antero-lateral markings indicative of segments, found so frequently in
these primitive fishes, must be interpreted as due not to gills but to
aponeuroses, due to the presence of muscles which moved prosomatic
appendages, muscles which arose from the dorsal region in very much the
same position as do the muscles of the lower lip in Ammocoetes; the latter,
as already argued, represent the tergo-coxal muscles of the last pair of
prosomatic appendages--the chilaria or metastoma. Such an interpretation of
these markings signifies that the first-formed fishes must have possessed
prosomatic appendages of a more definite character than the tentacles of
Ammocoetes, something intermediate between those of the palæostracan and
Ammocoetes.
For my part I should not be in the least surprised were I to hear that
something of the nature of appendages in this region had been found,
especially in view of the well-known existence of the pair of appendages in
the members of the Asterolepidæ--large, oar-like appendages which may well
represent the ectognaths.
{343}THE RELATIONSHIP OF THE OSTRACODERMS.
Of the three groups of fishes--the Heterostraci, the Osteostraci, and the
Antiarcha--the last is Devonian, and therefore the latest in time of the
three, while the earliest is the first group, as both Pteraspis and
Cyathaspis have been found in lower levels of the Silurian age than any of
the Osteostraci, and, indeed, Cyathaspis has been discovered in Sweden in
the lower Silurian. This, the earliest of all groups of fishes, is confined
to two forms only--Pteraspis and Cyathaspis,--for Scaphaspis is now
recognized to be the ventral shield of Pteraspis.
Hitherto a strong tendency has existed in the minds both of the comparative
anatomist and the palæontologist to look on the elasmobranchs as the
earliest fishes, and to force, therefore, these strange forms of fish into
the elasmobranch ranks. For this purpose the same device is often used as
has been utilized in order to account for the existence of the
Cyclostomata, viz. that of degeneration. The evidence I have put forward is
very strongly in favour of a connection between the cyclostomes and the
cephalaspids, and agrees therefore with all the rest of the evidence that
the jawless fishes are more ancient than those which bore jaws--the
Gnathostomata.
This is no new view. It was urged by Cope, who classified the Heterostraci,
Osteostraci, and Antiarcha under one big group--the Agnatha--from which
subsequently the Gnathostomata arose. Cope's arguments have not prevailed
up to the present time, as is seen in the writings of Traquair, one of the
chief authorities on the subject in Great Britain. He is still an advocate
of the elasmobranch origin of all these earliest fishes, and claims that
the latest discoveries of the Silurian deposits (_Thelodus Pagei_) and
other members of the Coelolepidæ confirm this view of the question.
This view may be summed up somewhat as follows:--
Cartilaginous jaws would not fossilize, and the Ostracoderms may have
possessed them.
They may have degenerated from elasmobranchs just as the cyclostomes are
supposed to have degenerated.
Seeing that bone succeeds cartilage, the presence of bony shields in
Cephalaspis, etc., indicates that their precursors were cartilaginous,
presumably elasmobranch fishes.
Of these arguments the strongest is based on the supposed bony
{344}covering of the Osteostraci, with the consequent supposition that
their ancestors possessed a cartilaginous covering. This argument is
entirely upset, if, as I have pointed out, the structure of the cephalaspid
shield is that of muco-cartilage and not of bone. If these plates are a
calcified muco-cartilage, then the whole argument for their ancestry from
animals with a cartilaginous skeleton falls to the ground, for
muco-cartilage is the precursor not only of bone, but also of cartilage
itself.
The evidence, then, points strongly in favour of Cope's view that the most
primitive fishes were Agnatha, after the fashion of cyclostomes, as is also
believed by Smith Woodward, Bashford Dean, and Jaekel.
Among living animals, as I have shown, the Limulus is the sole survivor of
the palæostracan type, and Ammocoetes alone gives a clue to the nature of
the cephalaspid, _i.e._ the osteostracan fish. Older than the latter is the
heterostracan, Pteraspis, and Cyathaspis. Is it possible from their
structure to obtain any clue as to the actual passage from the palæostracan
to the vertebrate?
Here again, as in the case of the Osteostraci, a relationship to the
elasmobranch has been supposed, for the following reasons:--
The latest discoveries in the Silurian and Devonian deposits have brought
to light strange forms such as Thelodus and Drepanaspis, of which the
latter from the Devonian must, according to Traquair, be included in the
Heterostraci. It possessed, as seen in Fig. 139, large plates, after the
fashion of Pteraspis, and also many smaller ones.
The former, from the upper Silurian, belongs to the Coelolepidæ, and was
covered over with shagreen composed of small scutes, after the fashion of
an elasmobranch. Traquair suggests that Thelodus arose from the original
elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis
occurred, and, with still further fusion, Pteraspis.
There are always two ways of looking at a question, and it seems to me
possible and more probable to turn the matter round and to argue that the
original condition of the surface-covering was that of large plates, as in
Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was
formed, and later on, by further splitting, the elasmobranch, Thelodus
being a stage on the way to the formation of an elasmobranch, and not a
backward stage from the elasmobranch towards Pteraspis.
{345}This method of looking at the problem seems to me to be more in
consonance with the facts than the reverse; for, as pointed out by Jaekel,
the fishes with large plates are the oldest, and in Cyathaspis, the very
oldest of all, the size of the plates is most conspicuous; he considers,
therefore, this preconceived view that large plates are formed by the
fusion of small ones must give way to the opposite belief.
[Illustration: FIG. 139.--DREPANASPIS. VENTRAL AND DORSAL ASPECTS. (After
LANKESTER.)
_A._, anus; _E._, lateral eyes.]
So also Rohon, as quoted by Traquair, who, in his first paper accepted
Lankester's view that the ridges of the pteraspidian shield were formed by
the fusion of a linear arrangement of numbers of placoid scales, suggests
in his second paper that these ridges may have been the most primitive
condition of the dermal skeleton of the vertebrate, out of which, by
differentiation, the dermal denticles (placoid scales) of the selachian, as
well as their modifications in the ganoids, teleosteans, and amphibians,
have arisen.
One thing is agreed upon on all sides; no sign of bone-corpuscles is to be
found in this dermal covering of Pteraspis. In the deeper layers are large
spaces, the so-called pulp-cavities leading into narrow canaliculi, the
so-called dentine canals. The structure is {346}looked upon as similar to
that of the pulp and dentine canals of many fish-scales.
On the other hand, this dermal covering of Pteraspis has been compared by
Patten with the arrangement of the chitinous structure of certain parts of
the external covering of Limulus, a comparison which to my mind presents a
great difficulty. The chitin-layers in Limulus are _external_ to the
epidermal cells, being formed by them; the layers in Pteraspis which look
like chitin must have been _internal_ to the epidermal layer; for each
vascular canal which passes from a pulp-cavity on its way to be distributed
into the dentine canals of the ridge gives off short side branches, which
open directly into the groove between the ridges. If these canals were
filled with blood they could not possibly open directly into the open
grooves between the ridges; these openings must, therefore, have been
covered over with an epithelial layer which covered over the surface of the
animal, and consequently the chitin-like structure must have been internal
to the epidermis, and not external, as on Patten's view. The comparison of
this structure with the dentine of fish-scales signifies the same thing,
for in the latter the epidermis is external to the dentine-plates, the hard
skeletal structure is in the position of the cutis, not of the cuticle.
The position appears to me to be this: the dermal cranial skeleton of
vertebrates, whether it takes the form of a bony skull or of the dorsal
plates of a cephalaspid or a pteraspid is, in all cases, not cuticular,
_i.e._ is not an external formation of the epidermal cells, but is formed
in tissue of the nature of connective tissue underlying the epidermis. On
the contrary, the hard part of the head-carapace of the palæostracan is an
external formation of the epidermal cells.
If, then, this tissue of Pteraspis is not to be looked upon as chitin, how
can we imagine its formation? It is certainly not bone, for there are no
bone-corpuscles; it is a very regular laminated structure resembling in
appearance chitin rather than anything else.
As in all cases of difficulty, turn to Ammocoetes and let us see what clue
there is to be found there. The skin of Ammocoetes is peculiar among
vertebrates in many respects. It consists of a number of epidermal cells,
as in Fig. 140, the varying function of which need not be considered here,
covered over with a cuticular layer which is extraordinarily thick for the
cuticle of a vertebrate skin; this cuticular layer is perforated with fine
canaliculi, through which the {347}secretion of the underlying cells
passes, as is seen in Fig. 140, A and B. This cuticle corresponds to the
chitinous covering of the arthropod, and like it is perforated with
canaliculi, and, according to Lwoff, possibly contains chitin. The
epidermal cells rest on a thick layer of most striking appearance (Fig.
141), for it resembles, in an extraordinary degree, when examined
superficially, a layer of chitin; it is called the laminated layer, and is
characterized by the extreme regularity of the laminæ. This appearance is
due, as the observations of Miss Alcock show, to alternate layers of
connective tissue fibres arranged at right angles to each other, each fibre
running a straight course and possessing its own nucleus. Although the
fibres in each layer are packed close together, they are sufficiently apart
to form with the fibres of the alternate layers a meshwork rather than a
homogeneous structure, and thus the surface view of this layer shows a
regular network of very fine spaces through which nerve-fibres and fluid
pass. This layer is easily dissolved in a solution of hypochlorite of soda,
a fluid which dissolves chitin. Any one looking at Ammocoetes would say
that the only part of its skin which resembles chitin is this laminated
layer, and therefore the only part of its skin which would afford an
indication of the nature of the skeleton of Pteraspis is this laminated
layer, which belongs to the cutis, and not to the cuticle. Yet another
significant peculiarity of this layer is its entire disappearance at
transformation. Miss Alcock, in a research not yet published, has shown
that this layer is completely broken up and absorbed at transformation; the
cutis of Petromyzon is formed entirely anew, and no longer presents any
regular laminated character, but resembles rather the sub-epidermal
connective tissue layer of the skin of higher vertebrates. This laminated
layer, then, just like the muco-cartilage, shows, by its complete
disappearance at transformation, its ancestral character.
[Illustration: FIG. 140.--EPITHELIAL CELLS OF AMMOCOETES TO SHOW THE
CANALICULI IN THE THICK CUTICLE (B). A, TRANSVERSE SECTION THROUGH THE
CUTICLE.]
Very suggestive is the arrangement of the different skeletal {348}tissues
in the head-region of Ammocoetes. Fig. 141 represents a section through the
head near the pineal eye. Most internally is _a_, a section of the
membranous cranium, then comes _b_, the muco-cartilaginous skeleton, then
_c_, the laminated layer, and finally _d_, the external cuticle. If in
Ammocoetes we possess an epitome of the history of the vertebrate, how
would these layers be represented in the past ages, supposing they could be
fossilized?
[Illustration: FIG. 141.--SECTION OF SKIN AND UNDERLYING TISSUES IN THE
HEAD-REGION OF AMMOCOETES.
_a_, cranial wall; _b_, muco-cartilage; _c_, laminated layer; _d_, external
cuticular layer.]
The most internal layer _a_, by the formation of cartilage and then bone,
represents the great mass of vertebrate fossils; the next layer _b_, by a
process of calcification, as previously argued, represents the head-shield
of the Osteostracan fishes; while the cuticular layer _d_, no longer thin,
is the remnant of the Palæostracan head-carapace. Between these two layers,
_b_ and _d_, lies the laminated layer _c_. Intermediate to the Palæostracan
and the Osteostracan comes the Heterostracan, with its peculiar
head-shield--a head-shield whose origin is more easily conceivable as
arising from something of the nature of the laminated layer than from any
other structure represented in Ammocoetes.
My present suggestion, then, is this: the transition from the skeletal
covering of the Palæostracan to that of the highest vertebrates was brought
about by the calcification of successive layers from without inwards, all
of which still remain in Ammocoetes and show how the external chitinous
covering of the arthropod was gradually replaced by the deep-lying internal
bony cranium of the higher vertebrates.
In Ammocoetes the layer which represents the covering of the
{349}Palæostracan has already almost disappeared. At transformation the
layers representing the stage arrived at by the Heterostracan and the
Osteostracan disappear; but the stage representing the higher vertebrates,
far from disappearing, by the formation of cartilage reaches a higher stage
and prepares the way for the ultimate stage of all--the formation of the
bony cranium.
So much for the evidence as to the nature of the structure of the
head-shield of the Pteraspidæ.
It suggests that these fishes were covered anteriorly with armoured plates
derived from the cutis layer of the skin, a layer which was specially
thickened and very vascular, apparently, to enable respiration to be very
largely, if not entirely, effected by the surface of the body. It is
difficult to understand how the sea-scorpions breathed, and it is easy to
see how the formation of ventral and dorsal plates enclosing the
mesosomatic appendages may at the outset have hindered the action of the
branchiæ. The respiratory chamber, according to my view, had at first the
double function of respiration and digestion. A new digestive apparatus was
the pressing need at the time; it would, therefore, be of distinct
advantage to remove, as much as possible, the burden of respiration from
this incipient alimentary canal.
