Chapter XIX: Act 1861: s. 31, any person may arrest any one whom he shall find (6)

On the other hand, the polymeniscous eye undergoes special elaboration
on its lines. The retinulae become elongated as deep and very narrow
pits (fig. 12 and explanation), and develop additional cells near the
mouth of the narrow pit. Those nearest to the lens are the corneagen
cells of this more elaborated eye, and those between the original
retinula cells and the corneagen cells become firm and transparent.
They are the crystalline cells or vitrella (see Watase, 7). Each such
complex of cells underlying the lenticle of a compound eye is called
an "ommatidium"; the entire mass of cells underlying a monomeniscous
eye is an "ommataeum." The ommataeum, as already stated, tends to
segregate into retinulae which correspond potentially each to an
ommatidium of the compound eye. The ommatidium is from the first
segregate and consists of few cells. The compound eye of the king-crab
(Limulus) is the only recognized instance of ommatidia in their
simplest state. Each can be readily compared with the single-layered
lateral eye of the scorpion. In Crustacea and Hexapoda of all grades
we find compound eyes with the more complicated ommatidia described
above. We do not find them in any Arachnida.

It is difficult in the absence of more detailed knowledge as to the
eyes of Chilopoda and Diplopoda to give full value to these facts in
tracing the affinities of the various classes of Arthropods. But they
seem to point to a community of origin of Hexapods and Crustacea in
regard to the complicated ommatidia of the compound eye, and to a
certain isolation of the Arachnida, which are, however, traceable, so
far as the eyes are concerned, to a distant common origin with
Crustacea and Hexapoda through the very simple compound eyes
(monostichous, polymeniscous) of Limulus.

_The Tracheae._--In regard to tracheae the very natural tendency of
zoologists has been until lately to consider them as having once
developed and once only, and therefore to hold that a group
"Tracheata" should be recognized, including all tracheate Arthropods.
We are driven by the conclusions arrived at as to the derivation of
the Arachnida from branchiate ancestors, independently of the other
tracheate Arthropods, to formulate the conclusion that tracheae have
been independently developed in the Arachnidan class. We are also, by
the isolation of Peripatus and the impossibility of tracing to it all
other tracheate Arthropoda, or of regarding it as a degenerate offset
from some one of the tracheate classes, forced to the conclusion that
the tracheae of the Onychophora have been independently acquired.
Having accepted these two conclusions, we formulate the generalization
that tracheae can be independently acquired by various branches of
Arthropod descent in adaptation to a terrestrial as opposed to an
aquatic mode of life. A great point of interest therefore exists in
the knowledge of the structure and embryology of tracheae in the
different groups. It must be confessed that we have not such full
knowledge on this head as could be wished for. Tracheae are
essentially tubes like blood-vessels--apparently formed from the same
tissue elements as blood-vessels--which contain air in place of blood,
and usually communicate by definite orifices, the tracheal stigmata,
with the atmosphere. They are lined internally by a cuticular deposit
of chitin. In Peripatus and the Diplopods they consist of bunches of
fine tubes which do not branch but diverge from one another; the
chitinous lining is smooth. In the Hexapods and Chilopods, and the
Arachnids (usually), they form tree-like branching structures, and
their finest branches are finer than any blood-capillary, actually in
some cases penetrating a single cell and supplying it with gaseous
oxygen. In these forms the chitinous lining of the tubes is thickened
by a close-set spiral ridge similar to the spiral thickening of the
cellulose wall of the spiral vessels of plants. It is a noteworthy
fact that other tubes in these same terrestrial Arthropoda--namely,
the ducts of glands--are similarly strengthened by a chitinous
cuticle, and that a spiral or annular thickening of the cuticle is
developed in them also. Chitin is _not_ exclusively an ectodermal
product, but occurs also in cartilaginous skeletal plates of
mesoblastic origin (connective tissue). The immediate cavities or pits
into which the tracheal stigmata open appear to be in many cases
ectodermic in sinkings, but there seems to be no reason (based on
embryological observation) for regarding the tracheae as an ingrowth
of the ectoderm. They appear, in fact, to be an air-holding
modification of the vasifactive connective tissue. Tracheae are
abundant just in proportion as blood-vessels become suppressed. They
are reciprocally exclusive. It seems not improbable that they are two
modifications of the same tissue-elements. In Peripatus the stigmatic
pits at which the tracheae communicate with the atmosphere are
scattered and not definite in their position. In other cases the
stigmata are definitely paired and placed in a few segments or in
several. It seems that we have to suppose that the vasifactive tissue
of Arthropoda can readily take the form of air-holding instead of
blood-holding tubes, and that this somewhat startling change in its
character has taken place independently in several instances--viz. in
the Onychophora, in more than one group of Arachnida, in Diplopoda,
and again in the Hexapoda and Chilopoda.

_The Malpighian Tubes._--This name is applied to the numerous fine
caecal tubes of noticeable length developed from the proctodaeal
invert of ectodermal origin in Hexapods. These tubes are shown to
excrete nitrogenous waste products similar to uric acid. Tubes of
renal excretory function in a like position occur in most terrestrial
Arthropoda--viz. in Chilopoda, Diplopoda and Arachnida. They are also
found in some of the semi-terrestrial and purely aquatic Amphipod
Crustaceans. But the conclusion that all such tubes are identical in
essential character seems to be without foundation. The Malpighian
tubes of Hexapods are outgrowths of the proctodaeum, but those of
Scorpion and the Amphipod Crustacea are part of the metenteron or
endodermal gut, though originating near its junction with the
proctodaeum. Hence the presence or absence of such tubes cannot be
used as an argument as to affinity without some discrimination. The
Scorpion's so-called Malpighian tubes are _not_ the same organs as
those so named in the other Tracheata. Such renal caecal tubes seem to
be readily evolved from either metenteron or proctodaeum when the
conditions of the out-wash of nitrogenous waste-products are changed
by the transference from aquatic to terrestrial life. The absence of
such renal caeca in Limulus and their presence in the terrestrial
Arachnida is precisely on a parallel with their absence in aquatic
Crustacea and their presence in the feebly branchiate Amphipoda.

