Chapter XXII: Act 1737: , and the Apportionment Act 1834, and is now allowed generally (13)

The Trilobites are known only as fossils, mostly Silurian and
prae-Silurian; a few are found in Carboniferous and Permian strata. As
many as two thousand species are known. Genera with small metasomatic
carapace, consisting of three to six fused segments distinctly marked
though not separated by soft membrane, are _Harpes_, _Paradoxides_ and
_Triarthrus_ (fig. 34). In _Calymene_, _Homalonotus_ and _Phacops_
(fig. 38) from six to sixteen segments are clearly marked by ridges
and grooves in the metasomatic tagma, whilst in _Illaenus_ the shield
so formed is large but no somites are marked out on its surface. In
this genus ten free somites (mesosoma) occur between the prosomatic
and metasomatic carapaces. _Asaphus_ and _Megalaspis_ (fig. 39) are
similarly constituted. In _Agnostus_ (fig. 40) the anterior and
posterior carapaces constitute almost the entire body, the two
carapaces being connected by a mid-region of only two free somites. It
has been held that the forms with a small number of somites marked in
the posterior carapace and numerous free somites between the anterior
and posterior carapace, must be considered as anterior to those in
which a great number of posterior somites are traceable in the
metasomatic carapace, and that those in which the traces of distinct
somites in the posterior or metasomatic carapace are most completely
absent must be regarded as derived from those in which somites are
well marked in the posterior carapace and similar in appearance to
the free somites. The genus _Agnostus_, which belongs to the last
category, occurs abundantly in Cambrian strata and is one of the
earliest forms known. This would lead to the supposition that the
great development of metasomatic carapace is a primitive and not a
late character, were it not for the fact that _Paradoxides_ and
_Atops_, with an inconspicuous telsonic carapace and numerous free
somites, are also Cambrian in age, the latter indeed anterior in
horizon to _Agnostus_.

(After Beecher, from Zittel.) ]

On the other hand, it may well be doubted whether the pygidial or
posterior carapace is primarily due to a fusion of the tergites of
somites which were previously movable and well developed. The
posterior carapace of the Trilobites and of _Limulus_ is probably
enough in origin a telsonic carapace--that is to say, is the tergum of
the last segment of the body which carries the anus. From the front of
this region new segments are produced in the first instance, and are
added during growth to the existing series. This telson may enlarge,
it may possibly even become internally and sternally developed as
partially separate somites, and the tergum may remain without trace of
somite formation, or, as appears to be the case in _Limulus_, the
telson gives rise to a few well-marked somites (mesosoma and two
others) and then enlarges without further trace of segmentation,
whilst the chitinous integument which develops in increasing thickness
on the terga as growth advances welds together the unsegmented telson
and the somites in front of it, which were previously marked by
separate tergal thickenings. It must always be remembered that we are
liable (especially in the case of fossilized integuments) to attach an
unwarranted interpretation to the mere discontinuity or continuity of
the thickened plates of chitinous cuticle on the back of an Arthropod.
These plates may fuse, and yet the somites to which they belong may
remain distinct, and each have its pair of appendages well developed.
On the other hand, an unusually large tergal plate, whether terminal
or in the series, is not always due to fusion of the dorsal plates of
once-separate somites, but is often a case of growth and enlargement
of a single somite without formation of any trace of a new somite. For
the literature of Trilobites see (22*).

(After Beecher.)]

(After Beecher.)]

(From Zittel's Palaeontology.)]

(From Zittel.)]

(From Zittel.)]

_Grade B (of the Aracknida) NOMOMERISTICA._--Arachnida in which,
excluding from consideration the eye-bearing prosthomere, the somites
are primarily (that is to say, in the common ancestor of the grade)
grouped in three regions of six--(a) the "prosoma" with palpiform
appendages, (b) the "mesosoma" with plate-like appendages, and (c) the
"metasoma" with suppressed appendages. A somite placed between the
prosoma and mesosoma --the prae-genital somite--appears to have
belonged originally to the prosomatic series (which with its ocular
prosthomere and palpiform limbs [Pantopoda], would thus consist of
eight somites), but to have been gradually reduced. In living
Arachnids, excepting the Pantopoda, it is either fused (with loss of
its appendages) with the prosoma (_Limulus_,[7] _Scorpio_), after
embryonic appearance, or is retained as a rudimentary, separate,
detached somite in front of the mesosoma, or disappears altogether
(excalation). The atrophy and total disappearance of ancestrally
well-marked somites frequently take place (as in all Arthropoda) at
the posterior extremity of the body, whilst excalation of somites may
occur at the constricted areas which often separate adjacent
"regions," though there are very few instances in which it has been
recognized. Concentration of the organ-systems by fusion of
neighbouring regions (prosoma, mesosoma, metasoma), previously
distinct, has frequently occurred, together with obliteration of the
muscular and chitinous structures indicative of distinct somites. This
concentration and obliteration of somites, often accompanied by
dislocation of important segmental structures (such as appendages and
nerve-ganglia), may lead to highly developed specialization
(individuation, H. Spencer), as in the Araneae and Opiliones, and, on
the other hand, may terminate in simplification and degeneration, as
in the Acari.

(From Korschelt and Heider.)]

From Korschelt and Heider, after Barrande.

A, Youngest stage.
B, Older stage with distinct pygidial carapace.
C, Stage with two free mesosomatic somites between the prosomatic
and telsonic carapaces.
D, Stace with seven free intermediate somites.
E, Stave with twelve free somites; the telsonic carapace has not
increased in size.
a, Lateral eye.
g, So-called facial "suture" (not really a suture).
p, Telsonic carapace.]