What can be said as to the shape of these ancient forms of fishes? Certain
parts of them are absolutely known, other parts are guesswork. They are
known to have possessed a dorsal shield, a ventral shield formerly looked
upon as belonging to a separate species, called Scaphaspis, and a spine
attached to the dorsal shield. The rest of their configuration, as given in
Smith Woodward's restoration (Fig. 142) is guesswork; the fish-like body
with its scales, the heterocercal tail, is based on the most insufficient
evidence of something of the nature of scales having being found near the
head-plates.
The dorsal shield is characterized by a pair of lateral eyes situated on
the edge of the shield, not as in Cephalaspis near the middle line. In the
middle line, where the rostrum meets the large dorsal plate, median eyes
were situated. But the slightest sign of any median single nasal opening,
such as is so characteristic of the head-shield of the Osteostraci and of
Ammocoetes has never been discovered. The olfactory organ must have been
situated on the ventral side as in the larval stage of Ammocoetes, or in
the Palæostraca. Many of these head-shields are remarkably well preserved,
{350}and it is difficult to believe that an olfactory opening would not be
seen if any such had existed, as it does in Thyestes.
[Illustration: FIG. 142.--RESTORATION OF PTERASPIS. (After SMITH
WOODWARD.)]
The difficulty of interpreting these types is the difficulty of
understanding their method of locomotion; that is largely the reason why
the spine has been placed as if projecting from the back, and a fish-like
body with a heterocercal tail-fin added. If, on the contrary, the spine is
a terminal tail-spine, then, as far as the fossilized remains indicate, the
animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to
which must be added two apparently lateral pieces and a few scales. If the
animal did not possess a flexible body with a tail-fin, but terminated in a
rigid spike after the fashion of a Limulus-like animal, then it must have
moved by means of {351}appendages. At present we have not sufficient
evidence to decide this question.
That the animal crawled about in the mud by means of free appendages is by
no means an impossible view, seeing how difficult it is to find the remains
of appendages in the fossils of this far-back time, even when we are sure
that they existed. Thus, for many generations, the appendages of
trilobites, which occur in such countless numbers, and in such great
variety of form, were absolutely unknown, until at last, in consequence of
a fortunate infiltration by pyrites, they were found by Beecher preserved
down to the minutest detail. Even to this day no trace of appendages has
been found in such forms as Hemiaspis, Bunodes, Belinurus, Prestwichia.
The whole question of the evidence of any prosomatic appendages in these
ancient fishes is one of very great interest, and of late years has been
investigated by Patten. It has long been known that forms such as
Pterichthys and Bothriolepis possessed two large, jointed locomotor
appendages, and Patten has lately obtained better specimens of Bothriolepis
than have ever been found before, which show not only the general
configuration of the fish, but also the presence of mandibles or gnathites
in the mouth-region resembling those of an arthropod. These mandibles had
been seen before (Smith Woodward), but Patten's specimens are more perfect
than any previously described, and cause him to conclude that these ancient
fish were of the nature of arthropods rather than of vertebrates.
Patten has also been able to obtain some excellent specimens of the under
surface of the head of Tremataspis, which, as evident in Fig. 143, show the
presence of a series of holes, ranging on each side from the mouth-opening,
in a semicircular fashion towards the middle line. He considers that these
openings indicate the attachments of appendages, in opposition to other
observers, such as Jaekel, who look upon them as gill-slits. To my mind,
they are not in the right position for gill-slits; they are certainly in a
prosomatic rather than in a mesosomatic position, and I should not be at
all surprised if further research justified Patten's position. So convinced
is he of the presence of appendages in all these old forms, that he
considers them to be arthropods rather than vertebrates, although, at the
same time, he looks upon them as indicating the origin of vertebrates from
arthropods. Here, perhaps, it is advisable to say a few words on Patten's
attitude towards this question.
{352}Two years after I had put forward my theory of the derivation of
vertebrates from arthropods, Patten published, in the _Quarterly Journal of
Microscopical Science_, simultaneously with my paper in that journal, a
paper entitled "The Origin of Vertebrates from Arachnids." In this paper he
made no reference to my former publications, but he made it clear that
there was an absolutely fundamental difference between our treatment of the
problem; for he took the old view that of necessity there must be a
reversal of surfaces in order that the internal organs should be in the
same relative positions in the vertebrate and in the invertebrate. He
simply, therefore, substituted Arachnid for Annelid in the old theory.
Because of this necessity for the reversal of surfaces he discarded the
terms dorsal and ventral as indicative of the surfaces of an animal, and
substituted hæmal and neural, thereby hopelessly confusing the issue and
making it often very difficult to understand his meaning.
[Illustration: FIG. 143.--UNDER-SURFACE OF HEAD-REGION IN TREMATASPIS.
(After PATTEN.)]
He still holds to his original opinion, and I am still waiting to find out
when the reversal of surfaces took place, for his investigations lead him,
as must naturally be the case, to compare the dorsal (or, as he would call
it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and of the
Pteraspidæ with the dorsal surface of the Palæostraca.
All these ancient fishes are, according to him, still in the arthropod
stage, have not yet turned over, though in a peculiarly unscientific manner
he argues elaborately that they must have swum on their back rather than on
their front, and so indicated the coming reversal. Because they were
arthropods they cannot have had a {353}frontal nose-organ; therefore,
Patten looks upon the nose and the two lateral eyes of the Osteostraci as a
complex median eye, regardless of the fact that the median eyes already
existed.
Every atom of evidence Patten has brought forward, every new fact he has
discovered, confirms my position and makes his still more hopelessly
confused. Keep the animal the right side uppermost, and the evidence of the
rocks confirms the transition from the Palæostracan to the Cyclostome;
reverse the surfaces, and the attempt to derive the vertebrate from the
palæostracan becomes so confused and hopelessly muddled as to throw
discredit on any theory of the origin of vertebrates from arthropods. For
my own part, I fully expect that appendages will be found not only in the
Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue
his researches with increasing success. I feel sure, however, his task will
be much simplified if he abandons his present position and views the
question from my standpoint.
SUMMARY.
The shifting of the nasal tube from a ventral to a dorsal position, as
seen in Ammocoetes, is, perhaps, the most important of all clues in
connection with the comparison of Ammocoetes to the Palæostracan on the
one hand, and to the Cephalaspid on the other; for, whereas the exact
counterpart of the opening of such a tube is always found on the dorsal
head-shield in all members of the latter group, nothing of the kind is
ever found on the dorsal carapace of the former group.
The reason for this difference is made immediately evident in the
development of Ammocoetes itself, for the olfactory tube originates as a
ventral tube--the tube of the hypophysis--in exactly the same position as
the olfactory tube of the Palæostracan, and later on in its development
takes up a dorsal position.
In fact, Ammocoetes in its development indicates how the Palæostracan
head-shield became transformed into that of the Cephalaspid.
In another most important character Ammocoetes indicates its relationship
to the Cephalaspidæ, for it possesses an external skeleton or head-shield
composed of muco-cartilage, which is the exact counterpart of the
so-called bony head-shield of the latter group; and still more strikingly
the structure of the cephalaspidian head-shield is remarkably like that
of muco-cartilage. In the one case, by the deposition of calcium salts, a
hard external skeleton, capable of being preserved as a fossil, has been
formed; in the other, by the absence of the calcium salts, a soft
chondro-mucoid matrix, in which the characteristic cells and fibrils are
embedded, distinguishes the tissue.
The recognition that the head-shields of these most primitive fishes were
not composed of bone, but of muco-cartilage, the precursor of both
cartilage and bone, immediately clears up in the most satisfactory manner
the whole {354}question of their derivation from elasmobranch fishes; for
the main argument in favour of the latter derivation is the exceedingly
strong one that bone succeeds cartilage--not _vice versâ_--therefore,
these forms, since their head-shield is bony, must have arisen from some
other fishes with a cartilaginous skeleton, most probably of an
elasmobranch nature. Seeing, however, that the structure of their shields
resembles muco-cartilage much more closely than bone, and that Ammocoetes
forms a head-shield of muco-cartilage closely resembling theirs, there is
no longer any necessity to derive the jawless fishes from the
gnathostomatous; but, on the contrary, we may look with certainty upon
the Agnatha as the most primitive group from which the others have been
derived.
The history of the rocks shows that the group of fishes, Pteraspis and
Cyathaspis, are older than the Cephalaspidæ--come, therefore,
phylogenetically between the Palæostraca and the latter group. In this
group the head-shields are of a very different character, without any
sign of any structure comparable with that of bone, and although they
possessed both lateral and median eyes, there is never in any case any
trace of a dorsal nasal orifice. Their olfactory passage, like that of
the Palæostraca, must have been ventral.
The remarkable comparison which exists between the head-shields of
Ammocoetes and Cephalaspis, enables us to locate the position of the
brain and cranium of the latter with considerable accuracy, and so to
compare the segmental markings found in many of these fossils with the
corresponding markings, found either in fossil Palæostraca or on the
head-carapaces of living scorpions and spiders, such as Phrynus and
Mygale. In all cases the cranial region was covered with a median plate,
often especially hard, which corresponded to the glabellum of the
trilobite; the growth of the cranium can be traced from its beginnings as
the upturned lateral flanges of the plastron to the membranous cranium of
Ammocoetes.
From such a comparison it follows that the segments, found in the
antero-lateral region of the head-shield, were not segments of the
cranium, but of parts beyond the region of the cranium, and from their
position must have been segments supplied by the trigeminal nerve, and
not by the vagus group; segments, therefore, which did not indicate gills
and gill-slits, but muscles, innervated by the trigeminal nerve; muscles
which, as indicated by the corresponding markings on the carapace of
Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic
appendages.
The discovery of the nature of these appendages in the Pteraspidæ and
Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and
Bothriolepis), is a problem of the future, though in the latter, not only
have the well-known oar-like appendages been long since discovered, but
Patten has recently found specimens of Bothriolepis which throw light on
the anterior masticating gnathite-like appendages which these ancient
forms possessed.
{355}CHAPTER XI
_THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE_
Lateral line organs.--Function of this group of organs.--Poriferous
sense-organs on the appendages in Limulus.--Branchial
sense-organs.--Prosomatic sense organs.--Flabellum.--Its structure and
position.--Sense-organs of mandibles.--Auditory organs of insects and
arachnids.--Poriferous chordotonal organs.--Balancers of
Diptera.--Resemblance to organs of flabellum.--Racquet-organs of
Galeodes.--Pectens of scorpions.--Large size of nerve to all these
special sense-organs.--Origin of parachordals and auditory
capsule.--Reason why VIIth nerve passes in and out of capsule.--Evidence
of Ammocoetes.--Intrusion of glandular mass round brain into auditory
capsule.--Intrusion of generative and hepatic mass round brain into base
of flabellum.--Summary.
When speaking of the tripartite arrangement of the cranial nerves, an
arrangement which gave the clue to the meaning of the cranial segments, I
spoke of the trigeminal as supplying the sensory nerves to the skin in the
head-region, and I compared this dorsal system of afferent nerves to the
system of epimeral nerves in Limulus which supply the prosomatic and
mesosomatic carapaces of Limulus with sensory fibres. I compared the
ventral system of eye-muscle nerves with the system of nerves supplying the
segmental dorso-ventral somatic muscles of the prosomatic region, and I
compared the lateral system of mixed nerves with the nerves supplying the
prosomatic and mesosomatic appendages of Limulus. I compared, also, the
optic nerves and the olfactory nerves with the corresponding nerves in the
same invertebrate group. My readers will see at once that one well-marked
group of nerves--the auditory and lateral line system--has been entirely
omitted up to the present, it has not even been mentioned in the scheme of
the cranial segments; I have purposely reserved its consideration until
now, because the organs these nerves supply, though situated in the skin,
are of such a special character {356}as to form a category by themselves.
These nerves cannot be classed among the afferent nerves of the skin any
more than the nerves of the optic and olfactory apparatus; they require
separate consideration. A very extensive literature has grown up on the
subject of this system of lateral line sense-organs and their innervation,
the outcome of which is decisively in favour of this system being classed
with the sense-organs supplied by the auditory nerve, so that in
endeavouring to understand the position of the auditory nerve, we must
always bear in mind that any theory as to its origin must apply to the
system of lateral line nerves as well.
Now, although the auditory apparatus is common to all vertebrates, the
lateral line system is not found in any land-dwelling animals; it belongs
essentially to the fishes, and is, therefore, an old system so far as
concerns the vertebrate group. Its sense-organs are arranged along the
lateral line of the fish, and, in addition, on the head-region in three
well-marked lines known as the supra-orbital, infra-orbital, and mandibular
line systems. These sense-organs lie in the skin in a system of canals, and
are innervated by a special nervous system different to that innervating
adjacent skin-areas. The great peculiarity of their innervation consists in
the fact that their nerves all belong to the branchial system of nerves; no
fibres arise in connection with the trigeminal, but all of them in
connection with the facial, glossopharyngeal and vagus nerves. In other
words, although organs in the skin, their nerve-supply belongs to the
lateral nervous system which supplies splanchnic and not somatic segments,
a system which, according to the theory advanced in this book, originated
in the nerves supplying appendages. The conclusion, therefore, is that in
order to obtain some clue as to the origin of the sense-organs of this
system in the assumed palæostracan ancestor, we must examine the
mesosomatic appendages and see whether they possess any special
sense-organs of similar function.