_Group Characters._--We shall now pass the groups of the Arthropoda in
review, attempting to characterize them in such a way as will indicate
their probable affinities and genetic history.

SUB-PHYLUM ARTHROPODA.--The characters of the sub-phylum and those of
the associated sub-phyla Chaetopoda and Rotifera have been given
above, as well as the general characters of the phylum Appendiculata
which comprises these great sub-phyla.

Grade A.--Hyparthropoda.

Hypothetical forms.

Grade B.--Protarthropoda.

(a) The integument is covered by a delicate soft cuticle (not firm or
plated) which allows the body and its appendages great range of
extension and contraction.

(b) The paired claws on the ends of the parapodia and the fang-like
modifications of these on the first post-oral appendages (mandibles)
are the only hard chitinous portions of the integument.

(c) The head is deuterognathous--that is to say, there is only one
prosthomere, and accordingly the first and only pair of hemignaths is
developed by adaptation of the appendages of the second somite.

(d) The appendages of the third somite (second post-oral) are clawless
oral papillae.

(e) The rest of the somites carry equi-formal simple appendages,
consisting of a corm or axis tipped with two chitinous claws and
devoid of rami.

(f) The segmentation of the body is anomomeristic, there being no
fixed number of somites characterizing all the forms included.

(g) The pair of eyes situated on the prosthomere are not of the
Euarthropod type, but resemble those of Chaetopods (hence
Nereid-ophthalmous).

(h) The muscles of the body-wall and gut do not consist of
transversely-striped muscular fibre, but of the unstriped tissue
observed also in Chaetopoda.

(i) A pair of coelomoducts is developed in every somite including the
prosthomere, in which alone it atrophies in later development.

(j) The ventral nerve-cords are widely separated--in fact, lateral in
position.

(k) There are no masses of nerve-cells forming a ganglion (neuromere)
in each somite. (In this respect the Protarthropoda are at a lower
stage than most of the existing Chaetopoda.)

(l) The genital ducts are formed by the enlargement of the
coelomoducts of the penultimate somite.

Class (Unica).--ONYCHOPHORA.

With the characters of the grade: add the presence within the body of
fine unbranched tracheal tubes, devoid of spiral thickening, opening
to the exterior by numerous irregularly scattered tracheal pits.

Genera--Eoperipatus, Peripatopsis, Opisthopatus, &c. (See PERIPATUS.)

Grade C (of the Arthropoda).--Euarthropoda.

(a) Integument heavily plated with firm chitinous cuticle, allowing no
expansion and retraction of regions of the body nor change of
dimensions, except, in some cases, a dorso-ventral bellows movement.
The separation of the heavier plates of chitin by grooves of delicate
cuticle results in the hinging or jointing of the body and its
appendages, and the consequent flexing and extending of the jointed
pieces.

(b) Claws and fangs are developed on the branches or rami of the
parapodia, not on the end of the axis or corm.

(c) The head is either deuterognathous, tritognathous, or
tetartognathous.

(d) Rarely only one, and usually at least two, of the somites
following the mandibular somite carry appendages modified as jaws
(with exceptions of a secondary origin).

(e) The rest of the somites may all carry appendages, or only a
limited number may carry appendages. In all cases the appendages
primarily develop rami or branches which form the limbs, the primitive
axis or corm being reduced and of insignificant size. In the most
primitive stock all the post-oral appendages had gnathobasic
outgrowths.

(f) The segmentation of the body is anomomeristic in the more archaic
members of each class, nomomeristic in the higher members.

(g) The two eyes of Chaetopod structure have disappeared, and are
replaced by the Euarthropod eyes.

(h) The muscles in all parts of the body consist of striped muscular
fibre, never of unstriped muscular tissue.

(i) The coelomoducts are suppressed in most somites, and retained only
as the single pair of genital ducts (very rarely more numerous) and in
some also as the excretory glands (one or two pairs).

(j) The ventral nerve-cords approach one another in the mid-ventral
line behind the mouth.

(k) The nerve-cells of the ventral nerve cords are segregated as
paired ganglia in each somite, often united by meristic dislocation
into composite ganglia.

(l) The genital ducts may be the coelomoducts of the penultimate or
ante-penultimate or adjacent somite, or of a somite placed near the
middle of the series, or of a somite far forward in the series.

Class 1 (of the Euarthropoda).--DIPLOPODA.

The head has but one prosthomere (monoprosthomerous), and is
accordingly deuterognathous. This carries short-jointed antennae (in
one case bi-ramose) and eyes, the structure and development of which
require further elucidation. Only one somite following the first
post-oral or mandibular segment has its appendages modified as jaws.

The somites of the body, except in Pauropus, either fuse after early
development and form double somites with two pairs of appendages
(Julus, &c.), or present legless and leg-bearing somites alternating.

Somites, anomomeristic, from 12 to 150 in the post-cephalic series.

The genital ducts open in the fourth, or between the fourth and fifth
post-oral somite.

Terrestrial forms with small-jointed legs formed by adaptation of a
single ramus of the appendage. Tracheae are present.

_Note._--The Diplopoda include the Juliformia, the Symphyla
(Scolopendrella), and Pauropoda (Pauropus). They were until recently
classified with the Chilopoda (Centipedes), with which they have no
close affinity, but only a superficial resemblance. (Compare the
definition of the class Chilopoda.)

The movement of the legs in Diplopoda is like that of those of
Peripatus, of the Phyllopod Crustacea, and of the parapodia of
Chaetopoda, symmetrical and identical on the two sides of the body.
The legs of Chilopoda move in alternating groups on the two sides of
the body. This implies a very much higher development of nerves and
muscles in the latter. (See MILLIPEDE.)