The most important general change which has affected the structure of
the nomomeristic Arachnida in the course of their historic development
is the transition from an aquatic to a terrestrial life. This has been
accompanied by the conversion of the lamelliform gill-plates into
lamelliform lung-plates, and later the development from the
lung-chambers, and at independent sites, of tracheae or air-tubes (by
adaptation of the vasifactive tissue of the blood-vessels) similar to
those independently developed in _Peripatus_, Diplopoda, Hexapoda and
Chilopoda. Probably tracheae have developed independently by the same
process in several groups of tracheate Arachnids. The nomomeristic
Arachnids comprise two sub-classes--one a very small degenerate
offshoot from early ancestors; the other, the great bulk of the class.

(from Korschelt and Heider, after Leuckart.)]

Sub-Class I. (of the Nomomeristica). PANTOPODA.--Nomomeristic
Arachnids, in which the somites corresponding to mesosoma and metasoma
have entirely aborted. The seventh, and sometimes the eighth,
leg-bearing somite is present and has its leg-like appendages fully
developed. Monomeniscous eyes with a double (really triple) cell-layer
formed by invagination, as in the Eu-arachnida, are present The
Pantopoda stand in the same relation to _Limulus_ and _Scorpio_ that
_Cyamus_ holds to the thoracostracous Crustacea. The reduction of the
organism to seven leg-bearing somites, of which the first pair, as in
so many Eu-arachnida, are chelate, is a form of degeneration connected
with a peculiar quasi-parasitic habit resembling that of the
crustacean Laemodipoda. The genital pores are situate at the base of
the 7th pair of limbs, and may be repeated on the 4th, 5th, and 6th.
In all known Pantopoda the size of the body is quite minute as
compared with that of the limbs: the alimentary canal sends a long
caecum into each leg (cf. the Araneae) and the genital products are
developed in gonocoels also placed in the legs.

From Parker and Harwell's _Text-book of Zoology_, after Hoek.]

The Pantopoda are divided into three orders, the characters of which
are dependent on variation in the presence of the full number of legs.

Order 1. (of the Pantopoda). Nymphonomorpha, Pocock (nov.) (fig.
43).--In primitive forms belonging to the family _Nymphonidae_ the
full complement of appendages is retained--the 1st (mandibular), the
2nd (palpiform), and the 3rd (ovigerous) pairs being well developed in
both sexes. In certain derivative forms constituting the family
_Pallenidae_, however, the appendages of the 2nd pair are either
rudimentary or atrophied altogether.

Two families: 1. Nymphonidae (genus _Nymphon_), and 2. Pallenidae
(genus _Pallene_).

Order 2. Ascorhynchomorpha, Pocock (nov.).--Appendages of the 2nd and
3rd pairs retained and developed, as in the more primitive types of
Nymphonomorpha; but those of the 1st pair are either rudimentary, as
in the _Ascorhynchidae_, or atrophied, as in the _Colossendeidae_. In
the latter a further specialization is shown in the fusion of the body
segments.

Two families. 1. Ascorhynchidae (genera _Ascorhynchus_ and
_Ammothea_); 2. Colossendeidae (genera _Colossendeis_ and
_Discoarachne_).

Order 3. Pycnogonomorpha, Pocock (nov.).--Derivative forms in which
the reduction in number of the anterior appendages is carried farther
than in the other orders, reaching its extreme in the _Pycnogonidae_,
where the 1st and 2nd pairs are absent in both sexes, and the 3rd pair
also are absent in the female. In the _Hannoniidae_, however, which
resemble the _Pycnogonidae_ in the absence of the 3rd pair in the
female and of the 2nd pair in both sexes, the 1st pair are retained in
both sexes.

Two families: 1. Hannoniidae (genus _Hannonia_); 2. Pycnogonidae
(genera _Pycnogonum_ and _Phoxichilus_).

_Remarks._--The Pantopoda are not known in the fossil condition. They
are entirely marine, and are not uncommon in the coralline zone of the
sea-coast. The species are few, not more than fifty (23). Some large
species of peculiar genera are taken at great depths. Their movements
are extremely sluggish. They are especially remarkable for the small
size of the body and the extension of viscera into the legs. Their
structure is eminently that of degenerate forms. Many frequent growths
of coralline Algae and hydroid polyps, upon the juices of which they
feed, and in some cases a species of gall is produced in hydroids by
the penetration of the larval Pantopod into the tissues of the polyp.

Sub-Class II. (of the Nomomeristic Arachnida). EU-ARACHNIDA.--These
start from highly developed and specialized aquatic branchiferous
forms, exhibiting a prosoma with six pediform pairs of appendages, an
intermediate prae-genital somite, a mesosoma of six somites bearing
lamelliform pairs of appendages, and a metasoma of six somites devoid
of appendages, and the last provided with a post-anal spine. Median
eyes are present, which are monomeniscous, with distinct retinal and
corneagenous cell-layers, and placed centrally on the prosoma. Lateral
eyes also may be present, arranged in lateral groups, and having a
single or double cell-layer beneath the lens. The first pair of limbs
is often chelate or prehensile, rarely antenniform; whilst the second,
third and fourth may also be chelate, or may be simple palps or
walking legs.

An internal skeletal plate, the so-called "entosternite" of
fibro-cartilaginous tissue, to which many muscles are attached, is
placed between the nerve-cords and the alimentary tract in the prosoma
of the larger forms (_Limulus_, _Scorpio_, _Mygale_). In the same and
other leading forms a pair of much-coiled glandular tubes, the coxal
glands (coelomocoels in origin), is found with a duct opening on the
coxa of the fifth pair of appendages of the prosoma. The vascular
system is highly developed (in the non-degenerate forms); large
arterial branches closely accompany or envelop the chief nerves;
capillaries are well developed. The blood-corpuscles are large
amoebiform cells, and the blood-plasma is coloured blue by
haemocyanin.

The alimentary canal is uncoiled and cylindrical, and gives rise
laterally to large gastric glands, which are more than a single pair
in number (two to six pairs), and may assume the form of simple caeca.
The mouth is minute and the pharynx is always suctorial, never
gizzard-like. The gonadial tubes (gonocoels or gonadial coelom) are
originally reticular and paired, though they may be reduced to a
simpler condition. They open on the first somite of the mesosoma. In
the numerous degenerate forms simplification occurs by obliteration of
the demarcations of somites and the fusion of body-regions, together
with a gradual suppression of the lamelliferous respiratory organs and
the substitution for them of tracheae, which, in their turn, in the
smaller and most reduced members of the group, may also disappear.