Further, considering that the auditory organ is to be regarded as a
specially developed member of this system, we must especially look for an
exceptionally developed organ in the region supplied by the auditory nerve.
The question of the origin of this system of lateral line sense-organs
possesses a special interest for all those who attempt to obtain a solution
of the origin of vertebrates, for the upholders of the view that the
vertebrates have descended from annelids have always {357}found its
strongest support in the similarity of two sets of segmental organs found
in annelids and vertebrates. On the one hand, great stress was laid upon
the similarity of the segmental excretory organs in the two groups of
animals, as will be discussed later; on the other, of the similarity of the
segmentally arranged lateral sense-organs.
These lateral sense-organs of the annelids have been specially described by
Eisig in the Capitellidæ, and, according to Lang, "there are many reasons
for considering these lateral organs to be homologous with the dorsal cirri
of the ventral parapodia of other Polychæta, and in the family of the
Glyceridæ we can follow, almost step by step, the transformation of the
cirri into lateral organs." Eisig describes them in the thoracic
prebranchial region as slightly different from those in the abdominal
branchial region; in the latter region, the ventral parapodia are
gill-bearing, so that these lateral organs are in the branchial region
closely connected with the branchiæ, just as is also the case in the
vertebrates. It is but a small step from the gill-bearing ventral parapodia
of the annelid to the gill-bearing appendages of the phyllopod-like
protostracan; so that if we assume that this is the correct line along
which to search for the origin of the vertebrate auditory apparatus, then,
on my theory of the origin of the vertebrates from a group resembling the
Protostraca, it follows that special sense-organs must have existed either
on or in close connection with the branchial and prebranchial appendages of
the protostracan ancestor of the vertebrates, which would form an
intermediate link between the lateral organs of the annelids and the
lateral and auditory organs of the vertebrates.
Further, these special sense-organs could not have been mere tactile hairs,
but must have possessed some special function, and their structure must
have been compatible with that function. Can we obtain any clear conception
of the original function of this whole system of sense-organs?
A large amount of experimental work has been done to determine the function
of the lateral line organs in fishes, and they have been thought at one
time or another to be supplementary organs for equilibration, organs for
estimating pressure, etc. The latest experimental work done by Parker
points directly to their being organs for estimating slow vibrations in
water in contradistinction to the quicker vibrations constituting sound. He
concludes that surface wave-movements, whether produced by air moving on
the water or {358}solid bodies falling into the water, are accompanied by
disturbances which are stimuli for the lateral line organs.
One of these segmental organs has become especially important and exists
throughout the whole vertebrate group, whether the animal lives on land or
in water--this is the auditory organ. Throughout, the auditory organ has a
double function--the function of hearing and the function of equilibration.
If, then, this is, as is generally supposed, a specialized member of the
group, it follows that the less specialized members must possess the
commencement of both these functions, just as the experimental evidence
suggests.
In our search, then, for the origin of the auditory organ of vertebrates,
we must look for special organs for the estimation of vibrations and for
the maintenance of the equilibrium of the animal, situated on the
appendages, especially the branchial or mesosomatic appendages; and,
further, we must specially look for an exceptional development of such
segmental organs at the junction of the prosomatic and mesosomatic regions.
Throughout this book the evidence which I have put forward has in all cases
pointed to the same conclusion, viz. that the vertebrate arose by way of
the Cephalaspidæ from some arthropod, either belonging to, or closely
allied to, the group called Palæostraca, of which the only living
representative is Limulus. If, then, my argument so far is sound, the
appendages of Limulus, both prosomatic and mesosomatic, ought to possess
special sense-organs which are concerned in equilibration or the
appreciation of the depth of the water, or in some modification of such
function, and among these we might expect to find that somewhere at the
junction of the prosoma and mesosoma such sense-organs were specially
developed to form the beginning of the auditory organ.
Now, it is a striking fact that the appendages of Limulus do possess
special sense-organs of a remarkable character, which are clearly not
simply tactile. Thus Gegenbaur, as already stated, has drawn attention to
the remarkable branchial sense-organs of Limulus; and Patten has pointed
out that special organs, which he considers to be gustatory in function,
are present on the mandibles of the prosomatic appendages. I myself, as
mentioned in my address to the British Association at Liverpool in 1896,
searched for some special sense-organ at the junction of the prosoma and
mesosoma, and was rewarded by finding that that extraordinary adjunct to
the {359}last locomotor appendage, known as the flabellum, was an elaborate
sense-organ. I now propose to show that all these special sense-organs are
constructed on a somewhat similar plan; that the structure of the branchial
sense-organs suggests that they are organs for the estimation of water
pressures; that among air-breathing arthropods sense-organs, built up on a
somewhat similar plan, are universally found, and are considered to be of
the nature of auditory and equilibration organs; and, what is especially of
importance, in view of the fact that the most prominent members of the
Palæostraca were the sea-scorpions, that the remarkable sense-organs of the
scorpions known as the pectens belong apparently to the same group.
THE PORIFEROUS SENSE-ORGANS OF THE APPENDAGES IN LIMULUS.
On all the branchial appendages in Limulus, special sense-organs are found
of a most conspicuous character. They form in the living animal bluish
convex circular patches, the situation of which on the appendages is shown
in Fig. 58. These organs are not found on the non-branchial operculum.
Gegenbaur, who was the first to describe them, has pointed out how the
surface of the organ is closely set with chitinous goblets shaped as seen
in Fig. 144, A, which do not necessarily project free on the surface, but
are extruded on the slightest pressure. Each goblet fits into a socket in
the chitinous covering, and is apparently easily protruded by variations of
pressure from within. The whole surface of the organ on the appendage is
slightly bulged in the living condition, and the chitin is markedly softer
here than in the surrounding part of the limb. Each of these organs is
surrounded by a thick protection of strongly branching spines. On the
surface of the organ itself no spines are found, only these goblets, so
that the surface-view presents an appearance as in Fig. 144, B. Each goblet
possesses a central pore, which is the termination of a very fine, very
tortuous, very brittle chitinous tubule (_ch.t._), which passes from the
goblet through the layers of the chitin into the subjacent tissue. The
goblets vary considerably in size, a few very large ones being scattered
here and there. The fine chitinous tubule is especially conspicuous in
connection with these largest goblets. In the smaller ones there is the
same appearance of a pore and a commencing tube, but I have not been able
to trace the tube through the chitinous layers, as in the case of the
larger goblets.
{360}[Illustration: FIG. 144.--A, A GOBLET FROM ONE OF THE BRANCHIAL
SENSE-ORGANS OF LIMULUS (_ch.t._, chitinous tubule); B, SURFACE VIEW OF A
PORTION OF A BRANCHIAL SENSE-ORGAN.]
[Illustration: FIG. 145.--THE ENDOGNATHS OF LIMULUS PUSHED OUT OF THE WAY
ON ONE SIDE IN ORDER TO SHOW THE POSITION OF THE FLABELLUM (_fl._)
PROJECTING TOWARDS THE CRACK BETWEEN THE PROSOMATIC AND MESOSOMATIC
CARAPACES.]
Gegenbaur, in his picture, draws a straight tubule passing from every
goblet among the fine canaliculi of the chitin. He says they are difficult
to see, except in the case of the larger goblets. The tubule from the
larger goblets is most conspicuous, and is in my sections always tortuous,
never straight, as represented by Gegenbaur. A special branch of the
appendage-nerve passes to these organs, and upon the fine branches of this
nerve groups of ganglion-cells are seen, very similar in appearance to the
groups described by Patten on the terminal branches of the nerves which
supply the mandibular organs. At present I can see no mechanism by which
the goblets are extruded or returned into place. In the case of the
Capitellidæ, Eisig describes retractor muscles by means of which the
lateral sense-organs are {361}brought below the level of the surface, and
he imagines that the protrusion is effected by hydraulic means, by the aid
of the vascular system. In the branchial sense-organs of Limulus there are
no retractor muscles, and it seems to me that both retraction and
protrusion must be brought about by alterations of pressure in the vascular
fluids. Certainly the cavity of the organ is very vascular. If this be so,
it seems likely enough that such an organ should be a very delicate organ
for estimating changes in the pressure of the external medium, for the
position of the goblets would depend on the relation between the pressure
of the fluid inside the organ and that on the surface of the appendage.
Whether the chitinous tubule contains a nerve-terminal or not I am unable
to decide from my specimens, but, judging from Patten's description of the
similar chitinous tubules belonging to the mandibular organs, it is most
highly probable that these tubules also contain a fine terminal
nerve-fibre.
These organs, then, represent segmental branchial sense-organs, of which it
can be said their structure suggests that they may be pressure-organs; but
the experimental evidence is at present wanting.
Passing now from the branchial to the prosomatic region, the first thing
that struck me was the presence of that most conspicuous projection at the
base of the last locomotor appendage, which is usually called the
flabellum, and has been described by Lankester as an exopodite of this
appendage. It is jointed on to the most basal portion of the limb (_cf._
Fig. 155), and projects dorsally from the limb into the open slit between
the prosomatic and mesosomatic carapace, as is seen in Fig. 145 (_fl._). Of
its two surfaces, the undermost is very convex and the uppermost nearly
flat from side to side, the whole organ being bent, so that when the animal
is lying half buried in sand, entirely covered over by the prosomatic and
mesosomatic carapaces except along this slit between the two, the upper
flat or slightly convex surface of the flabellum is exposed to any movement
of water through this slit, and owing to its possessing a joint, the
direction of the whole organ can be altered to a limited extent. The whole
of this flat upper surface is one large sense-organ of a striking
character, thus forming a great contrast to the convex under surface, which
is remarkably free from tactile spines or special sense-organs.
The nerve going to the flabellum is very large, almost as large as the
nerve to the rest of the appendage, and the very large majority {362}of the
nerve-fibres turn towards the flat, uppermost side, where the sense-organ
is situated. Between the nerve-fibres (_n._) and the chitinous surface
containing the special sense-tubes masses of cells (_gl._) are seen, as in
Fig. 146, apparently nerve-cells, which form a broad border between the
nerve-fibres and the pigmented chitinogenous layer (_p._). On the opposite
side, nothing of the sort intervenes between the pigmented layer and the
blood-spaces and nerve-fibres which constitute the central mass of the
flabellum.
[Illustration: FIG. 146.--SECTION THROUGH FLABELLUM.
_ch._, chitinous layers; _s.o._, sense-organs; _sp._, spike-organ; _p._,
pigment layer; _gl._, ganglion cell layer; _bl._ and _n._, blood-spaces and
nerves.]
[Illustration: FIG. 147.--SECTION PARALLEL TO THE SURFACE OF FLABELLUM,
SHOWING THE POROUS TERMINATIONS OF THE SENSE-ORGANS AND THE ARRANGEMENT OF
THE CANALICULI ROUND THEM.]
At present I am inclined to look upon this mass of cells as constituting a
large ganglion, which extends over the whole length and breadth of the
upper surface of the flabellum. At the same {363}time, my preparations are
not sufficiently clear to enable me to trace out the connections of these
cells, especially their connections with the special sense-organs.
[Illustration: FIG. 148.--SECTION THROUGH THE THREE SENSE-ORGANS OF
FLABELLUM.
_bl._, blood-spaces; _n._, nerve; _gl._, layer of ganglion-cells; _p._,
pigment layer; _ch._, 1, 2, 3, the three layers of chitin; _ch.t._,
chitinous tubule in large tube of sense-organ; _cap._, capitellum or
swollen extremity of large tube; _can._, very fine porous canals or
canaliculi of chitin.]
In Fig. 148 I give a magnified representation of a section through three of
these flabellar sense-organs. As is seen, the section divides itself into
four zones: (1) the chitinous layer (_ch._); (2) the layer of pigment
(_p._) and hypodermal cells; (3) the layer of ganglion-cells (_gl._); and
(4) the layer of nerve-fibres (_n._) and blood-spaces (_bl._). The
chitinous layer is composed of the usual three zones of the Limulus
surface--externally (Fig. 148), a thin homogeneous layer, followed by a
thick layer of chitin (3), in which the fine vertical tubules or canaliculi
are well marked; the external portion (2) of this layer is differentiated
from the rest by the presence of well-marked horizontal layers in addition
to the canaliculi.