Class 2 (of the Euarthropoda).--ARACHNIDA.

Head tritognathous and diprosthomerous--that is to say, with two
prosthomeres, the first bearing typical eyes, the second a pair of
appendages reduced to a single ramus, which is in more primitive forms
antenniform, in higher forms chelate or retrovert. The ancestral stock
was pantognathobasic--i.e. had a gnathobase or jaw process on every
parapodium. As many as six pairs of appendages following the mouth may
have an enlarged gnathobase actually functional as a jaw or hemignath,
but a ramus is well developed on each of these appendages either as a
simple walking leg, a palp or a chela. In the more primitive forms the
appendage of every post-oral somite has a gnathobase and two rami; in
higher specialized forms the gnathobases may be atrophied in every
appendage, even in the first post-oral.

The more primitive forms are anomomeristic; the higher forms
nomomeristic, showing typically three groups or tagmata of six somites
each.

The genital apertures are placed on the first somite of the second
tagma or mesosoma. Their position is unknown in the more primitive
forms. The more primitive forms have branchial respiratory processes
developed on a ramus of each of the post-oral appendages. In higher
specialized forms these branchial processes become first of all
limited to five segments of the mesosoma, then sunk beneath the
surface as pulmonary organs, and finally atrophied, their place being
taken by a well-developed tracheal system.

A character of great diagnostic value in the more primitive Arachnida
is the tendency of the chitinous investment of the tergal surface of
the telson to unite during growth with that of the free somites in
front of it, so as to form a pygidial shield or posterior carapace,
often comprising as many as fifteen somites (Trilobites, Limulus).

A pair of central monomeniscous diplostichous eyes is often present on
the head. Lateral eyes also are often present which are monostichous
with aggregated lenses (_Limulus_) or with isolated lenses (Scorpio),
or are diplostichous with simple lens (_Pedipalpi_, Araneae, &c.).

Class 3 (of the Euarthropoda).--CRUSTACEA.

Head tetartognathous and triprosthomerous--that is to say, with three
prosthomeres; the first bearing typical eyes, the second a pair of
antenniform appendages (often bi-ramose), the third a pair of
appendages usually antenniform, sometimes claw-like. The ancestral
stock was (as in the Arachnida) pantognathobasic, that is to say, had
a gnathobase or jaw-process on the base of every post-oral appendage.

Besides the first post-oral or mandibular pair, at least two
succeeding pairs of appendages are modified as jaws. These have small
and insignificant rami, or none at all, a feature in which the
Arachnida differ from them. The appendages of four or more additional
following somites may be turned upwards towards the mouth and assist
in the taking of food.

The more primitive forms (Entomostraca) are anomomeristic, presenting
great variety as to number of somites, form of appendages, and
tagmatic grouping; the higher forms (Malacostraca) are nomomeristic,
showing in front of the telson twenty somites, of which the six hinder
carry swimmerets and the five next in front ambulatory limbs. The
genital apertures are neither far forward nor far backward in the
series of somites, e.g. on the fourteenth post-oral in Apus, on the
ninth post-oral in female Astacus and in Cyclops.

With rare exceptions, branchial plates are developed either by
modification of a ramus of the limbs or as processes on a ramus, or
upon the sides of the body. No tracheate Crustacea are known, but some
terrestrial Isopoda develop pulmonary in-sinkings of the integument. A
characteristic, comparable in value to that presented by the pygidial
shield of Arachnida, is the frequent development of a pair of long
appendages by the penultimate somite, which with the telson form a
trifid, or, when that is small, a bifid termination to the body.

The lateral eyes of Crustacea are polymeniscous, with highly
specialized retinulae like those of Hexapoda, and unlike the simpler
compound lateral eyes of lower Arachnida. Monomeniscous eyes are
rarely present, and when present, single, minute, and central in
position.

_Note._--The Crustacea exhibit a longer and more complete series of
forms than any other class of Arthropoda, and may be regarded as
preserving the most completely represented line of descent.

Class 4.--CHILOPODA.

Head triprosthomerous[3] and tetartognathous. The two somites
following the mandibular or first post-oral or buccal somite carry
appendages modified as maxillae. The fourth post-oral somite has its
appendages converted into very large and powerful hemignaths, which
are provided with poison-glands. The remaining somites carry
single-clawed walking legs, a single pair to each somite. The body is
anomomeristic, showing in different genera from 17 (inclusive of the
anal and genital) to 175 somites behind that which bears the poison
jaws. No tagmata are developed. The genital ducts open on the
penultimate somite.

Tracheae are developed which are dendriform and with spiral thickening
of their lining. Their trunks open at paired stigmata placed laterally
in each somite of the trunk or in alternate somites. Usually the
tracheae open by paired stigmata placed upon the sides of a greater or
less number of the somites, but never quite regularly on alternating
somites. At most they are present on all the pedigerous somites
excepting the first and the last. In _Scutigera_ there are seven
unpaired dorsal stigmata, each leading into a sac whence a number of
air-holding tubes project into the pencardial blood-sinus.

Renal caecal tubes (Malpighian tubes) open into the proctodaeum. (See
CENTIPEDE.)

Class 5.--HEXAPODA.