The Eu-arachnida are divided into two grades with reference to the
condition of the respiratory organs as adapted to aquatic or
terrestrial life.

Grade a (of the Eu-arachnida). DELOBRANCHIA (Hydropheustea).

Mesosomatic segments furnished with large plate-like appendages, the
1st pair acting as the genital operculum, the remaining pairs being
provided with branchial lamellae fitted for breathing oxygen dissolved
in water. The prae-genital somite partially or wholly obliterated in
the adult. The mouth lying far back, so that the basal segments of all
the prosomatic appendages, excepting those of the 1st pair, are
capable of acting as masticatory organs. Lateral eyes consisting of a
densely packed group of eye-units ("compound" eyes).

Order 1. Xiphosura.--The prae-genital somite fuses in the embryo with
the prosoma and disappears (see fig. 19). Not free-swimming, none of
the prosomatic appendages modified to act as paddles; segments of the
mesosoma and metasoma (=opisthosoma) not more than ten in number,
distinct or coalesced.

Family--Limulidae (_Limulus_).
" *Belinuridae (_Belinurus_, _Aglaspis_, _Prestwichia_).
" *Hemiaspidae (_Hemiaspis_, _Bunodes_).

(From Parker and Haswell, _Text book of Zoology_ after Leuckart.)]

_Remarks_.--The Xiphosura are marine in habit, frequenting the shore.
They are represented at the present day by the single genus _Limulus_
(figs. 44 and 45; also figs. 7, 9, 11, to 15 and 20), often termed the
king-crab, which occurs on the American coast of the Atlantic Ocean,
but not on its eastern coasts, and on the Asiatic coast of the
Pacific. The Atlantic species (_L. polyphemus_) is common on the
coasts of the United States, and is known as the king-crab or
horse-shoe crab. A single specimen was found in the harbour of
Copenhagen in the 18th century, having presumably been carried over by
a ship to which it clung.

A species of _Limulus_ is found in the Buntersandstein of the Vosges;
_L. Walchi_ is abundant in the Oolitic lithographic slates of Bavaria.

1 to 6, The six prosomatic pairs of appendages.
abd, the solid opisthosomatic carapace.
tels, the post-anal spine (not the telson as the lettering would
seem to imply, but only its post-anal portion).
operc, the fused first pair of mesosomatic appendages forming the
genital operculum.

(From Parker and Haswell, _Text book of Zoology_, after Leuckart.)]

The genera _Belinurus_, _Aglaspis_, _Prestwichia_, _Hemiaspis_ and
_Bunodes_ consist of small forms which occur in Palaeozoic rocks. In
none of them are the appendages known, but in the form of the two
carapaces and the presence of free somites they are distinctly
intermediate between _Limulus_ and the Trilobitae. The young form of
_Limulus_ itself (fig. 40) is also similar to a Trilobite so far as
its segmentation and trilobation are concerned. The lateral eyes of
_Limulus_ appear to be identical in structure and position with those
of certain Trilobitae.

(From Zittel's _Text-book of Palaeontology_, The Macmillan Co, New
York, 1896.)]

Order 2. Gigantostraca (figs. 46, 47).--Free-swimming forms, with the
appendages of the 6th or 5th and 6th pairs flattened or lengthened to
act as oars; segments of mesosoma and metasoma (= opisthosoma), twelve
in number.

Appendages of anterior pair very large and chelate.

Sub-order Pterygotomorpha, Pterygotidae (_Pterygotus_).

Appendages of anterior pair minute and chelate.

/ Stylonuridae (_Stylonurus_).
Sub-order Eurypteromorpha < Eurypteridae (_Eurypterus_,
\ _Slimonia_).

FIG. 47.--_Pterygotus osiliensis_, Schmidt. Silurian of Rootzikil.
Restoration of the ventral surface, about a third natural size, after
Schmidt.

a, Camerostome or epistoma.
m, Chilarium or metasternite of the prosoma (so-called metastoma).
oc, The compound eyes.
1 to 8, Segments of the sixth prosomatic appendage.
I' to V', First five opisthosomatic somites.
7', Sixth opisthosomatic somite.

[Observe the powerful gnathobases of the sixth pair of prosomatic
limbs and the median plates behind m. The dotted line on somite I
indicates the position of the genital operculum which was probably
provided with branchial lamellae.]]

_Remarks._--The Gigantostraca are frequently spoken of as "the
Eurypterines." Not more than thirty species are known. They became
extinct in Palaeozoic times, and are chiefly found in the Upper
Silurian, though extending upwards as far as the Carboniferous. They
may be regarded as "macrourous" Xiphosura; that is to say, Xiphosura
in which the nomomeristic number of eighteen well-developed somites is
present and the posterior ones form a long tail-like region of the
body. There still appears to be some doubt whether in the sub-order
Eurypteromorpha the first pair of prosomatic appendages (fig. 46) is
atrophied, or whether, if present, it has the form of a pair of
tactile palps or of minute chelae. Though there are indications of
lamelliform respiratory appendages on mesosomatic somites following
that bearing the genital operculum, we cannot be said to have any
proper knowledge as to such appendages, and further evidence with
regard to them is much to be desired. (For literature see Zittel,
22*.)

Grade b (of the Eu-arachnida). EMBOLOBRANCHIA (Aeropneustea).

In primitive forms the respiratory lamellae of the appendages of the
3rd, 4th, 5th and 6th, or of the 1st and 2nd mesosomatic somites are
sunk beneath the surface of the body, and become adapted to breathe
atmospheric oxygen, forming the leaves of the so-called lung-books. In
specialized forms these pulmonary sacs are wholly or partly replaced
by tracheal tubes. The appendages of the mesosoma generally
suppressed; in the more primitive forms one or two pairs may be
retained as organs subservient to reproduction or silk-spinning. Mouth
situated more forwards than in Delobranchia, no share in mastication
being taken by the basal segments of the 5th and 6th pairs of
prosomatic appendages. Lateral eyes, when present, represented by
separate ocelli.