In this chitinous layer the special sense-organs are found. They consist of
a large tube which passes through all the layers of the chitin except the
thin homogeneous most external layer. {364}This tube is conical in shape,
its base, which rests on the pigmented layer, being so large and the organs
so crowded together that a section of the chitin across the base of the
tubes gives the appearance of a honeycomb, the septa of which is all that
remains of the chitin. This large tube narrows down to a thin elongated
neck as it passes through the chitin, and then, at its termination, bulges
out again into an oval swelling (_cap._) situated always beneath the
homogeneous most external layer of chitin. Within this tube a fine
chitinous tubule (_ch. t._) is situated similar to that seen in the
branchial sense-organs; it lies apparently free in the tube, not straight,
but sinuous, and it passes right through all the chitinous layers to open
at the surface as a pore; in the last part of its course, where it passes
through the most external layer (1) of chitin, it lies always at right
angles to the surface.
If the flabellum be stained with methylene blue and acid fuchsin, then all
the canaliculi in the chitin show up as fine red lines, and present the
appearance given in Fig. 148, and it is seen that each of the terminations
of the tubules is surrounded in the homogeneous layer of chitin by a
thick-set circular patch of canaliculi which pass to the very surface of
the chitin, while the canaliculi in other parts terminate at the
commencement of the homogeneous layer and do not reach the surface.
Further, the contents of the oval swelling, and, indeed, of the tube as a
whole, are stained blue, the chitinous tubule being either unstained or
slightly pink in colour. We see, then, that the chitinous tubule alone
reaches the surface, while the large tube, which contains the tubule,
terminates in an oval swelling, which often presents a folded or wrinkled
appearance, as in Fig. 149 (see also Patten's Fig. 1, Plate I.). This
terminal bulging of the tube is reminiscent of the bulging in the chitinous
tubes of the lyriform organs of the Arachnida, as described by Gaubert, and
of the poriferous chordotonal organs in insects, as described by Graber
(see Fig. 150). This terminal swelling is filled with a homogeneous
refringent mass staining blue with methylene blue, in which I have seen no
trace of a nucleus; through this the chitinous tubule makes its way without
any sign of bulging on its part. Patten, in his description of the
sense-organs on the mandibles of Limulus, which are evidently the same in
structure as those on the flabellum, refers to this homogeneous mass as a
coagulum. I doubt whether this is an adequate description; it appears to me
to stain rather more {365}readily than a blood-coagulum, yet in the sense
of being structureless it resembles a coagulum.
The enormous number of these organs crowded together over the whole flat
surface of the flabellum produces a very striking appearance when viewed on
the surface. Such a view presents an appearance resembling that of the
surface-view of the branchial sense-organs; in both cases the surface is
covered with a great number of closely set circular plaques, in the centre
of each of which is seen a well-marked pore. The circular plaques in the
case of the flabellum are much smaller than those of the branchial
sense-organs, and clearly are not protrusible as in the latter organs, the
appearance as of a plaque being due to the ring of thickly-set canaliculi
round the central tubule, as already described. When stained with methylene
blue, the surface view of the flabellum under a low power presents an
appearance of innumerable circular blue masses, from each of which springs
a fine bent hair, terminating in a pore at the surface. The blue masses are
the homogeneous substance (_cap._) of the bulgings seen through the
transparent external layer of chitin, and the hairs are the terminal part
of the chitinous tubules. Patten has represented their appearance in the
mandibles in his Fig. 2, Plate I.
The large tubes in the chitin alter in shape according to their position.
Those in the middle of the sensory surface of the flabellum, in their
course through the chitinous layers, are hardly bent at all; as they
approach the two lateral edges of this surface, their long thin neck
becomes bent more and more, the bending always being directed towards the
middle of the surface (see Fig. 146); in this way the chitinous tubules
increase more or less regularly in length from the centre of the organ to
the periphery. The large basal part of the conical tube contains, besides
the chitinous tubule, a number of nuclei which are confined to this part of
the tube; some of these nuclei look like those belonging to nerve-fibres,
others are apparently the nuclei of the chitinogenous membrane lining the
tube. I have never seen any sign of nerve-cells in the tube itself.
The only other kind of sense-organ I have found in connection with these
sense-organs are a few spike-like projections, the appearance of which is
given in Fig. 149. I have always seen these in the position given in Fig.
146 (_sp._), _i.e._ at the junction of the surface which contains the
sense-organs and the surface which is free from them. They are, so far as I
have seen, not very numerous; I have {366}not, however, attempted to
examine the whole sense-organ for the purpose of estimating their number
and arrangement.
As is seen in Fig. 149, they possess a fine tubule of the same character as
that of the neighbouring sense-organs, which apparently terminates at the
apex of the projecting spike. They appear to belong to the same group as
the other poriferous sense-organs, and are of special interest, because in
their appearance they form a link between the latter and the poriferous
sense-organs which characterize the pecten of the scorpion (_cf._ Fig. 152,
C).
[Illustration: FIG. 149.--SPIKE-ORGAN OF FLABELLUM.
_ch.t._, chitinous tubule.]
Such, then, is the structure of this remarkable sense-organ of the
flabellum, as far as I have been able to work it out with the materials at
my disposal. It is evident that the flabellar organs, apart from the
spike-organs, are of the same kind as those described by Patten on the
mandibles and chelæ of Limulus, and therefore it is most probable that the
nerve-terminals in the chitinous tubules, and the origin of the latter, are
similar in the two sets of organs.
These organs, as Patten has described them, are situated in lines on the
spines of the mandibles of the prosomatic locomotor appendages, and are
grouped closely together to form a compact sense-organ on the surface of
the inner mandible (Lankester's epicoxite) (_i.m._ in Fig. 155), so that a
surface-view of the organ here gives the characteristic appearance of these
poriferous sense-patches. Precisely similar organs are found on the
chilaria, which are, in function at all events, simply isolated mandibles,
to use Patten's terminology.
On the digging appendage (ectognath), as the comparison of Fig. 155, A and
C, shows, the mandibular spines are almost non-existent, and the inner
mandible or epicoxite is not present, so that {367}the special sense-organ
of this appendage is represented solely by the flabellum.
This sketch of the special sense-organs of Limulus shows that all the
appendages of Limulus possess special sense-organs, with the exception of
the operculum. All these sense-organs are formed on the same plan, in that
they possess a fine chitinous tubule passing through the layers of chitin
into the underlying hypodermal and nervous tissues, which terminates on the
surface in a pore. The surface of the chitin where these pores are situated
is perfectly smooth, although, in the case of the branchial sense-organs,
the goblet-shaped masses of chitin, each of which contains a pore, are able
to be pressed out beyond the level of the surface.
As to their functions, we unfortunately do not know much that is definite.
Patten considers that he has evidence of a gustatory function in the case
of the mandibular organs, and suggests also a temperature-sense in the case
of some of these organs. The large organ of the flabellum and the branchial
organs he has not taken into consideration. The situation of these organs
puts the suggestion of any gustatory function, as far as they are
concerned, out of the question; and I do not think it probable that such
large specialized organs would exist only for the estimation of
temperature, when one sees how, in the higher animals, the
temperature-nerves and the nerves of common sensation are universally
distributed over the body. As already stated, the structure of the
branchial organs seems to me to point to organs for estimating varying
pressures more than anything else, and I am strongly inclined to look upon
the whole set of organs as the derivatives of the lateral sense-organs of
annelids, such as are described by Eisig in the Capitellidæ. This is
Patten's opinion with respect to the mandibular organs; and from what I
have shown, these organs cannot be separated in type of structure from
those of the flabellum and the branchial sense-organs.
In our search, then, for the origin of the vertebrate auditory organ in
Limulus and its allies, we see so far the following indications:--
1. The auditory organ of the vertebrate is regarded as a special organ
belonging to a segmentally arranged set of lateral sense-organs, whose
original function was co-ordination and equilibration.
2. Such a set of segmentally arranged lateral sense-organs is found in
annelids in connection with the dorsal cirri of the ventral parapodia.
{368}3. If, as has been supposed, there is a genetic connection between
(1) and (2) and if, as I suppose, the vertebrates did not arise from the
annelids directly, but from a protostracan group, then it follows that the
lateral sense-organs, one of which gave rise to the auditory organ, must
have been situated on the protostracan appendages.
4. In Limulus, which is the sole surviving representative of the
palæostracan group, such special sense-organs are found on both the
prosomatic and mesosomatic appendages, and therefore may be expected to
give a direct clue to the origin of the vertebrate auditory organ.
5. Both from its position, its size, and its specialization, the
flabellum, _i.e._ an organ corresponding to the flabellum, must be looked
upon as more likely to give a direct clue to the origin of the auditory
organ than the sense-organs of the branchial appendages, or the so-called
gustatory organs of the mandibles.
THE AUDITORY ORGANS OF ARACHNIDS AND INSECTS.
The difficulty of the investigating these organs consists in the fact that
so little is known about them in those Arthropoda which live in the water;
the only instance of any organ apparently of the nature of an auditory
organ, is the pair of so-called auditory sacs at the base of the antennæ in
various decapods. We are in a slightly better position when we turn to the
land-living arthropods; here the presence of stridulating organs in so many
instances carries with it the necessity of an organ for appreciating sound.
It has now been shown that such stridulating organs are not confined to the
Insecta, but are present also in the scorpion group, and I myself have
added to their number by the discovery of a distinct stridulating apparatus
in various members of the Phrynidæ. We may then take it for granted that
arachnids as well as insects hear. Where is the auditory organ?
Many observers believe that certain surface-organs found universally among
the spiders, to which Gaubert has given the name of lyriform organs, are
auditory in function. His investigations show that they are universally
present on the limbs and pro-meso-sternite of all spiders; that they are
present singly, not in groups, on the limbs of Thelyphonus, and that a
group of them exists on the second segment of each limb in the members of
the Phrynus tribe. In the latter case this organ is the most elaborate of
all described by him.
{369}It is especially noticeable that they do not exist in Galeodes or in
the scorpions, but in the former special sense-organs are found in the
shape of the so-called 'racquet-organs,' on the basal segments of the most
posterior pair of appendages, and also, according to Gaubert, on the
extremity of the palps and the first pair of feet, while in the latter they
occur in the shape of the pectens.
This observation of Gaubert suggests that the place of the lyriform organs
in other arachnids is taken in Galeodes by the racquet-organs, and in the
scorpions by the pectens. Bertkau, Schimkéwitsch, and Wagner, as quoted by
Gaubert, all suggest that the lyriform organs of the arachnids belong to
the same group of sense-organs as the porous chordotonal organs of the
Insecta, sense-organs which have been found in every group of Insecta, and
are generally regarded as auditory organs. Gaubert does not agree with
this, and considers the lyriform organs to be concerned with the
temperature-sense rather than with audition.
The chordotonal organs of insects have been specially studied by Graber. He
divides them into two groups, the poriferous and the non-poriferous, the
former being characterized by the presence of pores on the surface arranged
in groups or lines. These poriferous chordotonal organs are remarkably
constant in position, being found only at the base of the wings on the
subcostal ridge, in marked contrast to the other group of chordotonal
organs which are found chiefly on the appendages in various regions. The
striking character of this fixity of position of these organs and the
universality of their presence in the whole group, led Graber to the
conclusion that in these poriferous chordotonal organs we are studying a
form of auditory apparatus which characterized the ancestor of the
insect-group. These organs are always well developed on the hind wings, and
in the large group of Diptera the auditory apparatus has usurped the whole
of the function of the wing; for the balancers or 'halteres,' as they are
called, are the sole representatives of the hind wings, and they are
usually considered to be of the nature of auditory organs. It is
instructive to find that such an auditory organ serves not only for the
purpose of audition, but also as an organ of equilibration; thus Lowne
gives the evidence of various observers, and confirms it himself, that
removal of the balancers destroys the power of orderly flight in the
animal.
A striking peculiarity of these organs in the Insecta, as described {370}by
Graber, is the bulging of the porous canal near its termination (Fig. 150,
C). This bulging is filled with a homogeneous, highly refractive material,
from which, according to Lowne, a chordotonal thread passes, to be
connected with a ganglion-cell and nerve. This sphere of refractive
material he calls the 'capitellum' of the chordotonal thread. The presence
of this material produces in a surface view an appearance as of a halo
around the terminal plaque with its central pore; Graber has attempted to
represent this by the white area round the central area (in Fig. 150, B). A
very similar appearance is presented by the surface view of the flabellum
in those parts where the tube runs straight to the surface, so that the
refractive material which fills the oval bulging shines through the
overlying chitin and appears to surround the terminal plaque with a
translucent halo.
[Illustration: FIG. 150 (from GRABER).--A, SECTION OF SUBCOSTAL NERVURE OF
HIND WING OF DYTISCUS TO SHOW PATCH OF PORIFEROUS ORGANS (_s.o._). B,
SURFACE VIEW OF PORIFEROUS ORGANS; THE WHITE SPACE ROUND EACH ORGAN
INDICATES THE DEEPER LYING REFRINGENT BODY WHICH FILLS THE BULGING OF THE
CANAL SEEN IN TRANSVERSE SECTION IN C.]
Such a peculiarity must have a very definite meaning, and suggests that the
canals in the flabellum of Limulus and in the hind wings of insects belong
to the same class of organ, the chitinous tubule with its nerve-terminal in
the former corresponding to the chordotonal thread in the latter. One
wonders whether this sphere of refractive material or 'capitellum' (to use
Lowne's phraseology) is so universally present in order to act as a damper
upon the vibrations of the chordotonal thread in the one case and of the
{371}chitinous tubule in the other, just as the _membrana tectoria_ and the
otoliths act in the case of the vertebrate ear.