Head shown by its early development to be triprosthomerous and
consequently tetartognathous. The first prosthomere has its appendages
represented by the compound eyes and a protocerebrum, the second has
the antennae for its appendages and a deutocerebral neuromere, the
third has suffered suppression of its appendages (which corresponded
to the second pair of antennae of Crustacea), but has a tritocerebrum
and coelomic chamber. The mandibular somite bears a pair of
gnathobasic hemignaths without rami or palps, and is followed by two
jaw-bearing somites (maxillary and labial). This enumeration would
give six somites in all to the head--three prosthomeres and three
opisthomeres. Recent investigations (Folsom, 4) show the existence in
the embryo of a prae-maxillary or supra-lingual somite which is
suppressed during development. This gives seven somites to the
Hexapod's head, the tergites of which are fused to form a cephalic
carapace or box. The number is significant, since it agrees with that
found in Edriophthalmous Crustacea, and assigns the labium of the
Hexapod to the same somite numerically as that which carries the
labium-like maxillipedes of those Crustacea.

The somites following the head are strictly nomomeristic and
nomotagmic. The first three form the thorax, the appendages of which
are the walking legs, tipped with paired claws or ungues (compare the
homoplastic claws of Scorpio and Peripatus). Eleven somites follow
these, forming the abdominal "tagma," giving thus twenty-one somites
in all (as in the higher Crustacea). The somites of the abdomen all
may carry rudimentary appendages in the embryo, and some of the hinder
somites may retain their appendages in a modified form in adult life.
Terminal telescoping of the abdominal somites and excalation may occur
in the adult, reducing the obvious abdominal somites to as few as
eight. The genital apertures are median and placed far back in the
series of somites, viz. the female on the seventh abdominal
(seventeenth of the whole series) and the male on the ninth or
ante-penultimate abdominal (nineteenth of the whole series). The
appendages of the eighth and tenth abdominal somites are modified as
gonapophyses. The eleventh abdominal segment is the telson, usually
small and soft; it carries the anus.

The Hexapoda are not only all confined to a very definite disposition
of the somites, appendages and apertures, as thus indicated, but in
other characters also they present the specialization of a
narrowly-limited highly-developed order of such a class as the
Crustacea rather than a range from lower more generalized to higher
more specialized forms such as that group and also the Arachnida
present. It seems to be a legitimate conclusion that the most
primitive Hexapoda were provided with wings, and that the term
Pterygota might be used as a synonym of Hexapoda. Many Hexapoda have
lost either one pair or both pairs of wings; cases are common of
wingless genera allied to ordinary Pterygote genera. Sdme Hexapods
which are very primitive in other respects happen to be also Apterous,
but this cannot be held to prove that the possession of wings is not a
primitive character of Hexapods (compare the case of the Struthious
Birds). The wings of Hexapoda are lateral expansions of the terga of
the second and third thoracic somites. They appear to be serial
equivalents (homogenous meromes) of the tracheal gills, which develop
in a like position on the abdominal segments of some aquatic Hexapods.

The Hexapoda are all provided with a highly developed tracheal system,
which presents considerable variation in regard to its stigmata or
orifices of communication with the exterior. In some a serial
arrangement of stigmata comparable to that observed in Chilopoda is
found. In other cases (some larvae) stigmata are absent; in other
cases again a single stigma is developed, as in the smaller Arachnida
and Chilopoda, in the median dorsal line or other unexpected position.
When the facile tendency of Arthropoda to develop tracheal air-tubes
is admitted, it becomes probable that the tracheae of Hexapods do not
all belong to one original system, but may be accounted for by new
developments within the group. Whether the primitive tracheal system
of Hexapoda was a closed one or open by serial stigmata in every
somite remains at present doubtful, but the intimate relation of the
system to the wings and tracheal gills cannot be overlooked.

The lateral eyes of Hexapoda, like those of Crustacea, belong to the
most specialized type of "compound eye," found only in these two
classes. Simple monomeniscous eyes are also present in many Hexapods.

Renal excretory caeca (Malpighian tubes) are developed from the
proctodaeum (not from mesenteron as in scorpion and Amphipoda).

_Concluding Remarks on the Relationships to one another of the
Classes of the Arthropoda._--Our general conclusion from a survey of
the Arthropoda amounts to this, that whilst Peripatus, the Diplopoda,
and the Arachnida represent terrestrial offshoots from successive
lower grades of primitive aquatic Arthropoda which are extinct, the
Crustacea alone present a fairly full series of representatives
leading upwards from unspecialized forms. The latter were not very far
removed from the aquatic ancestors (Trilobites) of the Arachnida, but
differed essentially from them by the higher specialization of the
head. We can gather no indication of the forefathers of the Hexapoda
or of the Chilopoda less specialized than they are, whilst possessing
the essential characteristics of these classes. Neither embryology nor
palaeontology assists us in this direction. On the other hand, the
facts that the Hexapoda and the Chilopoda have triprosthomerous heads,
that the Hexapoda have the same total number of somites as the
nomomeristic Crustacea, and the same number of opisthomeres in the
head as the more terrestrial Crustacea, together with the same
adaptation of the form of important appendages in corresponding
somites, and that the compound eyes of both Crustacea and Hexapoda are
extremely specialized and elaborate in structure and identical in that
structure, all lead to the suggestion that the Hexapoda, and with
them, at no distant point, the Chilopoda, have branched off from the
Crustacean main stem as specialized terrestrial lines of descent. And
it seems probable that in the case of the Hexapoda, at any rate, the
point of departure was subsequent to the attainment of the
nomomeristic character presented by the higher grade of Crustacea. It
is on the whole desirable to recognize such affinities in our schemes
of classification.

We may tabulate the facts as to head-structure in Chaetopoda and
Arthropoda as follows:--

Grade x (below the Arthropoda).--AGNATHA, APROSTHOMERA.

Without parapodial jaws; without the addition of originally post-oral
somites to the prae-oral region, which is a simple prostomial lobe of
the first somite; the first somite is perforated by the mouth and its
parapodia are not modified as jaws.

= CHARTOPODA.

Grade 1 (of the Arthropoda).--MONOGNATHA, MONOPROSTHOMERA.

With a single pair of parapodial jaws carried by the somite which is
perforated by the mouth; this is not the first somite, but the second.
The first somite has become a prosthomere, and carries a pair of
extensile antennae.