The prae-genital somite, after appearing in the embryo, either is
obliterated (_Scorpio, Galeodes, Opilio_and others) or is retained as
a reduced narrow region of the body, the "waist," between prosoma and
mesosoma. It is represented by a full-sized tergal plate in the
Pseudo-scorpiones.

Section [alpha]. _Pectinifera._--The primitive distinction between the
mesosoma and the metasoma retained, the latter consisting of six
somites and the former of six somites in the adult, each of which is
furnished during growth with a pair of appendages. Including the
prae-genital somite (fig. 16), which is suppressed in the adult, there
are thirteen somites behind the prosoma. The appendages of the 1st and
2nd mesosomatic somites persisting as the genital operculum and
pectones respectively, those of the 3rd, 4th, 5th and 6th somites (?
in _Palaeophonus_) sinking below the surface during growth in
connexion with the formation of the four pairs of pulmonary sacs (see
fig. 17). Lateral eyes monostichous.

FIG. 48.--Dorsal view of a restoration of _Palaeophonus nuncius_,
Thorell. The Silurian scorpion from Gothland.]

Order 1. Scorpiones.--Prosoma covered by a single dorsal shield,
bearing typically median and lateral eyes; its sternal elements
reduced to a single plate lodged between or behind the basal segments
of the 5th and 6th pairs of appendages. Appendages of 1st pair
tri-segmented, chelate; of 2nd pair chelate, with their basal segments
subserving mastication; of 3rd, 4th, 5th and 6th pairs similar in form
and function, except that in recent and Carboniferous forms the basal
segments of the 3rd and 4th are provided with sterno-coxal (maxillary)
lobes, those of the 4th pair meeting in the middle line and underlying
the mouth. The five posterior somites of the metasoma constricted to
form a "tail," the post-anal sclerite persisting as a weapon of
offence and provided with a pair of poison glands (see figs. 8, 10,
12, 13, 14, 15, 21 and 22).

Sub-order Apoxypoda.--The 3rd, 4th, 5th and 6th pairs of appendages
short, stout, tapering, the segments about as wide as long, except the
apical, which is distally slender, pointed, slightly curved, and
without distinct movable claws.

Family--Palaeophonidae, _Palaeophonus_ (figs. 48 and 49).

Sub-order Dionychopoda.--The 3rd, 4th, 5th and 6th pairs of appendages
slender, not evenly tapering, the segments longer than wide; the
apical segment short, distally truncate, and provided with a pair of
movable claws. Basal segments of the 5th and 6th pairs of appendages
abutting against the sternum of the prosoma (see fig. 10 and figs. 51,
52 and 53).

Family--Pandinidae (_Pandinus, Opisthophthalmus, Urodacus_).
" Vejovidae (_Vaejovis, Jurus, Euscorpius, Broteas_).
" Bothriuridae (_Bothriurus, Cercophonius_).
" Buthidae (_Buthus, Centrums_).
" *Cyclophthalmidae (_Cydophthalmus_) ( Carboniferous.
" *Eoscorpiidae (_Eoscorpius, Centromachus_) (

(See Pocock, _Quart Jour. Micr. Sci._, 1901.)]

_Remarks on the Order Scorpiones._--The Scorpion is one of the great
animals of ancient lore and tradition. It and the crab are the only
two invertebrates which had impressed the minds of early men
sufficiently to be raised to the dignity of astronomical
representation. It is all the more remarkable that the scorpion proves
to be the oldest animal form of high elaboration which has persisted
to the present day. In the Upper Silurian two specimens of a scorpion
have been found (figs. 48, 49), one in Gothland and one in Scotland,
which would be recognized at once as true scorpions by a child or a
savage. The Silurian scorpion _Palaeophonus_, differs, so far as
obvious points are concerned, from a modern scorpion only in the
thickness of its legs and in their terminating in strong spike-like
joints, instead of being slight and provided with a pair of terminal
claws. The legs of the modern scorpion (fig. 10; fig. 51) are those of
a terrestrial Arthropod, such as a beetle; whilst those of the
Silurian scorpion are the legs of an aquatic Arthropod, such as a crab
or lobster. It is probable that the Silurian scorpion was an aquatic
animal, and that its respiratory lamellae were still projecting from
the surface of the body to serve as branchiae. No trace of "stigmata,"
the orifices of the lung-chambers of modern scorpions, can be found in
the Scottish specimen of _Palaeophonus_, which presents the ventral
surface of the animal to view. On the other hand, no trace of
respiratory appendages excepting the pectens can be detected in the
specimen (see fig. 49).

(After Pocock, _Q. J. Mic. Sci._, 1901.)]

FIG. 51.--Drawing from life of the desert scorpion, _Buthusaustralis_,
Lin., from Biskra, N. Africa.]