Patten says that the only organs which seem to him to be comparable with
the gustatory porous organs of Limulus are the sense-organs in the
extremities of the palps and of the first pair of legs of Galeodes, as
described by Gaubert. I imagine that he was thinking only of arachnids, for
the comparison of his drawings with those of Graber show what a strong
family resemblance exists between the poriferous sense-organs of Limulus
and those of the insects. On the course of the terminal nerve-fibres,
between the nerve-cell and their entrance into the porous chitinous canal,
Graber describes the existence of rods or scolophores. On the course of the
terminal fibres in the Limulus organ, between the nerve-cells and their
entrance into the porous chitinous canal, Patten describes a spindle-shaped
swelling, containing a number of rod-like thickenings among the fibrils in
the spindle, which present an appearance reminiscent of the rods described
by Graber.
It appears as though a type of sense-organ, characterized by the presence
of pores on the surface and a fine chitinous canal which opens at these
pores, was largely distributed among the Arthropoda. According to Graber,
this kind of organ represents a primitive type of sense-organ, which was
probably concerned with audition and equilibration, and he expresses
surprise that similar organs have not been discovered among the Crustacea.
It is, therefore, a matter of great interest to find that so ancient a type
of animal as Limulus, closely allied to the primitive crustacean stock,
_does_ possess poriferous sense-organs upon its appendages which are
directly comparable with these poriferous chordotonal organs of the
Insecta.
THE PECTENS OF SCORPIONS.
Among special sense-organs such as those with which I am now dealing, the
pectens of scorpions and the 'racquet-organs' of Galeodes must, in all
probability, be classed. I have given my reasons for this conclusion in my
former paper.[2] At present such reasons are based entirely upon the
structure of the organs; experimental {372}evidence as to their function is
entirely wanting. With respect to the pectens of the scorpion (Fig. 151),
it has been suggested that they are of the nature of copulatory organs, a
suggestion which may be dismissed without hesitation, for they are not
constructed after the fashion of claspers, but are simply elaborate
sense-organs, and, as such, are found equally in male or female. The only
observer who has hitherto specially studied the structure of the
sense-organs in the pecten is, as far as I know, Gaubert, and he describes
their structure together with that of the sense-organs of the racquets of
Galeodes, in connection with the lyriform organs of arachnids, as though he
recognized a family resemblance between the three sets of organs.
[Illustration: FIG. 151.--UNDER SURFACE OF SCORPION (ANDROCTONUS).
The operculum is marked out with dots, and on each side of it is seen one
of the pectens.]
The pecten of the scorpions is an elaborate sense-organ, or rather group of
sense-organs, the special organ being developed on each tooth of the comb;
its surface, which is frequently flattened, being directed backwards and
inwards, when the axis of the pecten is horizontal at right angles to the
length of the body. The surface view of this part of the tooth resembles
that of the branchial organs or of the flabellum in Limulus, in that it is
thickly covered with circular patches, in the centre of which an
ill-defined appearance as of a fine pore is seen. In Fig. 152, B, I give a
sketch of the surface view of a part of the organ.
Transverse sections of a tooth of the comb of _Scorpio Europæus_ present
the appearance given in Fig. 152, A, and show that each of these circular
patches is the surface-view of a goblet-shaped chitinous organ, Fig. 152,
C, from the centre of which a short, somewhat cylindrical chitinous spike
projects. Within this spike, and running through the goblet into the
subjacent tissue, is a fine tubule. The series of goblets gives rise to the
appearance of the circular plaques on the surface-view, while the spike
with its tubule {373}is the cause of the ill-defined appearance of the
central pore, just as the terminal pore is much less conspicuous on
surface-view in the spike-organs of the flabellum than in the purely
poriferous organs, no part of which projects beyond the level of the
chitinous surface.
[Illustration: FIG. 152.--A, SECTION THROUGH TOOTH OF PECTEN OF SCORPION;
B, SURFACE VIEW OF SENSE-ORGANS; C, GOBLET OF SENSE-ORGAN MORE HIGHLY
MAGNIFIED.
_bl._ and _n._, region of blood-spaces and nerves; _gl._, ganglion-cell
layer; _ch._, modified chitinous layer; _s.o._, sense-organ.]
The fine tubule is soon lost in the thickened but soft modification of the
chitinous layer (_ch._) which is characteristic of the sense-organ; at all
events, I have not succeeded in tracing it through this layer with any more
success than in the corresponding case of the tubules belonging to the
smaller goblets of the branchial sense-organ of Limulus already described.
At the base of the modified chitinous layer a series of cells is seen,
many, if not all, of which belong to the chitinogenous layer. Next to these
is the marked layer of ganglion-cells (_gl._), similar to those seen in the
flabellum of Limulus. The rest of the space in the section of the tooth is
filled up with nerves (_n._) and blood-spaces (_bl._) just as in the
section, Fig. 146, of the flabellum of Limulus.
Gaubert does not appear to have seen the goblets at all clearly; {374}he
describes them simply as conical eminences, and states that they
"recouvrent un pore analogue a celui des poils mais plus petit; il est
rempli par le protoplasma de la couche hypodermique." From the ganglion,
according to him, nervous prolongations pass, which traverse the
chitinogenous layer and terminate at the base of the conical eminences.
Each of these prolongations "présente sur son trajet, mais un peu plus près
du ganglion que de sa terminaison périphérique, une cellule nerveuse
fusiforme (_g._) offrant, comme celles du ganglion, un gros noyau." He
illustrates his description with the following, Fig. 153, taken from his
paper.
[Illustration: FIG. 153 (from GAUBERT).--SECTION OF A TOOTH OF PECTEN OF
SCORPION.
_n._, nerve; _gl._, ganglion.]
I have not been able to obtain any evidence of a fusiform nerve-cell on the
course of the terminal nerve-fibres as depicted by him; fusiform cells
there are in plenty, as depicted in my drawing, but none with a large
nucleus resembling those of the main ganglion. In no case, either in the
flabellum or in the branchial organs of Limulus, or in the pecten-organs,
have I ever seen a ganglion-cell within the chitin-layer; all the nuclei
seen there resemble those of the cells of the hypodermis or else the
elongated nuclei characteristic of the presence of nerve-fibres. Gaubert's
drawing is a striking one, and I have looked through my specimens to see
whether there was anything similar, but have hitherto failed to obtain any
definite evidence of anything of the kind.
I feel, myself, that an exhaustive examination of the structure and
function of the pecten of scorpions ought to be undertaken. At present I
can only draw the attention of my readers to the similarity of the
arrangement of parts, and of the nature of the end-organs, in the
sense-organs of the flabellum of Limulus and of the pecten of the scorpion.
In both cases the special nerve-fibres terminate in a massive ganglion,
situated just below the chitinogenous layer. In both cases the terminal
fibres from these ganglion-cells pass through the modified chitinous layer
to supply end-organs of a striking character; and although the end-organ of
the pecten of the scorpion does {375}not closely resemble the majority of
the end-organs of the flabellum, yet it does resemble, on the one hand, the
isolated poriferous spikes found on the flabellum (Fig. 149) and, on the
other, the poriferous goblets found on the sense-patches of the branchial
appendages of Limulus (Fig. 144, A), so that a combination of these two
end-organs would give an appearance very closely resembling that of the
pecten of the scorpion.
Finally, the special so-called 'racquet-organs' of Galeodes, which are
found on the most basal segments of the last pair of prosomatic appendages,
ought also to be considered here. Gaubert has described their structure,
and shown how the nerve-trunk in the handle of the racquet splits up into a
great number of separate bundles, which spread out fan-shaped to the free
edge of the racquet; each of these separate bundles supplies a special
sense-organ, which terminates as a conical eminence on the floor of a deep
groove, running round the whole free edge of the racquet. This groove is
almost converted into a canal, owing to the projection of its two sides.
Gaubert imagines that the sense-organs are pushed forward out of the groove
to the exterior by the turgescence of the whole organ; each of the
nerve-fibres forming a bundle is, according to Gaubert, connected with a
nerve-cell before it reaches its termination.
This sketch of the special sense-organs on the appendages of Limulus, of
the scorpions, of Galeodes, and other arachnids, and their comparison with
the porous chordotonal organs of insects, affords reason for the belief
that we are dealing here with a common group of organs, which, although
their nature is not definitely known, have largely been accredited with the
functions of equilibration and audition, a group of organs among which the
origin of the auditory organ of vertebrates must be sought for, upon any
theory of the origin of vertebrates from arthropods.
Whenever in any animal these organs are concentrated together to form a
special organ, it is invariably found that the nerve going to this organ is
very large, out of all proportion to the size of the organ, and also that
the nerve possesses, close to its termination in the organ, large masses of
nerve-cells. Thus, although the whole hind wing in the blow-fly has been
reduced to the insignificant balancers or 'halteres,' yet, as Lowne states,
the nerves to them are the largest in the body.
The pectinal nerve in the scorpion is remarkable for its size, and {376}so,
also, is the nerve to the flabellum in Limulus, while the large size of the
auditory nerve in the vertebrate, in distinction to the size of the
auditory apparatus, has always aroused the attention of anatomists.
Throughout this book my attention has been especially directed to both
Limulus and the scorpion group in endeavouring to picture to myself the
ancestor of the earliest vertebrates, because the Eurypteridæ possessed
such marked scorpion-like characteristics; so that in considering the
origin of a special sense-organ, such as the vertebrate auditory organ near
the junction of the prosoma and mesosoma, it seems to me that the presence
of such marked special sense-organs as the flabellum on the one hand and
the pecten on the other, must both be taken into account, even although the
former is an adjunct to a prosomatic appendage, while the latter
represents, according to present ideas, the whole of a mesosomatic
appendage.
From the point of view that the VIIIth nerve represents a segment
immediately posterior to that of the VIIth, it is evident that an organ in
the situation of the pecten, immediately posterior to the operculum, _i.e._
according to my view, posterior to the segment originally represented by
the VIIth nerve, is more correctly situated than an organ like the
flabellum, which belongs to a segment anterior to the operculum.
On the other hand, from the point of view of the relationship between the
scorpions and the king-crabs, it is a possibly debatable question whether
the pecten really belongs to a segment posterior to the operculum. The
position of any nerve in a series depends upon its position of origin in
the central nervous system, rather than upon the position of its peripheral
organ. Now, Patten gives two figures of the brain of the scorpion built up
from serial sections. In both he shows that the main portion of the
pectinal nerve arises from a swelling, to which he gives the name _ganglion
nodosum_. This swelling arises on each side in close connection with the
origin of the most posterior prosomatic appendage-nerve, according to his
drawings, and posteriorly to such origin he figures a small nerve which he
says supplies the distal parts of the sexual organs. This nerve is the only
nerve which can be called the opercular nerve, and apparently arises
posteriorly to the main part of the pectinal nerve. If this is so, it would
indicate that the pectens arose from sense-organs which were originally,
like the flabella, pre-opercular in position, but have shifted to a
post-opercular position.
{377}THE ORIGIN OF THE PARACHORDALS AND AUDITORY CARTILAGINOUS CAPSULE.
In addition to what I have already said, there is another reason why a
special sense-organ such as the pecten is suggestive of the origin of the
vertebrate auditory organ, in that such a suggestion gives a clue to the
possible origin of the parachordals and auditory cartilaginous capsules.
In the lower vertebrates the auditory organ is characterized by being
surrounded with a cartilaginous capsule which springs from a special part
of the axial cartilaginous skeleton on each side, known as the pair of
parachordals. The latter, in Ammocoetes, form a pair of cartilaginous bars,
which unite the trabecular bars with the branchial cartilaginous
basket-work. They are recognized throughout the Vertebrata as distinct from
the trabecular bars, thus forming a separate paired cartilaginous element
between the trabeculæ and the branchial cartilaginous system, which of
itself indicates a position for the auditory capsule between the prosomatic
trabeculæ and the mesosomatic branchial cartilaginous system.
The auditory capsule and parachordals when formed are made of the same kind
of cartilage as the trabeculæ, _i.e._ of hard cartilage, and are therefore
formed from a gelatin-containing tissue, and not from muco-cartilage.
Judging from the origin already ascribed to the trabeculæ, viz. their
formation from the great prosomatic entochondrite or plastron, this would
indicate that a second entochondrite existed in the ancestor of the
vertebrate in the region of the junction of the prosoma and mesosoma, which
was especially connected with the sense-organ to which the auditory organ
owes its origin. This pair of entochondrites becoming cartilaginous would
give origin to the parachordals, and subsequently to the auditory capsules,
their position being such that the nerve to the operculum would be
surrounded at its origin by the growth of cartilage.