= ONYCHOPHORA (_Peripatus, &c._).

Grade 2 (of the Arthropoda).--DIGNATHA, MONOPROSTHOMERA.

The third somite as well as the second develops a pair of parapodial
jaws; the first somite is a prosthomere carrying jointed antennae.

= DIPLOPODA.

Grade 3 (of the Arthropoda).--PANTOGNATHA, DIPROSTHOMERA.

A gnathobase is developed (in the primitive stock) on every pair of
post-oral appendages; two prosthomeres present, the second somite as
well as the first having passed in front of the mouth, but only the
second has appendages.

= ARACHNIDA.

Grade 4 (of the Arthropoda).--PANTOGNATHA, TRIPROSTHOMERA.

The original stock, like that of the last grade, has a gnathobase on
every post-oral appendage, but three prosthomeres are now present, in
consequence of the movement of the oral aperture from the third to the
fourth somite. The later eyes are polymeniscous, with specialized
vitrellae and retinulae of a definite type peculiar to this grade.

= CRUSTACEA, CHILOPODA, HEXAPODA.

According to older views the increase of the number of somites in
front of the mouth would have been regarded as a case of intercalation
by new somite-budding of new prae-oral somites in the series. We are
prohibited by a general consideration of metamerism in the Arthropoda
from adopting the hypothesis of intercalation of somites. However
strange it may seem, we have to suppose that one by one in the course
of long historical evolution somites have passed forwards and the
mouth has passed backwards. In fact, we have to suppose that the
actual somite which in grades 1 and 2 bore the mandibles lost those
mandibles, developed their rami as tactile organs, and came to occupy
a position in front of the mouth, whilst its previous jaw-bearing
function was taken up by the next somite in order, into which the oral
aperture had passed. A similar history must have been slowly brought
about when this second mandibulate somite in its turn became agnathous
and passed in front of the mouth. The mandibular parapodia may be
supposed during the successive stages of this history to have had,
from the first, well-developed rami (one or two) of a palp-like form,
so that the change required when the mouth passed away from them would
merely consist in the suppression of the gnathobase. The solid
palpless mandible such as we now see in some Arthropoda is,
necessarily, a late specialization. Moreover, it appears probable that
the first somite never had its parapodia modified as jaws, but became
a prosthomere with tactile appendages before parapodial jaws were
developed at all, or rather _pari passu_ with their development on the
second somite. It is worth while bearing in mind a second possibility
as to the history of the prosthomeres, viz. that the buccal
gnathobasic parapodia (the mandibles) were in each of the three grades
of prosthomerism only developed after the recession of the mouth and
the addition of one, of two, or of three post-oral somites to the
prae-oral region had taken place. In fact, we may imagine that the
characteristic adaptation of one or more pairs of post-oral parapodia
to the purposes of the mouth as jaws did not occur until after
ancestral forms with one, with two, and with three prosthomeres had
come into existence. On the whole the facts seem to be against this
supposition, though we need not suppose that the gnathobase was very
large or the rami undeveloped in the buccal parapodia which were
destined to lose their mandibular features and pass in front of the
mouth.

REFERENCES.--1. Bateson, _Materials for the Study of Variation_
(Macmillan, 1894), p. 85; 2. Lankester, "Primitive Cell-layers of the
Embryo." _Annals and Mag. Nat. Hist._ (1873), p. 336; 3. Korschelt and
Heider, _Entwickelungsgeschichte_ (Jena, 1892), cap. xv. p. 389; 4.
Folsom, "Development of the Mouth Parts of Anurida," _Bulletin Mus.
Comp. Zool. Harvard College_, vol. xxxvi. No. 5 (1900), pp. 142-146;
5. Lankester, "Observations and Reflections on the Appendages and
Nervous System of Apus Cancriformis," _Quart. Journ. Micr. Sci._ vol.
xxi. (1881); 6. Hofer, "Ein Krebs mit einer Extremitat statt eines
Stielauges," _Verhandl. d. deutschen zool. Gesellsch._ (1894); 7.
Watase, "On the Morphology of the Compound Eyes of Arthropods,"
_Studies from the Biol. Lab. of the Johns Hopkins University_, vol.
iv. pp. 287-334; 8. Benham describes backward shifting of the oral
aperture in certain Chaetopods, _Proc. Zoolog. Soc. London_ (1900),
No. lxiv. p. 976. N.B.--References to the early literature concerning
the group Arthropoda will be found in Carus, _Geschichte der
Zoologie_. The more important literature up to 1892 is given in the
admirable treatise on Embryology by Professors Korschelt and Heider.
Detailed references will be found under the articles on the separate
groups of Arthropoda. (E. R. L.)

FOOTNOTES:

[1] The group Arthropoda itself, thus constituted, was precisely
identical in its area with the Insecta of Linnaeus, the Entoma of
Aristotle. But the word "Insect" had become limited since the days of
Linnaeus to the Hexapod Pterygote forms, to the exclusion of his
Aptera. Lamarck's penetrating genius is chiefly responsible for the
shrinkage of the word Insecta, since it was he who, forty years after
Linnaeus's death, set up and named the two great classes Crustacea
and Arachnida (included by Linnaeus under Insecta as the order
"Aptera"), assigning to them equal rank with the remaining Insecta of
Linnaeus, for which he proposed the very appropriate class-name
"Hexapoda." Lamarck, however, appears not to have insisted on this
name Hexapoda, and so the class of Pterygote Hexapods came to retain
the group-name Insecta, which is, historically or etymologically, no
more appropriate to them than it is to the classes Crustacea and
Arachnida. The tendency to retain the original name of an old and
comprehensive group for one of the fragments into which such group
becomes divided by the advance of knowledge--instead of keeping the
name for its logical use as a comprehensive term, including the new
divisions, each duly provided with a new name--is most curiously
illustrated in the history of the word physiology. Cicero says,
"Physiologia naturae ratio," and such was the meaning of the name
_Physiologus_, given to a cyclopaedia of what was known and imagined
about earth, sea, sky, birds, beasts and fishes, which for a thousand
years was the authoritative source of information on these matters,
and was translated into every European tongue. With the revival of
learning, however, first one and then another special study became
recognized--anatomy, botany, zoology, mineralogy, until at last the
great comprehensive term physiology was bereft of all its
once-included subject-matter, excepting the study of vital processes
pursued by the more learned members of the medical profession.
Professional tradition and an astute perception on their part of the
omniscience suggested by the terms, have left the medical men in
English-speaking lands in undisturbed but illogical possession of the
words physiology, physic and physician.