Fossil scorpions of the modern type are found in the Coal Measures. At
the present day scorpions of various genera are found in all the warm
regions of the world. In Europe they occur as far north as Bavaria and
the south of France. The largest species measure 9 in. from the front
of the head to the end of the sting, and occur in tropical India and
Africa. Between 200 and 300 species are known. The scorpions use their
large chelae for seizing prey and for fighting with one another. They
never use the sting when (as frequently happens) they attack another
scorpion, because, as was ascertained by A.G. Bourne (24), the poison
exuded by the sting has no injurious effect on another scorpion nor on
the scorpion itself. The stories of a scorpion stinging itself to
death when placed in a circle of burning coals are due to erroneous
observation. When placed in such a position the scorpion faints and
becomes inert. It is found (Bourne, 24) that some species of scorpion
faint at a temperature of 40 deg. Cent. They recover on being removed
to cooler conditions. A scorpion having seized its prey (usually a
large insect, or small reptile or mammal) with the large chelae brings
its tail over its head, and deliberately punctures the struggling
victim twice with its sting (fig. 52). The poison of the sting is
similar to snake-poison (Calmette), and rapidly paralyses animals
which are not immune to it. It is probably only sickly adults or young
children of the human race who can be actually killed by a scorpion's
sting. When the scorpion has paralysed its prey in this way, the two
short chelicerae are brought into play (fig. 53). By the crushing
action of their pincers, and an alternate backward and forward
movement, they bring the soft blood-holding tissues of the victim
close to the minute pin-hole aperture which is the scorpion's mouth.
The muscles acting on the bulb-like pharynx now set up a pumping
action (see Huxley, 26); and the juices--but no solid matter,
excepting such as is reduced to powder--are sucked into the scorpion's
alimentary canal. A scorpion appears to prefer for its food another
scorpion, and will suck out the juices of an individual as large as
itself. When this has taken place, the gorged scorpion becomes
distended and tense in the mesosomatic region. It is certain that the
absorbed juices do not occupy the alimentary canal alone, but pass
also into its caecal off-sets which are the ducts of the gastric
glands (see fig. 33).

FIG. 52.--Drawing from life of the Italian scorpion _Euscorpius
italicus_, Herbst, holding a blue-bottle fly with its left chela, and
carefully piercing it between head and thorax with its sting. Two
insertions of the sting are effected and the fly is instantly
paralysed by the poison so introduced into its body.]

FIG. 53.--The same scorpion carrying the now paralysed fly held in its
chelicerae, the chelae liberated for attack and defence. Drawn from
life.]

All Arachnida, including _Limulus_, feed by suctorial action in
essentially the same way as _Scorpio_.

Scorpions of various species have been observed to make a hissing
noise when disturbed, or even when not disturbed. The sound is
produced by stridulating organs developed on the basal joints of the
limbs, which differ in position and character in different genera (see
Pocock, 27). Scorpions copulate with the ventral surfaces in contact.
The eggs are fertilized, practically in the ovary, and develop _in
situ_. The young are born fully formed and are carried by the mother
on her back. As many as thirty have been counted in a brood. For
information as to the embryology of scorpions, the reader is referred
to the works named in the bibliography below. Scorpions do not possess
spinning organs nor form either snares or nests, so far as is known.
But some species inhabiting sandy deserts form extensive burrows. The
fifth pair of prosomatic appendages is used by these scorpions when
burrowing, to kick back the sand as the burrow is excavated by the
great chelae.

References to works dealing with the taxonomy and geographical
distribution of scorpions are given at the end of this article (28).

Section [beta]. _Epectinata._--The primitive distinction between the
mesosoma and the metasoma wholly or almost wholly obliterated, the two
regions uniting to form an opisthosoma, which never consists of more
than twelve somites and never bears appendages or breathing-organs
behind the 4th somite. The breathing-organs of the opisthosoma, when
present, represented by two pairs of stigmata, opening either upon the
1st and 2nd (Pedipalpi) or the 2nd and 3rd somites (Solifugae,
Pseudo-scorpiones), or by a single pair upon the 3rd (? 2nd) somite
(Opiliones) of the opisthosoma, there being rarely an additional
stigma on the 4th (some Solifugae). The appendages of the 2nd somite
of the opisthosoma absent, rarely minute and bud-like (some
Amblypygi), never pectiniform. A prae-genital somite is often present
either in a reduced condition forming a waist (Pedipalpi, Araneae,
Palpigradi) or as a full-sized tergal plate (Pseudo-scorpiones); in
some it is entirely atrophied (Solifugae, Holosomata, and
Rhynchostomi). Lateral eyes when present diplostichous.

_Remarks._--The Epectinate Arachnids do not stand so close to the
aquatic ancestors of the Embolobranchia as do the Pectiniferous
scorpions. At the same time we are not justified in supposing that the
scorpions stand in any way as an intermediate grade between any of the
existing Epectinata and the Delobranchia. It is probable that the
Pedipalpi, Araneae, and Podogona have been separately evolved as
distinct lines of descent from the ancient aquatic Arachnida. The
Holosomata and Rhynchostomi are probably offshoots from the stem of
the Araneae, and it is not unlikely (in view of the structure of the
prosomatic somites of the Tartarides) that the Solifugae are connected
in origin with the Pedipalpi. The appearance of tracheae in place of
lung-sacs cannot be regarded as a starting-point for a new line of
descent comprising all the tracheate forms; tracheae seem to have
developed independently in different lines of descent. On the whole,
the Epectinata are highly specialized and degenerate forms, though
there are few, if any, animals which surpass the spiders in rapidity
of movement, deadliness of attack and constructive instincts.

FIG. 54.--_Thelyphonus_, one of the Pedipalpi.

A, Ventral view.
I, Chelicera (detached).
II, Chelae.
III, Palpiform limb.
IV to VI, The walking legs.
stc, Sterno-coxal process (gnathobase) of the chelae.
st^1, Anterior sternal plate of the prosoma.
st^2, Posterior sternal plate of the prosoma.
pregen, Position of the prae-genital somite (not seen).
l, l, Position of the two pulmonary sacs of the right side.
1 to 11, Somites of the opisthosoma (mesosoma plus metasoma).
msg, Stigmata of the tergo-sternal muscles.
an, Anus.
B, Dorsal view of the opisthosoma of the same.
pregen, The prae-genital somite.
p, The tergal stigmata of the tergo-sternal muscles.
paf, Post-anal segmented filament corresponding to the post-anal
spine of Limulus.]

