the present time the evidence is in
favour of at least six, probably seven, as the number of segments
supplied by the motor part of the trigeminal.
So, also, the original evidence for the number of dorsal or somatic
segments limits the number to three, innervated respectively by the
oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather
two, since the last nerve belongs to the facial segment. The muscles
which these three nerves supply are derived respectively from the walls
of the premandibular, mandibular, and hyoid coelomic cavities.
Later evidence points strongly to the conclusion that the oculomotor
nerve and the premandibular cavity represent not one segment but the
fusion of four, while the mandibular cavity represents two segments. In
addition to these, Miss Platt has discovered a still more anterior
head-cavity, which she has named the anterior cavity, so that the
pro-otic segments on this reckoning are seven in number, viz.: (1) the
anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the
mandibular cavity. The somatic muscles belonging to these dorsal segments
are the eye-muscles, which are all dorso-ventral in position, and are not
the same as the longitudinal somatic muscles, but belong to a distinct
dorso-ventral segmental group, the only representative of which at
present known in the mesosomatic region is the external rectus innervated
by the VIth nerve.
These head-cavities, and these muscles of the vertebrate, resemble the
corresponding cavities and muscles of the invertebrate to an
extraordinary {285}degree, so that it becomes easy to see how the
dorso-ventral muscles of the prosomatic segments of the latter have
become converted into the eye-musculature of the former. The most
powerful proof of all that such a conversion has taken place is that a
natural and simple explanation is at once given of the extraordinary
course taken by the IVth or trochlear nerve. Ever since neurology began,
the course of this nerve has arrested the attention of anatomists. Why
should just this one pair of nerve-roots of all those in the whole body
be directed dorsalwards instead of ventralwards, and cross each other in
the valve of Vieussens, each to supply a simple eye-muscle (the superior
oblique) belonging to the other side? For generations anatomists have
wondered and found no solution, and yet, without any straining of
hypotheses, in consequence simply of the investigation of the anatomy of
the corresponding pair of muscles in the scorpion group, the solution is
immediately apparent.
This pair of muscles alone, of all the musculature attached to the
carapace, crosses the mid-dorsal line to be attached to the other side,
thus carrying its nerve with it to the other side; by a continuation of
the same process the relation of the trochlear to the superior oblique
muscle can be explained.
The comparison of the eye-muscles of the vertebrate with the
dorso-ventral segmented muscles of the invertebrate makes the number and
nature of the pro-otic segments much clearer.
{286}CHAPTER IX
_THE PROSOMATIC SEGMENTS OF AMMOCOETES_
The prosomatic region in Ammocoetes.--The suctorial apparatus of the
adult Petromyzon.--Its origin in Ammocoetes.--Its derivation from
appendages.--The segment of the lower lip or metastomal segment.--The
tentacular segments.--The tubular muscles.--Their segmental
arrangement.--Their peculiar innervation.--Their correspondence with the
system of veno-pericardial muscles in Limulus.--The old mouth or
palæostoma.--The pituitary gland.--Its comparison with the coxal gland of
Limulus.--Summary.
In the last chapter it was seen not to be incompatible with both the
anatomical and morphological evidence to look upon the trigeminal nerves as
having originally supplied the seven prosomatic pairs of appendages of the
invertebrate ancestor, the foremost of which, the cheliceræ, and the four
pairs of endognaths dwindled away and became insignificant, leaving as
trace of their former presence the descending root of the Vth nerve; while
the two hindmost pairs, the ectognaths and the chilaria, or metastoma,
remained vigorous and developed, leaving as proof of their presence the
_nucleus masticatorius_. Evidence in favour of this suggestion and of the
nature of the dwindling process is afforded when we examine what the
trigeminus does supply in Ammocoetes. In all vertebrates this nerve
supplies the great muscles of mastication which, in all gnathostomatous
fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws;
they take in their food by attaching themselves to their prey and by means
of rasping teeth situated in serried rows within the circular mouth,
combined with a powerful suctorial apparatus, they suck the juices of the
fish they feed upon. Not possessing jaws, they feed by suction on the
living animal, a method of feeding which gives them no more claim to be
classed as parasitic animals than the whole group of spiders which feed in
a similar manner on living flies.
{287}THE ORIGIN OF THE SUCTORIAL APPARATUS OF PETROMYZON.