[2] H. Milne-Edwards, who was followed by Huxley, long ago formulated
the conclusion that the eye-stalks of Crustacea are modified
appendages, basing his argument on a specimen of Palinurus (figured
in Bateson's book (1), in which the eye-stalk of one side is replaced
by an antenniform palp. Hofer (6) in 1894 described a similar case in
Astacus.

[3] Embryological evidence of this is still wanting. In the other
classes of Arthropoda we have more or less complete embryological
evidence on the subject. It appears from observation of the embryo
that whilst the first prosthomere of Centipedes has its appendages
reduced and represented only by eye-patches (as in Arachnida,
Crustacea and Hexapoda). the second has a rudimentary antenna, which
disappears, whilst the third carries the permanent antennae, which
accordingly correspond to the second antennae of Crustacea, and are
absent in Hexapoda.

ARTHUR (Fr. _Artus_), the central hero of the cycle of romance known as the _Matiere de Bretagne_ (see ARTHURIAN LEGEND). Whether there was an historic Arthur has been much debated; undoubtedly for many centuries after the appearance of Geoffrey of Monmouth's _Historia Britonum_ (circ. 1136), the statements therein recorded of a mighty monarch, who ruled over Britain in the 5th-6th centuries, and carried his conquests far afield, even to the gates of Rome, obtained general, though not universal, credence. Even in the 12th century there were some who detected, and derided, the fictitious character of Geoffrey's "History." As was naturally to be expected, the pendulum swung to the other extreme, and in a more critical age the existence of Arthur was roundly denied. The truth probably lies midway between the two. The words of Wace, the Norman poet who translated the _Historia_ into verse, are here admirably to the point. Speaking of the tales told of Arthur, he says:--

"Ne tot mencunge, ne tot veir,
Ne tot fable, ne tot saveir,
Tant ont li conteor conte,
Et li fableor tant fable
Por lor contes embeleter
Que tout ont fait fable sembler."[1]

The opinion now generally accepted by scholars is that the evidence of Nennius, whose _Historia Britonum_ preceded that of Geoffrey by some 400 years, is in the main to be relied on. He tells us that Arthur was _Dux bellorum_, and led the armies of the British kings against the Saxon invaders, whom he defeated in twelve great battles. _Tunc Arthur pugnabat cum regibus Britonum, sed ipse dux erat bettorum._

The traditional site of these battles covers a very wide area, and it is supposed that Arthur held a post analogous to that of the general who, under the Roman occupation, was known as _Comes Britanniae_, and held a roving commission to defend the island wherever attacked, in contradistinction to the _Dux Britanniarum_, who had charge of the forces in the north, and the _Comes Littoris Saxonici_, whose task it was to defend the south-east line. The Welsh texts never call Arthur _gwledig_ (prince), but _amheradawr_ (Latin _imperator_) or emperor, a title which would be bestowed on the highest official in the island. The truth thus appears to be that, while there was never a _King_ Arthur, there was a noted chieftain and general of that name. If we say that he carried on a successful war against the Saxons, was probably betrayed by his wife and a near kinsman, and fell in battle, we have stated all which can be claimed as an historical nucleus for his legend. It is now generally admitted that the representation of Arthur as world conqueror, _Welt-Kaiser_, is due to the influence of the Charlemagne cycle. In the 12th century the _Matiere de France_ was waning, the _Matiere de Bretagne_ waxing in popularity, and public opinion demanded that the central figure of the younger cycle (for whatever the date of the subject matter, as a literary cycle the Arthurian is the younger) should not be inferior in dignity and importance to that of the earlier. When we add to this the fact that the writers of the 12th century represented the personages and events of the 6th in the garb, and under the conditions, of their own time, we can understand the reason of the manifold difficulties which beset the study of the cycle.

But into the figure of Arthur as we know him, other elements have entered; he is not merely an historic personality, but at the same time a survival of pre-historic myth, a hero of romance, and a fairy king; and all these threads are woven together in one fascinating but bewildering web. It is only possible here to summarize the leading features which may be claimed as characteristic of each phase.

_Mythic._--Certain elements of the story point to Arthur as a culture hero; as such his name has been identified with the _Mercurius Artaius_ of the Gauls. In this role he slays monsters, the boar Twrch Trwyth, the giant of Mont St Michel and the Demon Cat of Losanne (Andre de Coutances tells us that Arthur was really vanquished and carried off by the Cat, but that one durst not tell that tale before Britons!). He never, it should be noted, rides on purely chivalric ventures, such as aiding distressed damsels, seeking the Grail, &c. His expeditions are all more or less warlike. The story of his youth belongs, as Alfred Nutt (_Folk-lore_, vol. iv.) has shown, to the group of tales classified as the _Aryan Expulsion and Return_ formula, found in all Aryan lands. Numerous parallels exist between the Arthurian and early Irish heroic cycles, notably the Fenian or Ossianic. This Fenian cycle is very closely connected with the Tuatha de Danaan, the Celtic deities of vegetation and increase; recent research has shown that two notable features of the Arthurian story, the Round Table and the Grail, can be most reasonably accounted for as survivals of this Nature worship, and were probably parts of the legend from the first.