Order 2. Pedipalpi (figs. 54 to 59).--Appendages of 1st pair
bisegmented, without poison gland; of 2nd pair prehensile, their basal
segments underlying the proboscis, and furnished with sterno-coxal
(maxillary) process, the apical segment tipped with a single movable
or immovable claw; appendages of 3rd pair different from the
remainder, tactile in function, with at least the apical segment
many-jointed and clawless. The ventral surface of the prosoma bears
prosternal, metasternal and usually mesosternal chitine-plates (fig.
55). A narrow prae-genital somite is present between opisthosoma and
prosoma (figs. 55, 57). Opisthosoma consisting of eleven somites,
almost wholly without visible appendages. Intromittent organ of male
beneath the genital operculum (=sternum of the 1st somite of
opisthosoma).

(From a drawing made by Pickard - Cambridge, under the direction of
R.I. Pocock.)]

_Note._--The possibility of another interpretation of the anterior
somites of the mesosoma and the prae-genital somite must be borne in
mind. Possibly, though not probably, the somites carrying the two
lung-sacs correspond to the first two lung-bearing somites of
_Scorpio_, and it is the genital opening which has shifted. The same
caution applies in the case of the Araneae. Excalation of one or of
two anterior mesosomatic somites, besides the prae-genital somite,
would then have to be supposed to have occurred also.

Sub-order a. Uropygi.--Prosoma longer than wide, its sternal area very
narrow, furnished with a large prosternal and metasternal plate, and
often with a small mesosternal sclerite. Appendages of 2nd pair with
their basal segments united in the middle line and incapable of
lateral movement; appendages of 3rd pair with only the apical segment
many-jointed. Opisthosoma without trace of appendages; its posterior
somites narrowed to form a movable tail for the support of the
post-anal sclerite, which has no poison glands.

. Ventral
surface of the anterior region of the opisthosoma, the first somite
being pushed upwards and forwards so as to expose the subjacent
structures. _opistho_ 1, First somite of the opisthosoma; opistho 2,
second do.; g, genital aperture; l, edges of the lamellae of the
lung-books; m, stigmata of tergo-sternal muscles.

(Original drawing by Pocock.)]

Tribe 1. Urotricha.--Dorsal area of prosoma covered with a single
shield (? two in _Geralinura_), bearing median and lateral eyes.
Post-anal sclerite modified as a long, many-jointed feeler. Appendages
of 2nd pair folding in a horizontal plane, completely chelate, the
claw immovably united to the sixth segment. Respiratory organs present
in the form of pulmonary sacs.

Family--Thelyphonidae (_Thelyphonus_ (fig. 54), _Hypoctonus_,
*_Geralinura_).

Tribe 2. Tartarides.--Small degenerate forms with the dorsal area of
the prosoma furnished with two shields, a larger in front covering the
anterior four somites, and a smaller behind covering the 5th and 6th
somites; the latter generally subdivided into a right and left
portion. There is also a pair of narrow tergal sclerites interposed
between the anterior and posterior shields. Eyes evanescent or absent.
Appendages of 2nd pair folding in a vertical plane, not chelate, the
claw long and movable. Post-anal sclerite short and undivided. No
distinct respiratory stigmata behind the sterna of the 1st and 2nd
somites of the opisthosoma.

Family--Hubbardiidae (_Schizomus_, _Hubbardia_) (figs. 57-59).

a, Prosternum of prosoma.
b, Metasternum of prosoma.
prae-gen, The prae-genital somite.
I opisth, First somite of the opisthosoma.
II opisth, Eleventh somite of the opisthosoma.
pa, Post-anal lobe of the female (compare the jointed filament in
_Thelyphonus_, fig. 54).

(Original drawing by Pickard-Cambridge, directed by Pocock.)]

I to VI. The prosomatic appendages.
a, Anterior plate.
b, Posterior plate of the prosomatic carapace.
prae-gen, Tergum of the prae-genital somite.
11, The eleventh somite of the opisthosoma.
pa, Post-anal lobe of the male--a conical body with narrow basal
stalk.

(Original as preceding.)]

Sub-order b. Amblypygi.--Prosoma wider than long, covered above by a
single shield bearing median and lateral eyes, which have
diplostichous ommatea. Sternal area broad, with prosternal, two
mesosternal, and metasternal plates, the prosternum projecting
forwards beneath the coxae of the 2nd pair of appendages. Appendages
of 2nd pair folding in a horizontal plane; their basal segments
freely movable; claw free or fused; basal segments of 4th and 5th
pairs widely separated by the sternal area; appendages of 3rd pair
with all the segments except the proximal three, forming a
many-jointed flagellum. Opisthosoma without post-anal sclerite and
posterior caudal elongation: with frequently a pair of small lobate
appendages on the sternum of the 3rd somite. Respiratory organs, as in
Urotricha.

Family -- Phrynichidae (_Phrynichus_, _Damon_).
" Admetidae (_Admetus_, _Heterophrynus_).
" Charontidae (_Charon_, _Sarax_).
(Family ?)--*_Graeophonus_.

(Original as preceding.)]

_Remarks._--The Pedipalpi are confined to the tropics and warmer
temperate regions of both hemispheres. Fossil forms occur in the
Carboniferous. The small forms known as _Schizomus_ and _Hubbardia_
are of special interest from a morphological point of view. The
Pedipalpi have no poison glands. (Reference to literature (29).)

(Original by Pickard-Cambridge and Pocock.)]

Order 3. Araneae (figs. 60 to 64.).--Prosoma covered with a single
shield and typically furnished with median and lateral eyes of
diplostichous structure, as in the Amblypygi. The sternal surface
wide, continuously chitinized, but with prosternal and metasternal
elements generally distinguishable at the anterior and posterior ends
respectively of the large mesosternurm. Prosternum underlying the
proboscis. Appendages of 1st pair have two segments, as in Pedipalpi,
but are furnished with poison gland, and are retroverts. Appendages of
2nd pair not underlying the mouth, but freely movable and, except in
primitive forms, furnished with a maxillary lobe; the rest of the limb
like the legs, tipped with a single claw and quite unmodified (except
in [symbol: male]). Remaining pairs of appendages similar in form and
function, each tipped with two or three claws. Opisthosoma when
segmented showing the same number of somites as in the Pedipalpi;
usually unsegmented, the prae-genital somite constricted to form the
waist; the appendages of its 3rd and 4th somites retained as spinning
mammillae. Respiratory organs (see fig. 63, _stg_), as in the
Amblypygi, or with the posterior pair, rarely the anterior pair as
well, replaced by tracheal tubes. Intromittent organ of male in the
apical segment of the 2nd prosomatic appendage.