On this line of argument it is very significant to find that the scorpions
do possess a second pair of entochondrites, viz. the supra-pectinal
entochondrites, situated between the nerve-cord and the pectens, so that if
the ancestor of the Cephalaspid was sufficiently scorpion-like to have
possessed a second pair of entochondrites and at the same time a pair of
special sense-organs of the nature either of {378}the pectens or flabella,
then the origin of the auditory apparatus would present no difficulty.
It is also easy to see that the formation of the parachordals from
entochondrites homologous with the supra-pectinal entochondrites, would
give a reason why the VIIth or opercular nerve is involved with the VIIIth
in the formation of the auditory capsule, especially if the special
sense-organ which gave origin to the auditory organ was originally a
pre-opercular sense-organ such as the flabellum, which subsequently took up
a post-opercular position like that of the pecten.
THE EVIDENCE OF AMMOCOETES.
As to the auditory apparatus itself, we see that the elaborate organ for
hearing--the cochlea--has been evolved in the vertebrate phylum itself. In
the lowest vertebrates the auditory apparatus tends more and more to
resolve itself into a simple epithelial sac, the walls of which in places
bear auditory hairs projecting into the sac, and in part form otoliths.
Such a simple sac forms the early stage of the auditory vesicle in
Ammocoetes, according to Shipley; subsequently, by a series of foldings and
growings together, the chambers of the ear of the adult Petromyzon, as
figured and described by Retzius, are formed. Further, we see that
throughout the Vertebrata this sac was originally open to the exterior, the
auditory vesicle being first an open pit, which forms a vesicle by the
approximating of its sides, the last part to close being known as the
_recessus labyrinthicus_; in many cases, as in elasmobranchs, this part
remains open, or communicates with the exterior by means of the _ductus
endolymphaticus_.
Judging, therefore, from the embryological evidence, it would appear that
the auditory organ originated as a special sense-organ, formed by modified
epithelial cells of the surface, which epithelial surface becoming
invaginated, came to line a closed auditory vesicle under the surface. This
special sense-organ was innervated from a large ganglionic mass of
nerve-cells, situated close against the peripheral sense-cells, the
axis-cylinder processes of which formed the sensory roots of the nerve.
Yet another peculiarity of striking significance is seen in connection with
the auditory organ of Ammocoetes. The opening of the cartilaginous capsule
towards the brain is a large one (Fig. 154), and {379}admits the passage
not only of the auditory and facial nerves, but also of a portion of the
peculiar tissue which surrounds the brain. The large cells of this tissue,
with their feebly staining nuclei and the pigment between them, make them
quite unmistakable; and, as I have already stated, nowhere else in the
whole of Ammocoetes is such a tissue found. When I first noticed these
cells within the auditory capsule, it seemed to me almost impossible that
my interpretation of them as the remnant of the generative and hepatic
tissue which surrounds the brain of animals such as Limulus could be true,
for it seemed too unlikely that a part of the generative system could ever
have become included in the auditory capsule. Still, they are undoubtedly
there; and, as already argued with respect to the substance round the
brain, they must represent some pre-existing tissue which was functional in
the ancestor of Ammocoetes. If my interpretation is right, this tissue must
be generative and hepatic tissue, and its presence in the auditory capsule
immediately becomes a most important piece of evidence, for it proves that
the auditory organ must have been originally so situated that a portion of
the generative and hepatic mass surrounding the cephalic region of the
nervous system followed the auditory nerve to the peripheral sense-organ.
[Illustration: FIG. 154.--TRANSVERSE SECTION THROUGH AUDITORY CAPSULES AND
BRAIN OF AMMOCOETES.
_Au._, auditory organ; _VIII_, auditory nerve; _gl._, ganglion cells of
VIIIth nerve; _Au. cart._, cartilaginous auditory capsule; _gen._, cells of
old generative tissue round brain and in auditory capsule; _bl._,
blood-vessels]
{380}Here there was a test of the truth of my theory ranking second only to
the test of the median eyes; the strongest possible evidence of the truth
of any theory is given when by its aid new and unexpected facts are brought
to light. The theory said that in the group of animals from which the
vertebrates arose, a special sense-organ of the nature of an auditory organ
must have existed on the base of one of the appendages situated at the
junction of the prosoma and mesosoma, and that into this basal part of the
appendage a portion of the cephalic mass of generative and hepatic material
must have made its way in close contiguity to the nerve of the special
organ.
The only living example which nearly approaches the ancient extinct forms
from which, according to the theory, the vertebrates arose, is Limulus,
and, as has already been shown, in this animal, in the very position
postulated by the theory, a large special sense-organ--the
flabellum--exists, which, as already stated, may well have given rise to a
sense-organ concerned with equilibration and audition. If, further, it be
found that a diverticulum of the generative and hepatic material does
accompany the nerve of the flabellum in the basal part of the appendage,
then the evidence becomes very strong that the auditory organ of
Ammocoetes, _i.e._ of the ancient Cephalaspids, was derived from an organ
homologous with the flabellum; that, therefore, the material round the
brain of Ammocoetes was originally generative and hepatic material; that,
in fact, the whole theory is true, for all the parts of it hang together so
closely that, if one portion is accepted, all the rest must follow. As
pointed out in my address at Liverpool, and at the meeting of the
Philosophical Society at Cambridge, it is a most striking fact that a mass
of the generative and hepatic tissue does accompany the flabellar nerve
into the basal part of this appendage. Into no other appendage of Limulus
is there the slightest sign of any intrusion of the generative and hepatic
masses; nowhere, except in the auditory capsule, is there any sign of the
peculiar large-celled tissue which surrounds the brain and upper part of
the spinal cord of Ammocoetes. The actual position of the flabellum on the
basal part of the ectognath is shown in Fig. 155, A, and in Fig. 155, B, I
have removed the chitin, to show the generative and hepatic tissue (_gen._)
lying beneath.
The reason why, to all appearance, the generative and hepatic mass
penetrates into the basal part of this appendage only is apparent {381}when
we see (as Patten and Redenbaugh have pointed out) to what part of the
appendage the flabellum in reality belongs.
[Illustration: FIG. 155.--A, THE DIGGING APPENDAGE OR ECTOGNATH OF LIMULUS;
B, THE MIDDLE PROTUBERANCE (2) OF THE ENTOCOXITE OPENED, TO SHOW THE
GENERATIVE AND HEPATIC TISSUE (_gen._) WITHIN IT; C, ONE OF THE PROSOMATIC
LOCOMOTOR APPENDAGES OR ENDOGNATHS OF LIMULUS, FOR COMPARISON WITH A.
_fl._, flabellum; _cox._, coxopodite; _ent._, entocoxite; _m._, mandible;
_i.m._, inner mandible or epicoxite.]
Patten and Redenbaugh, in their description of the prosomatic appendages of
Limulus, describe the segments of the limbs as (1) the dactylopodite, (2)
the propodite, (3) the mero- and carpo-podites, (4) the ischiopodite, (5)
the basipodite, and (6) the coxopodite (_cox._ in Fig. 155). Still more
basal than the coxopodite is situated the entocoxite (_ent._ in Fig. 155),
which is composed of three sclerites {382}or sensory knobs, to use Patten's
description. The middle one of these three sclerites enlarges greatly in
the digging appendage, and grows over the coxopodite to form the base from
which the flabellum springs. Thus, as they have pointed out, the flabellum
does not belong to the coxopodite of the appendage, but to the middle
sensory knob of the entocoxite. Upon opening the prosomatic carapace, it is
seen that the cephalic generative and hepatic masses press closely against
the internal surface of the prosomatic carapace and also of the entocoxite,
so that any enlargement of one of the sensory knobs of the entocoxite would
necessarily be filled with a protrusion of the generative and hepatic
masses. This is the reason why the generative and hepatic material
apparently passes into the basal segment of the ectognath, and not into
that of the endognaths; it does not really pass into the coxopodite of the
appendage, but into an enlarged portion of the entocoxite, which can hardly
be considered as truly belonging to the appendage. Kishinouye has stated
that a knob arises in the embryo at the base of each of the prosomatic
locomotor appendages, but that this knob develops only in the last or
digging appendage (ectognath) forming the flabellum. Doubtless the median
sclerites of the entocoxites of the endognaths represent Kishinouye's
undeveloped knobs.
I conclude, therefore, that the flabellum, together with its basal part, is
an adjunct to the appendage rather than a part of it, and might, therefore,
easily remain as a separate and well-developed entity, even although the
appendage itself dwindled down to a mere tentacle.
The evidence appears to me very strong that the flabellum of Limulus and
the pecten of scorpions are the most likely organs to give a clue to the
origin of the auditory apparatus of vertebrates. At present both the
Eurypterids and Cephalaspids have left us in the lurch; in the former there
is no sign of either flabellum or pecten; in the latter, no sign of any
auditory capsule beyond Rohon's discovery of two small apertures situated
dorsally on each side of the middle line in Tremataspis, which he considers
to be the termination of the _ductus endolymphaticus_ on each side. In both
cases it is probable, one might almost say certain, that any such special
sense-organ, if present, was not situated externally, but was sunk below
the surface as in Ammocoetes.
The method by which such a sense-organ, situated externally on {383}the
surface of the animal, comes phylogenetically to form the lining wall of an
internally situated membranous capsule is given by the ontogeny of this
capsule, which shows step by step how the sense-organ sinks in and forms a
capsule, and finally is entirely removed from the surface except as regards
the _ductus endolymphaticus_.
SUMMARY.
The special apparatus for hearing is of a very different character from
that for vision or for smell, for its nerve belongs to the
infra-infundibular group of nerves, and not to the supra-infundibular, as
do those of the other two special senses. Of the five special senses the
nerves for touch, taste, and hearing, all belong to the
infra-infundibular segmental nerve-groups. The invertebrate origin, then,
of the vertebrate auditory nerve must be sought for in the
infra-oesophageal segmental group of nerves, and not in the
supra-oesophageal.
The organs supplied by the auditory nerve are only partly for the purpose
of hearing; there is always present also an apparatus--the semicircular
canals--concerned with equilibration and co-ordination of movements. Such
equilibration organs are not confined to the auditory nerve, but in the
water-living vertebrates are arranged segmentally along the body, forming
the organs of the lateral line in fishes; the auditory organ is but one
of these lateral line organs, which has been specially developed.
These lateral line organs have been compared to similar segmental organs
found in connection with the appendages in worms, especially the
respiratory appendages. In accordance with this suggestion we see that
they are all innervated from the region of the respiratory nerves--the
vagus, glosso-pharyngeal, and facial--nerves which originally supplied
the respiratory appendages of the palæostracan ancestor.
The logical conclusion is that the appendages of the Palæostraca
possessed special sense-organs concerned with the perception of special
vibrations, especially in the mesosomatic or respiratory region, and that
somewhere at the junction of the prosoma and mesosoma, one of these
sense-organs was specially developed to form the origin of the vertebrate
auditory apparatus.
Impressed by this reasoning I made search for some specially striking
sense-organ at the base of one of the appendages of Limulus, at the
junction of the prosoma and mesosoma, and was immediately rewarded by the
discovery of the extraordinary nature of the flabellum, which revealed
itself as an elaborate sense-organ supplied with a nerve out of all
proportion to its size. Up to this time no one had the slightest
conception that this flabellum was a special sense-organ; the discovery
of its nature was entirely due to the logical following out of the theory
of the origin of vertebrates described in this book.
The structure of this large sense-organ is comparable with that of the
sense-organs of the pectens of the scorpion, and of many other organs
found on the appendages of various members of the scorpion group, of
arachnids and {384}other air-breathing arthropods. Many of these organs,
such as the lyriform organs of arachnids, and the 'halteres' or balancers
of the Diptera, are usually regarded as auditory and equilibration
organs.
On all the mesosomatic appendages of Limulus very remarkable sense-organs
are found, apparently for estimating pressures, which, when the
appendages sank into the body to form with their basal parts the
branchial diaphragms of Ammocoetes, could easily be conceived as
remaining at the surface, and so giving rise to the lateral line organs.
Further confirmation of the view that an organ, such as the flabellum,
must be looked upon as the originator of the vertebrate auditory organ,
is afforded by the extraordinary coincidence that in Limulus a
diverticulum of the generative and hepatic mass accompanies the flabellar
nerve into the basal part of the digging appendage, while in Ammocoetes,
accompanying the auditory nerve into the auditory capsule, there is seen
a mass of cells belonging to that peculiar tissue which fills up the
space between the brain and the cranial walls, and has already, on other
grounds, been homologized with the generative and hepatic masses which
fill up the encephalic region of Limulus.
For all these reasons special sense-organs, such as are found in the
flabellum of Limulus and in the pectens of scorpions, may be looked upon
as giving origin to the vertebrate auditory apparatus. In such case it is
highly probable that the parachordals, with the auditory capsules
attached, arose from a second entochondrite of the same nature as the
plastron; a probability which is increased by the fact that the scorpion
does possess a second entochondrite, which, owing to its special
relations to the pecten, is known as the supra-pectinal entochondrite.