_Romantic._--The character of Arthur as a romantic hero is, in reality, very different from that which, mainly through the popularity of Tennyson's _Idylls_, English people are wont to suppose. In the earlier poems he is practically a lay figure, his court the point of departure and return for the knights whose adventures are related in detail, but he himself a passive spectator. In the prose romances he is a monarch, the splendour of whose court, whose riches and generosity, are the admiration of all; but morally he is no whit different from the knights who surround him; he takes advantage of his _bonnes fortunes_ as do others. He has two sons, neither of them born in wedlock; one, Modred, is alike his son and his nephew. In certain romances, the _Perlesvaus_ and _Diu Crone_, he is a veritable _roi faineant_, overcome by sloth and luxury. Certain traits of his story appear to show the influence of Northern romance. Such is the story of his begetting, where Uther takes upon him the form of Gorlois to deceive Yguerne, even as Siegfried changed shapes with Gunther to the undoing of Brunnhilde. The sword in the perron (stone pillar or block), the withdrawal of which proves his right to the kingdom, is the sword of the Branstock. Morgain carries him off, mortally wounded, to Avalon, even as the Valkyr bears the Northern hero to Valhal. Morgain herself has many traits in common with the Valkyrie; she is one of nine sisters, she can fly through the air as a bird (Swan maiden); she possesses a marvellous ointment (as does Hilde, the typical Valkyr). The idea of a slumbering hero who shall awake at the hour of his country's greatest need is world-wide, but the most famous instances are Northern, e.g. Olger Danske and Barbarossa, and depend ultimately on an identification with the gods of the Northern Pantheon, notably Thor. W. Larminie cited an instance of a rhyme current in the Orkneys as a charm against nightmare, which confuses Arthur with Siegfried and his winning of the Valkyr.

_Fairy._--We find that at Arthur's birth (according to Layamon, who here differs from Wace), three ladies appeared and prophesied his future greatness. This incident is also found in the first continuation to the _Perceval_, where the prediction is due to a lady met with beside a forest spring, clearly here a water fairy. In the late romance of _La Bataille de Loquifer_ Avalon has become a purely fairy kingdom, where Arthur rules in conjunction with Morgain. In _Huon de Bordeaux_ he is Oberon's heir and successor, while in the romance of _Brun de la Montagne_, preserved in a unique MS. of the Bibliotheque Nationale, we have the curious statement that all fairy-haunted places, wherever found, belong to Arthur:--

"Et touz ces lieux faes
Sont Artus de Bretagne."

This brief summary of the leading features of the Arthurian tradition will indicate with what confused and complex material we are here dealing. (See also ARTHURIAN LEGEND, GRAIL, MERLIN, ROUND TABLE; and CELT: _Celtic literature_.)

_Texts_. Historic:--Nennius, _Historia Britonum_; H. Zimmer, _Nennius
Vindicatus_ (Berlin, 1893), an examination into the credibility of
Nennius; Geoffrey of Monmouth, _Historia Britonum_ (translations of
both histories are in Bohn's Library); Wace, the _Brut_ (ed. by Leroux
de Lincey); Layamon (ed. by Sir Fred. Madden).

Romantic:--_Merlin_--alike in the Ordinary, or Vulgate (ed. Sommer),
the _Suite_ or "Huth" _Merlin_, the 13th century _Merlin_ (ed. by G.
Paris and J. Ulrich), and the unpublished and unique version of _Bibl.
nat. fonds francais_, 337 (cf. Freymond's analysis in _Zeitschrift fur
franz. Sprache_, xxii.)--devotes considerable space to the elaboration
of the material supplied by the chronicles, the beginning of Arthur's
reign, his marriage and wars with the Saxons. The imitation of the
Charlemagne romances is here evident; the Saxons bear names of Saracen
origin, and camels and elephants appear on the scene. The _Morte
Arthur_, or _Mort au roi Artus_, a metrical romance, of which a unique
English version exists in the Thornton collection (ed. for Early
English Text Society), gives an expanded account of the passing of
Arthur; in the French prose form it is now always found incorporated
with the _Lancelot_, of which it forms the concluding section. The
remains of the Welsh tradition are to be found in the _Mabinogion_
(cf. Nutt's edition, where the stories are correctly classified), and
in the Triads. Professor Rhys' _Studies in the Arthurian Legend_ are
largely based on Welsh material, and may be consulted for details,
though the conclusions drawn are not in harmony with recent research.
These are the only texts in which Arthur is the central figure; in the
great bulk of the romances his is but a subordinate role.
(J. L. W.)

FOOTNOTE:

[1] Nor all a lie, nor all true, nor all fable, nor all known, so
much have the story-tellers told, and the fablers fabled, in order to
embellish their tales, that they have made all seem fable.

ARTHUR I. (1187-1203), duke of Brittany, was the posthumous son of Geoffrey, the fourth son of Henry II. of England, and Constance, heiress of Conan IV., duke of Brittany. The Bretons hoped that their young prince would uphold their independence, which was threatened by the English. Henry II. tried to seize Brittany, and in 1187 forced Constance to marry one of his favourites, Randulph de Blundevill, earl of Chester (d. 1232). Henry, however, died soon afterwards (1189). The new king of England, Richard Coeur de Lion, claimed the guardianship of the young Arthur, but in 1190 Richard left for the Crusade. Constance profited by his absence by governing the duchy, and in 1194 she had Arthur proclaimed duke of Brittany by an assembly of barons and bishops. Richard invaded Brittany in 1196, but was defeated in 1197 and became reconciled to Constance. On his death in 1189, the nobles of Anjou, Maine and Touraine refused to recognize John of England, and did homage to Arthur, who declared himself the vassal of Philip Augustus. In 1202 war was resumed between the king of England and the king of France. The king of France recognized Arthur's right to Brittany, Anjou, Maine and Poitou. While Philip Augustus was invading Normandy, Arthur tried to seize Poitou. But, surprised at Mirebeau, he fell into the hands of John, who sent him prisoner to Falaise. In the following year he was transferred to Rouen, and disappeared suddenly. It is thought that John killed him with his own hand. After this murder John was condemned by the court of peers of France, and stripped of the fiefs which he possessed in France.