(Original as above.)]

Sub-order a. Mesothelae (see figs. 60 to 62).--Opisthosoma distinctly
segmented, furnished with 11 tergal plates, as in the Amblypygi; the
ventral surface of the 1st and 2nd somites with large sternal plates,
covering the genital aperture and the two pairs of pulmonary sacs, the
sternal plates from the 6th to the 11th somites represented by
integumental ridges, weakly chitinized in the middle. The two pairs of
spinning appendages retain their primitive position in the middle of
the lower surface of the opisthosoma far in advance of the anus on the
3rd and 4th somites, each appendage consisting of a stout,
many-jointed outer branch and a slender, unsegmented inner branch.
Prosoma as in the Mygalomorphae, except that the mesosternal area is
long and narrow.

Family--Liphistiidae (_Liphistius_, *_Arthrolycosa_).

Sub-order b. Opisthothelae (see fig. 63).--Opisthosoma without trace
of separate terga and sterna, the segmentation merely represented
posteriorly by slight integumental folds and the sterna of the 1st and
2nd somites by the opercular plates of the pulmonary sacs. The
spinning appendages migrate to the posterior end of the opisthosoma
and take up a position close to the anus; the inner branches of the
anterior pair either atrophy or are represented homogenetically by a
plate, the cribellum, or by an undivided membranous lobe, the colulus.

Tribe 1. Mygalomorphae.--The plane of the articulation of the
appendages of the 1st pair to the prosoma (the retrovert) vertical,
the basal segment projecting straight forwards at its proximal end,
the distal segment or fang closing backwards in a direction
subparallel to the long axis of the body. Two pairs of pulmonary sacs.

Families--Theraphosidae (_Avicularia_, _Poecilotheria_). Barychelidae
(_Barychelus_, _Plagiobothrus_). Dipluridae (_Diplura_, _Macrothele_).
Ctenizidae (_Cteniza_, _Nemesia_). Atypidae (_Atypus_, _Calommata_).

I to VI, Appendages of the prosoma cut off at the base.
o, Ocular tubercle.
praegen, The prae-genital somite.
1 and 2, Sternites of the first and second opisthosomatic somites.
3 and 4, Appendages of the third and fourth opisthosomatic somites,
which are the spinning organs, and in this genus occupy their
primitive position instead of migrating to the anal region as in
other spiders.
5, Tergite of the fifth opisthosomatic somite.
11, Eleventh opisthosomatic somite; an, Anus.

(Original.)]

Tribe 2. Arachnomorpnae.--The plane of the articulation of the
appendages of the 1st pair to the prosoma horizontal, the basal
segment projecting vertically downwards, at least at its proximal end,
the distal segment or fang closing inwards nearly or quite at right
angles to the long axis of the body. The posterior pulmonary sacs
(except in _Hypochilus_) replaced by tracheal tubes; the anterior and
posterior pairs replaced by tracheal tubes in the Caponiidae.

Principal families--Hypochilidae (_Hypochilus_). Dysderidae
(_Dysdera_, _Segestria_). Caponiidae (_Caponia_, _Nops_). Filistatidae
(_Filistata_). Uloboridae (_Uloborus_, _Dinopis_). Argiapidae
(_Nephila_, _Gasteracantha_). Pholcidae (_Pholcus_, _Artema_).
Agelenidae (_Tegenuria_). Lycosidae (_Lycosa_). Clubionidae
(_Clubiona_, _Olios_, _Sparassus_) Gnaphosidae (_Gnaphosa_,
_Hemiclaea_). Thomisidae (_Thomisus_). Attidae (_Salticus_).
Urocteidae (_Uroctea_). Eresidae (_Eresus_).

_Remarks on the Araneae._--The Spiders are the most numerous and
diversified group of the Arachnida; about 2000 species are known. No
noteworthy fossil spiders are known; the best-preserved are in amber
of Oligocene age. _Protolycosa_ and _Arthrolycosa_ occur in the
Carboniferous. Morphologically, the spiders are remarkable for the
concentration and specialization of their structure, which is
accompanied with high physiological efficiency. The larger species of
Bird's Nest Spiders (_Avicularia_), the opisthosoma of which is as
large as a bantam's egg, undoubtedly attack young birds, and M'Cook
gives an account of the capture in its web by an ordinary house spider
of a small mouse. The "retrovert" or bent-back first pair of
appendages is provided with a poison gland opening on the fang or
terminal segment. Spiders form at least two kinds of
constructions--snares for the capture of prey and nests for the
preservation of the young. The latter are only formed by the female,
which is a larger and more powerful animal than the male. Like the
scorpions the spiders have a special tendency to cannibalism, and
accordingly the male, in approaching the female for the purpose of
fertilizing her, is liable to be fallen upon and sucked dry by the
object of his attentions. The sperm is removed by the male from the
genital aperture into a special receptacle on the terminal segment of
the 2nd prosomatic appendage. Thus held out at some distance from the
body, it is cautiously advanced by the male spider to the genital
aperture of the female.

I to VI, The six pairs of prosomatic appendages.
a, Copulatory apparatus of the second appendage.
b, Process of the fifth joint of the third appendage.
M, Mouth.
pro, Prosternite of the prosoma.
mes, Mesosternite of the prosoma: observe the contact of the coxae
of the sixth pair of limbs behind it; compare _Liphistius_ (fig.
61) where this does not occur.
stg, Lung aperture.
gn, Genital aperture.
a, Anus with a pair of backwardly migrated spinning appendages on
each side of it; compare the position of these appendages in
_Liphistius_ (fig. 61).