{385}CHAPTER XII
_THE REGION OF THE SPINAL CORD_
Difference between cranial and spinal regions.--Absence of lateral
root.--Meristic variation.--Segmentation of coelom.--Segmental excretory
organs.--Development of nephric organs; pronephric, mesonephric,
metanephric.--Excretory organs of Amphioxus.--Solenocytes.--Excretory
organs of Branchipus and of Peripatus, appendicular and
somatic.--Comparison of coelom of Peripatus and of
vertebrate.--Pronephric organs compared to coxal glands.--Origin of
vertebrate body-cavity (metacoele).--Segmental duct.--Summary of
formation of excretory organs.--Origin of somatic
trunk-musculature.--Atrial cavity of Amphioxus.--Pleural folds.--Ventral
growth of pleural folds and somatic musculature.--Pleural folds of
Cephalaspidæ and of Trilobita.--Significance of the ductless
glands.--Alteration in structure of excretory organs which have lost
their duct in vertebrates and in invertebrates.--Formation of lymphatic
glands.--Segmental coxal glands of arthropods and of vertebrates.--Origin
of adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless
glands.--Summary.
The consideration of the auditory nerve and the auditory apparatus
terminates the comparison between the cranial nerves of the vertebrate and
the prosomatic and mesosomatic nerves of the arthropod, and leaves us now
free to pass on to the consideration of the vertebrate spinal nerves and
the organs they supply. Before doing so, it is advisable to pass in review
the conclusions already attained.
Starting with the working hypothesis that the central nervous system of the
vertebrate has arisen from the central nervous system of the arthropod, but
has involved and enclosed the alimentary canal of the latter in the
process, so that there has been no reversal of surfaces in the derivation
of the one form from the other, we have been enabled to compare closely all
the organs of the head-region in the two groups of animals, and in no
single case have we been compelled to make any startling or improbable
assumptions. The simple following out of this clue has led in every case in
the most natural {386}manner to the interpretation of all the organs in the
head-region of the vertebrate from the corresponding organs of the
arthropod.
That it is possible to bring together all the striking resemblances between
organs in the two classes of animals, such as I have done in preceding
chapters, has been ascribed to a perverted ingenuity on my part--a
suggestion which is flattering to my imaginative powers, but has no
foundation of fact. There has been absolutely no ingenuity on my part; all
I have done is to compare organs and their nerve-supply, as they actually
exist in the two groups of animals, on the supposition that there has been
no turning over on to the back, no reversal of dorsal and ventral surfaces.
The comparison is there for all to read; it is all so simple, so
self-evident that, given the one clue, the only ingenuity required is on
the part of those who fail to see it.
The great distinction that has arisen between the two head-regions is the
disappearance of appendages as such, never, however, of important organs on
those appendages. If the olfactory organs of the one group were originally
situated on antennules, the olfactory organs still remain, although the
antennules as such have disappeared. The coxal excretory organs at the base
of the endognaths remain and become the pituitary body. A special
sense-organ, such as the flabellum of Limulus or the pecten of scorpion,
remains and gives rise to the auditory organ. A special glandular organ,
the uterus in the base of the operculum, remains, and gives rise to the
thyroid gland. The branchiæ and sense-organs on the mesosomatic appendages
remain, and even the very muscles to a large extent. As will be seen later,
the excretory organs at the base of the metasomatic appendages remain. It
is merely the appendage as such which vanishes either by dwindling away, or
by so great an alteration as no longer to be recognizable as an appendage.
This dwindling process was already in full swing before the vertebrate
stage; it is only a continuation of a previous tendency, as is seen in the
dwindling of the prosomatic appendages in the Merostomata and the inclusion
of the branchiæ within the body of the scorpion. Already among the
Palæostraca, swimming had largely taken the place of crawling. The whole
gradual transformation from the arthropod to the vertebrate is associated
with a transformation from a crawling to a swimming animal--with the
concomitant loss of locomotor appendages as such, and the alteration of the
shape of {387}the animal into the lithe fish-like form. The consideration
of the manner in which this latter change was brought about, takes us out
of the cranial into the spinal region.
If we take Limulus as the only living type of the Palæostraca, we are
struck with the fact that the animal consists to all intents and purposes
of prosomatic and mesosomatic regions only; the metasoma consisting of the
segments posterior to the mesosoma is very insignificant, so that the large
mass of the animal consists of what has become the head-region in the
vertebrate; the spinal region, which has become in the higher vertebrates
by far the largest region of the body, can hardly be said to exist in such
an animal as Limulus. As to the Eurypterids and others, similar remarks may
be made, though not to the same extent, for in them a distinct metasoma
does exist.
In this book I have considered up to the present the cranial region as a
system of segments, and shown how such segments are comparable, one by one,
with the corresponding segments in the prosoma and mesosoma of the presumed
arthropod ancestor.
In the spinal region such direct comparison is not possible, as is evident
on the face of it; for even among vertebrates themselves the spinal
segments are not comparable one by one, so great is the variation, so
unsettled is the number of segments in this region. This meristic
variation, as Bateson calls it, is the great distinctive character of the
spinal region, which distinguishes it from the cranial region with its
fixed number of nerves, and its substantive rather than meristic variation.
At the borderland, between the two regions, we see how the one type merges
into the other; how difficult it is to fix the segmental position of the
spino-occipital nerves; how much more variable in number are the segments
supplied by the vagus nerves than those anterior to them.
This meristic variation is a sign of instability, of want of fixedness in
the type, and is evidence, as already pointed out, that the spinal region
is newer than the cranial. This instability in the number of spinal
segments does not necessarily imply a variability in the number of segments
of the metasoma of the invertebrate ancestor; it may simply be an
expression of adaptability in the vertebrate phylum itself, according to
the requirements necessitated by the conversion of a crawling into a
swimming animal, and the subsequent conversion of the swimming into a
terrestrial or flying animal.
{388}However many may have been the original number of segments belonging
to the spinal region, one thing is certain--the segmental character of this
region is remarkably clearly shown, not only by the presence of the
segmental spinal nerves, but also by the marked segmentation of the
mesoblastic structures. The question, therefore, that requires elucidation
above all others is the origin of the spinal mesoblastic segments, _i.e._
of the coelomic cavities of the trunk-region, and the structures derived
from their walls.
Proceeding on the same lines as in the case of the cranial segments, it is
necessary in the first instance to inquire of the vertebrate itself as to
the scope of the problem in this region. In addition to the variability in
the number of segments so characteristic of the spinal region, the complete
absence in each spinal segment of a lateral root affords another marked
difference between the two regions. Here, except, of course, at the
junction of the spinal and cranial regions, each segmental nerve arises
from two roots only, dorsal and ventral, and these roots are separately
sensory and motor, and not mixed in function as was the lateral root of
each cranial segment. Now, these lateral roots were originally the nerves
supplying the prosomatic and mesosomatic appendages with motor as well as
sensory fibres. The absence, therefore, of lateral roots in the spinal
region implies that in the vertebrate none of the musculature belonging to
the metasomatic appendages has remained. Consequently, as far as muscles
are concerned, the clue to the origin of the spinal segments must be sought
for in the segmentation of the body-muscles.
Here, in contradistinction to the cranial region, the segmentation is most
marked, for the somatic spinal musculature of all vertebrates can be traced
back to a simple sheet of longitudinal ventral and dorsal muscles, such as
are seen in all fishes. This sheet is split into segments or myotomes by
transverse connective tissue septa or myo-commata; each myotome
corresponding to one spinal segment.
In addition to the evidence of segmentation afforded by the
body-musculature in all the higher vertebrates, similar evidence is given
by the segmental arrangement of parts of the supporting tissue to form
vertebræ. Such segments have received the name of sclerotomes, and each
sclerotome corresponds to one spinal segment.
Yet another marked peculiarity of this region is the segmental arrangement
of the excretory organs. Just as our body-musculature {389}has arisen from
the uniformly segmented simple longitudinal musculature of the lowest fish,
so, as we pass down the vertebrate phylum, we find more and more of a
uniform segmental arrangement in the excretory organs.
The origin of all these three separate segmentations may, in accordance
with the phraseology of the day, be included in the one term--the origin of
the spinal mesoblastic segments--_i.e._ of the coelomic cavities of the
trunk-region and the structures derived from their walls.
THE ORIGIN OF THE SEGMENTAL EXCRETORY ORGANS.
Of these three clues to the past history of the spinal region, the
segmentation manifested by the presence of vertebræ is the least important,
for in Ammocoetes there is no sign of vertebræ, and their indications only
appear at transformation. Especially interesting is the segmentation due to
the excretory organs, for the evidence distinctly shows that such excretory
organs have steadily shifted more and more posteriorly during the evolution
of the vertebrate.
In Limulus the excretory organs are in the prosomatic region--the coxal
glands; these become in the vertebrate the pituitary body.
In Amphioxus the excretory organs are in the mesosomatic region,
segmentally arranged with the gills.
In vertebrates the excretory organs are in the metasomatic region posterior
to the gills, and are segmentally arranged in this region. Their
investigation has demonstrated the existence of three distinct stages in
these organs: 1. A series of segmental excretory organs in segments
immediately following the branchial segments. This is the oldest of the
three sets, and to these organs the name of the _pronephros_ is given. 2. A
second series which extends more posteriorly than the first, overlaps them
to an extent which is not yet settled, and takes their place; to them is
given the name of the _mesonephros_. 3. A third series continuous with the
mesonephric is situated in segments still more posterior, supplants the
mesonephros and forms the kidneys of all the higher vertebrates. This forms
the _metanephros_.
These three sets of excretory organs are not exactly alike in their origin,
in that the pronephric tubules are formed from a different portion of the
coelomic walls to that from which the meso- and {390}metanephric tubules
are formed, and the former alone gives origin to a duct, which forms the
basis for the generative and urinary ducts, and is called the _segmental
duct_. The mesonephric tubules, called also the Wolffian body, open into
this duct.
In order to make the embryology of these excretory organs quite clear, I
will make use of van Wijhe's phraseology and also of his illustrations. He
terms the whole coelomic cavity the _procoelom_, which is divisible into a
ventral unsegmented part, the body-cavity or _metacoelom_, and a dorsal
segmented part, the _somite_. This latter part again is divided into a
dorsal part--the _epimere_--and a part connecting the dorsal part with the
body-cavity, to which therefore he gives the name of _mesomere_.
The cavity of the epimere disappears, and its walls form the muscle and
cutis plates of the body. The part which forms the muscles is known as the
_myotome_, which separates off from the mesomere, leaving the latter as a
blind sac--the _mesocoelom_--communicating by a narrow passage with the
body cavity or _metacoelom_. At the same time, from the mesomere is formed
the _sclerotome_, which gives rise to the skeletal tissues of the vertebræ,
etc., so that van Wijhe's epimere and mesomere together correspond to the
original term, protovertebra, or somite of Balfour; and when the myotome
and sclerotome have separated off, there is still left the intermediate
cell-mass of Balfour and Sedgwick, _i.e._ the sac-like mesocoele of van
Wijhe, the walls of which give origin to the mesonephrotome or
_mesonephros_. Further, according to van Wijhe, the dorsal part of the
unsegmented metacoelom is itself segmented, but not, as in the case of the
mesocoele, with respect to both splanchnopleuric and somatopleuric walls.
The segmentation is manifest only on the somatopleuric side, and consists
of a distinct series of hollow somatopleuric outgrowths, called by him
_hypomeres_, which give rise to the _pronephros_ and the segmental duct.
Van Wijhe considers that the whole metacoelom was originally segmented,
because in the lower vertebrates the segmentation reaches further
ventral-wards, so that in Selachia the body-cavity is almost truly
segmental. Also in the gill-region of Amphioxus the cavities which are
homologous with the body-cavity arise segmentally.
{391}[Illustration: FIG. 156.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF
THE VERTEBRATE COELOM. (After VAN WIJHE.)
_N._, central nervous system; _Nc._, notochord; _Ao._, aorta; _Mg._,
midgut. A, _My._, myocoele; _Mes._, mesocoele; _Met._, metacoele; _Hyp._,
hypomere (pronephric). B and C, _My._, myotome; _Mes._, mesonephros;
_S.d._, segmental duct (pronephric); _Met._, body cavity.]
{392}As is well known, Balfour and Semper were led, from their
embryological researches, to compare the nephric organs of vertebrates with
those of annelids, and, indeed, the nature of the vertebrate segmental
excretory organs has always been the fact which has kept alive the belief
in the origin of vertebrates from a segmented annelid. These segmental
organs thus compared were the mesonephric tubules, and doubts arose,
especially in the mind of Gegenbaur, as to the validity of such a
comparison, because the mesonephric tubules did not open to the exterior,
but into a duct--the segmental duct--which was an unsegmented structure
opening into the cloaca; also because the segmental duct, which was the
excretory duct of the pronephros, was formed first, and the mesonephric
tubules only opened into it after it was fully formed. Further, the
pronephros was said to arise from an outbulging of the somatopleuric
mesoblast, which extended over a limited number of metameres, and was not
segmental, but continuous. Gegenbaur and others therefore argued that the
original prevertebrate excretory organ was the pronephros and its duct, not
the mesonephros, from which they concluded that the vertebrate must have
been derived from an unsegmented type of animal, and not from the segmented
annelid type.