See Ralph of Coggeshall, "Chronicon Anglicanum," in the _Monumenta
Britanniae historica_; Dom Lobineau, _Histoire de Bretagne_ (1702);
Dom Morice, _Histoire de Bretagne_ (1742-1756); A. de la Borderie,
_Histoire de Bretagne_, vol. iii. (1899); Bemont, "De la condamnation
de Jean-sans-Terre par la Cour des Pairs de France," in the _Revue
historique_ (1886), vol. xxxii.

ARTHUR III. (1393-1458), earl of Richmond, constable of France, and afterwards duke of Brittany, was the third son of John IV., duke of Brittany, and Joan of Navarre, afterwards the wife of Henry IV. of England. His brother, John V., gave him his earldom of Richmond in England. While still very young, he took part in the civil wars which desolated France during the reign of Charles VI. From 1410 to 1414 he served on the side of the Armagnacs, and afterwards entered the service of Louis the dauphin, whose intimate friend he became. He profited by his position at court to obtain the lieutenancy of the Bastille, the governorship of the duchy of Nemours, and the confiscated territories of Jean Larcheveque, seigneur of Parthenay. His efforts to reduce the latter were, however, interrupted by the necessity of marching against the English. At Agincourt he was wounded and captured, and remained a prisoner in England from 1415 to 1420. Released on parole, he gained the favour of King Henry V. by persuading his brother, the duke of Brittany, to conclude the treaty of Troyes, by which France was handed over to the English king. He was rewarded with the countship of Ivry.

In 1423 Arthur married Margaret of Burgundy, widow of the dauphin Louis, and became thus the brother-in-law of Philip the Good of Burgundy, and of the regent, the duke of Bedford. Offended, however, by Bedford's refusal to give him a high command, he severed his connexion with the English, and in March 1425 accepted the constable's sword from King Charles VII. He now threw himself with ardour into the French cause, and persuaded his brother, John V. of Brittany, to conclude with Charles VII. the treaty of Saumur (October 7, 1425). But though he saw clearly enough the measures necessary for success, he lacked the means to carry them out. In the field he met with a whole series of reverses; and at court, where his rough and overbearing manners made him disliked, his influence was overshadowed by that of a series of incompetent favourites. The peace concluded between the duke of Brittany and the English in September 1427 led to his expulsion from the court, where Georges de la Tremoille, whom he himself had recommended to the king, remained supreme for six years, during which Richmond tried in vain to overthrow him. In the meantime, in June 1429, he joined Joan of Arc at Orleans, and fought in several battles under her banner, till the influence of La Tremoille forced his withdrawal from the army. On the 5th of March 1432 Charles VII. concluded with him and with Brittany the treaty of Rennes; but it was not until June of the following year that La Tremoille was overthrown. Arthur now resumed the war against the English, and at the same time took vigorous measures against the plundering bands of soldiers and peasants known as _routiers_ or _ecorcheurs_. On the 20th of September 1435, mainly as a result of his diplomacy, was signed the treaty of Arras between Charles VII. and the duke of Burgundy, to which France owed her salvation.

On the 13th of April 1436, Arthur took Paris from the English; but he was ill seconded by the king, and hampered by the necessity for leading frequent expeditions against the _ecorcheurs_; it was not till May 1444 that the armistice of Tours gave him leisure to carry out the reorganization of the army which he had long projected. He now created the _compagnies d'ordonnance_, and endeavoured to organize the militia of the _francs archers_. This reform had its effect in the struggles that followed. In alliance with his nephew, the duke of Brittany, he reconquered, during September and October 1449, nearly all the Cotentin; on the 15th of April 1450 he gained over the English the battle of Formigny; and during the year he recovered for France the whole of Normandy, which for the next six or seven years it was his task to defend from English attacks. On the death of his nephew Peter II., on the 22nd of September 1457, he became duke of Brittany, and though retaining his office of constable of France, he refused, like his predecessors, to do homage to the French king for his duchy. He reigned little more than a year, dying on the 26th of December 1458, and was succeeded by his nephew Francis II., son of his brother Richard, count of Etampes.

Arthur was three times married: (1) to Margaret of Burgundy, duchess of Guienne (d. 1442); (2) to Jeanne d'Albret, daughter of Charles II. of Albret (d. 1444); (3) to Catherine of Luxemburg, daughter of Peter of Luxemburg, count of St Pol, who survived him. He left no legitimate children.

AUTHORITIES.--The main source for the life of Duke Arthur III. is the
chronicle of Guillaume Gruel (c. 1410-1474-1482). Gruel entered the
service of the earl of Richmond about 1425, shared in all his
campaigns, and lived with him on intimate terms. The chronicle covers
the whole period of the duke's life, but the earlier part, up to 1425,
is much less full and important than the later, which is based on
Gruel's personal knowledge and observation. In spite of a perhaps
exaggerated admiration for his hero, Gruel displays in his work so
much good faith, insight and originality that he is accepted as a
thoroughly trustworthy authority. It was first published at Paris in
1622. Of the numerous later editions, the best is that of Achille le
Vavasseur, _Chronique d'Arthur de Richemont_ (Paris, 1890). See also
E. Cosneau, _Le Connetable de Richemont_ (Paris, 1886); G. du Fresne
de Beaucourt, _Histoire de Charles VII._ (Paris, 1881, seq.).