Such a view, however, has no further reason for acceptance, as it was based
on wrong premises, for Rückert has shown that the pronephros does arise as
a series of segmental nephric tubules, and is not unsegmented. He also has
pointed out that in Torpedo the anterior part of the pronephric duct shows
indications of being segmented, a statement fully borne out by the
researches of Maas on Myxine, who gives the clearest evidence that in this
animal the anterior part of the pronephric duct is formed by the fusion of
a series of separate ducts, each of which in all probability once opened
out separately to the exterior.
Rückert therefore concludes that Balfour and Semper were right in deriving
the segmental organs of vertebrates from those of annelids, but that the
annelid organs are represented in the vertebrate, not by the mesonephric
tubules, but by the pronephric tubules and their ducts, which originally
opened separately to the exterior. By the fusion of such tubules the
anterior part of the segmental duct was formed, while its posterior part
either arose by a later coenogenetic lengthening, or is the only remnant of
a series of pronephric tubules which originally extended the whole length
of the body, as suggested also by Maas and Boveri. Rückert therefore
supposed that the mesonephric tubules were a secondary set of nephric
organs, which were not necessarily directly derived from the annelid
nephric organs.
{393}At present, then, Rückert's view is the one most generally
accepted--the original annelid nephric organs are represented by the
pronephric tubules and the pronephric duct, not by the mesonephric tubules,
which are a later formation. This latter statement would hold good if the
mesonephric tubules were found entirely in segments posterior to those
containing the pronephric tubules; such, however, is said not to be the
case, for the two sets of organs are said to overlap in some cases; even
when they exist in the same segments, the former are said always to be
formed from a more dorsal part of the coelom than the pronephros, always to
be a later formation, and never to give any indication of communicating
with the exterior except by way of the pronephric duct.
The recent observations of Brauer on the excretory organs of the
Gymnophiona throw great doubt on the existence of mesonephric and
pronephric tubules in the same segment. He criticizes the observations on
which such statements are based, and concludes that, as in Hypogeophis, the
nephrotome which is cut off after the separation of the sclero-myotome
gives origin to the pronephros in the more anterior regions, just as it
gives origin to the mesonephros in the more posterior regions. In fact, the
observations of van Wijhe and others do not in reality show that two
excretory organs may be formed in one segment, the one mesonephric from the
remains of the mesomere and the other pronephric from the hypomere, but
rather that in such cases there is only one organ--the pronephros--part of
which is formed from the mesomere and part from the hypomere. Brauer goes
further than this, and doubts the validity of any distinction between
pronephros and mesonephros, on the ground of the former arising from a more
ventral part of the procoelom than the latter; for, as he says, it is only
possible to speak of one part of the somite as being more ventral than
another part when both parts are in the same segment; so that if pronephric
and mesonephric organs are never in the same segment, we cannot say with
certainty that the former arises more ventrally than the latter.
These observations of Brauer strongly confirm Sedgwick's original statement
that the pronephric and mesonephric organs are homodynamous organs, in that
they are both derived from the original serially situated nephric organs,
the differences between them being of a subordinate nature and not
sufficient to force us to believe that the mesonephros is an organ of quite
different origin to the {394}pronephros. So, also, Price, from his
investigations of the excretory organs of Bdellostoma, considers that in
this animal both pronephros and mesonephros are derived from a common
embryonic kidney, to which he gives the name _holonephros_.
Brauer also is among those who conclude that the vertebrate excretory
organs were derived from those of annelids; he thinks that the original
ancestor possessed a series of similar organs over the whole pronephric and
mesonephric regions, and that the anterior pronephric organs, which alone
form the segmental duct, became modified for a larval existence--that their
peculiarities were adaptive rather than ancestral. This last view seems to
me very far-fetched, without any sufficient basis for its acceptance.
According to the much more probable and reasonable view, the pronephros
represents the oldest and original excretory organs, while the mesonephros
is a later formation. Brauer's evidence seems to me to signify that the
pronephros, mesonephros, and metanephros are all serially homologous, and
that the pronephros bears much the same relation to the mesonephros that
the mesonephros does to the metanephros. The great distinction of the
pronephros is that it, and it alone, forms the segmental duct.
We may sum up the conclusions at which we have now arrived as follows:--
1. The pronephric tubules and the pronephric duct are the oldest part of
the excretory system, and are distinctly in evidence for a few segments
only in the most anterior part of the trunk-region immediately following
the branchial region. They differ also from the mesonephric tubules by not
being so clearly segmental with the myotomes.
2. The mesonephric tubules belong to segments posterior to those of the
pronephros, are strictly segmental with the myotomes, and open into the
pronephric duct.
3. All observers are agreed that the two sets of excretory organs resemble
each other in very many respects, as though they arose from the same series
of primitive organs, and, according to Sedgwick and Brauer, no distinction
of any importance does exist between the two sets of organs. Other
observers, however, consider that the pronephric organs, in part at all
events, arise from a part of the nephrocoele more ventral than that which
gives origin to the mesonephric organs, and that this difference in
position of origin, combined {395}with the formation of the segmental duct,
does constitute a true morphological distinction between the two sets of
organs.
4. All the recent observers are in agreement that the vertebrate excretory
organs strongly indicate a derivation from the segmental organs of
annelids.
The very strongest support has been given to this last conclusion by the
recent discoveries of Boveri and Goodrich upon the excretory organs of
Amphioxus. According to Boveri, the nephric tubules of Amphioxus open into
the dorsal coelom by one or more funnels. Around each funnel are situated
groups of peculiar cells, called by him 'Fadenzellen,' each of which sends
a long process across the opening of the funnel. Goodrich has examined
these 'Fadenzellen,' and found that they are typical pipe-cells, or
solenocytes, such as he has described in the nephridial organs of various
members of the annelid group Polychæta. Also, just as in the Polychæta, the
ciliated nephric tubule has no internal funnel-shaped opening into the
coelom, but terminates in these groups of solenocytes. "Each solenocyte
consists of a cell-body and nucleus situated at the distal free extremity
of a delicate tube; the proximal end of the tube pierces the wall of the
nephridial canal and opens into its lumen. A single long flagellum arising
from the cells works in the tube and projects into the canal."
The exceedingly close resemblance between the organs of Amphioxus and those
of Phyllodoce, as given in his paper, is most striking, and, as he says,
leads to the conclusion that the excretory organs of Amphioxus are
essentially identical with the nephridia of certain polychæte worms.
It is to me most interesting to find that the very group of annelids, the
Polychæta, which possess solenocytes so remarkably resembling those of the
excretory organs of Amphioxus, are the highest and most developed of all
the Annelida. I have argued throughout that the law of evolution consists
in the origination of successive forms from the dominant group then alive,
dominance signifying the highest type of brain-power achieved up to that
time. The highest type among Annelida is found in the Chætopoda; from them,
therefore, the original arthropod type must have sprung. This original
group of Arthropoda gave rise to the two groups of Crustacea and Arachnida,
in my opinion also to the Vertebrata, and, as already mentioned, it is
convenient to give it a generalized {396}name, the Protostraca, from which
subsequently the Palæostraca arose.
The similarity between the excretory organs of Amphioxus and those of
Phyllodoce suggests that the protostracan ancestor of the vertebrates arose
from the highest group of the Chætopoda--the Polychæta. The evidence which
I have already given points, however, strongly to the conclusion that the
vertebrate did not arise from members of the Protostraca near to the
polychæte stock, but rather from members in which the arthropod characters
had already become well developed--members, therefore, which were nearer
the Trilobita than the Polychæta. Such early arthropods would very probably
have retained in part excretory organs of the same character as those found
in the original polychæte stock, and thus account for the presence of
solenocytes in the excretory organs of Amphioxus.
In connection with such a possibility, I should like to draw attention to
the observations of Claus and Spangenberg on the excretory organs of
Branchipus--that primitive phyllopod, which is recognized as the nearest
approach to the trilobites at present living. According to Claus, an
excretory apparatus exists in the neighbourhood of each nerve-ganglion, and
Spangenberg finds a perfectly similar organ in the basal segment of each
appendage--a system, therefore, of excretory organs as segmentally arranged
as those of Peripatus. Claus considers that although these organs formed an
excretory system, it is not possible to compare them with the annelid
segmental organs, because he thought the cells in question arose from
ectoderm. Now, the striking point in the description of the excretory cells
in these organs, as described both by Claus and Spangenberg, is that they
closely resemble the pipe-cells or solenocytes of Goodrich; each cell
possesses a long tube-like projection, which opens on the surface. They
appear distinctly to belong to the category of flame-cells, and resemble
solenocytes more than anything else. According to Goodrich, the solenocyte
is probably an ectodermal cell, so that even if it prove to be the case, as
Claus thought, that these pipe-cells of Branchipus are ectodermal, they
would still claim to be derived from the segmental organs of annelids,
especially of the Polychæta, being, to use Goodrich's nomenclature, true
nephridial organs, as opposed to coelomostomes.
These observations of Claus and Spangenberg suggest not only that the
primitive arthropod of the trilobite type possessed segmental {397}organs
in every segment directly derived from those of a polychæte ancestor, but
also that such organs were partly somatic and partly appendicular in
position. Such a suggestion is in strict accord with the observations of
Sedgwick on the excretory organs of the most primitive arthropod known,
viz. Peripatus, where also the excretory organs, which are true segmental
organs, are partly somatic and partly appendicular. Further, the excretory
organs of the Scorpion and Limulus group are again partly somatic and
partly appendicular, receiving the name of coxal glands, because there is a
ventral projection of the gland into the coxa of the corresponding
appendage.
Judging from all the evidence available, it is probable that when the
arthropod stock arose from the annelids, simultaneously with the formation
of appendages, the segmental somatic nephric organs of the latter extended
ventrally into the appendage, and thus formed a segmental set of excretory
organs, which were partly somatic, partly appendicular in position, and
might therefore be called coxal glands.
As already stated, all investigators of the origin of the vertebrate
excretory organs are unanimous in considering them to be derived from
segmental organs of the annelid type. I naturally agree with them, but, in
accordance with my theory, would substitute the words "primitive arthropod"
for the word "annelid," for all the evidence I have accumulated in the
preceding chapters points directly to that conclusion. Further, the most
primitive of the three sets of vertebrate segmental organs--the pronephros,
mesonephros, and metanephros--is undoubtedly the pronephros; consequently
the pronephric tubules are those which I consider to be more directly
derived from the coxal glands of the primitive arthropod ancestor. Such a
derivation appears to me to afford an explanation of the difficulties
connected with the origin of the pronephros and mesonephros respectively,
which is more satisfactory than that given by the direct derivation from
the annelid.
The only living animal which we know of as at all approaching the most
primitive arthropod type is, as pointed out by Korschelt and Heider,
Peripatus; and Peripatus, as is well known, possesses a true coelom and
true coelomic excretory organs in all the segments of the body. Sedgwick
shows that at first a true coelom, as typical as that of the annelids, is
formed in each segment of the body, and that then this coelom (which
represents in the vertebrate van Wijhe's pro-coelom) {398}splits into a
dorsal and a ventral part. In the anterior segments of the body the dorsal
part disappears (presumably its walls give origin to the mesoblast from
which the dorsal body-muscles arise), while the ventral part remains and
forms a nephrocoele, giving origin to the excretory organs of the adult.
According to von Kennel, the cavity becomes divided into three spaces,
which for a time are in communication--a lateral (I.), a median (II.), and
a dorso-median (III.). The dorso-median portion becomes partitioned off,
and this, as well as the greater part of the lateral portion, which lies
principally in the foot, is used up in providing elements for the formation
of the body- and appendage-muscles respectively and the connective tissue.
In Fig. 157 I reproduce von Kennel's diagram of a section across a
Peripatus embryo, in which I. represents the lateral appendicular part of
the coelom, II. the ventral somatic part, and III. the dorsal part which
separates off from the ventral and lateral parts, and, as its walls give
origin largely to the body-muscles, may be called the myocoele. The muscles
of the appendages are formed from the ventral part of the original
procoelom, just as I have argued is the case with the muscles of the
splanchnic segmentation in vertebrates.
Sedgwick states that the ventral part of the coelom extends into the base
of each appendage, and there forms the end-sac of each nephric tubule, into
which the nephric funnel opens, thus forming a coxal gland; this end-sac or
vesicle in the appendage is called by him the internal vesicle (_i.v._),
because later another vesicle is formed from the ventral coelom in the body
itself, close against the nerve-cord on each side, which he calls the
external vesicle (_e.v._). (_Cf._ Fig. 158, taken from Sedgwick.) This
second vesicle is, according to him, formed later in the development from
the nephric tubule of the internal vesicle, so that it discharges its
contents to the exterior by the same opening as the original tubule. Of
course, as he points out, the whole system of internal and external
vesicles and nephric tubules are all simply derivatives of the original
ventral part of the coelom or nephrocoele.