(From Lankester, "Limulus an Arachnid.")]

(Original drawing by Pocock.)]

For an account of the courtship and dancing of spiders, of their webs
and floating lines, the reader is referred to the works of M'Cook (30)
and the Peckhams (31), whilst an excellent account of the nests of
trap-door spiders is given by Moggridge (32). References to systematic
works will also be found at the end of this article (33).

Order 4. Palpigradi = Microthelyphonidae (see fig. 65).--Prosoma
covered above by three plates, a larger representing the dorsal
elements of the first four somites, and two smaller representing the
dorsal elements of the 5th and 6th.

Its ventral surface provided with one prosternal, two mesosternal and
one metasternal plate. Appendages of 1st pair consisting of three
segments, completely chelate, without poison gland; of 2nd pair
slender, leg-like, tipped with three claws, the basal segment without
sterno-coxal process taking no share in mastication, and widely
separated from its fellow of the opposite side; 3rd, 4th, 5th and 6th
appendages similar in form to the 2nd and to each other.

Proboscis free, not supported from below by either the prosternum or
the basal segments of the appendages of the 2nd pair.

A, Ventral view of prosoma and anterior region of opisthosoma with
the appendages cut off near the base; a and b, prosternites; c,
mesosternite; and d, metasternite of the prosoma; f, ventral
surface of the prae-genital somite; g, sternite of the genital
somite (first opisthosomatic somite).
B, Dorsal view. I to VI, prosomatic appendages; 1 opisth, genital
somite (first opisthosomatic somite).
C, Lateral view, I to VI, prosomatic appendages; a, b, c, the three
tergal plates of the prosoma; prae-gen, the prae-genital somite; 1
to 10, the ten somites of the opisthosoma.
D, Chelicera.

(Original drawing by Pocock and Pickard-Cambridge, after Hansen and
Sorensen.)]

Opisthosoma consisting of only ten somites, which have no tergal and
sternal elements, the prae-genital somite contracted to form a
"waist," as in the Pedipalpi; the last three narrowed to form a caudal
support for the many-jointed flagelliform telson, as in the Urotricha.
Respiratory organs atrophied.

Family--Koeneniidae (_Koenenia_).

_Remarks._--An extremely remarkable minute form originally described
by Grassi (34) from Sicily, and since further described by Hansen
(35). Recently the genus has been found in Texas, U.S.A. Only one
genus of the order is known.

Order 5. Solifugae = Mycetophorae (see figs. 66 to 69).--Dorsal area
of prosoma covered with three distinct plates, two smaller
representing the terga of the 5th and 6th somites, and a larger
representing those of the anterior four somites, although the reduced
terga of the 3rd and 4th are traceable behind the larger plate. The
latter bears a pair of median eyes and obsolete lateral eyes on each
side. Sternal elements of prosoma almost entirely absent, traces of a
prosternum and metasternum alone remaining. Rostrum free, not
supported by either the prosternum or the basal segments of the
appendages. Appendages of 1st pair large, chelate, bisegmented,
articulated to the sides of the head-shield; appendages of 2nd pair
simple, pediform, with protrusible (? suctorial) organ, and no claws
at the tip; their basal segments united in the middle line and
furnished with sterno-coxal process. Remaining pairs of appendages
with their basal segments immovably fixed to the sternal surface,
similar in form, the posterior three pairs furnished with two claws
supported on long stalks; the basal segments of the 6th pair bearing
five pairs of tactile sensory organs or malleoli. The prae-genital
somite is suppressed. Opisthosoma composed of ten somites. Respiratory
organs tracheal, opening upon the ventral surface of the 2nd and 3rd,
and sometimes also of the 4th somite of the opisthosoma. A
supplementary pair of tracheae opening behind the basal segment of the
4th appendage of the prosoma.

(? Intromittent organ of male lodged on the dorsal side of the 1st
pair of prosomatic appendages.)

Families--Hexisopodidae (_Hexisopus_). Solpugidae (_Solpuga_,
_Rhagodes_). Galeodidae (_Galeodes_).

_Remarks._--These most strange-looking Arachnids occur in warmer
temperate, and tropical regions of Asia, Africa and America. Their
anatomy has not been studied, as yet, by means of freshly-killed
material, and is imperfectly known, though the presence of the coxal
glands was determined by Macleod in 1884. The proportionately enormous
chelae (chelicerae) of the first pair of appendages are not provided
with poison glands; their bite is not venomous.

I to VI, The six leg-bearing somites of the prosoma.
opisth 1, First or genital somite of the opisthosoma.
ge, Site of the genital aperture.
st, Thoracic tracheal aperture.
l^2, Anterior tracheal aperture of the opisthosoma in somite 2 of the
opisthosoma.
l^3, Tracheal aperture in somite 3 of the opisthosoma.
a, Anus.

(From Lankester, "Limulus an Arachnid.")]

I to VI, Prosomatic appendages.
s, Prosomatic stigma or aperture of the tracheal system.
1, First opisthosomatic sternite covering the genital aperture g.
2, Second opisthosomatic sternite covering the second pair of
tracheal apertures sp1.
sp2, The third pair of tracheal apertures.
10, The tenth opisthosomatic somite.
an, The anal aperture.

(Original by Pickard-Cambridge and Pocock.)]

I to VI, Bases of the prosomatic appendages.
o, Eyes.
a, Lateral region of the cephalic plate to which the first pair of
appendages are articulated.
b, Cephalic plate with median eye.
c, Dorsal element of somites bearing third and fourth pairs of
appendages.
d, Second plate of the prosoma with fifth pair of appendages.
e, Third or hindermost plate of the prosoma beneath which the sixth
pair of legs is articulated.
1, 2, 9, 10, First, second, ninth and tenth somites of the
opisthosoma.
an, Anus.

(Original.)]

_Galeodes_ has been made the means of a comparison between the
structure of the Arachnida and Hexapod insects by Haeckel and other
writers, and it was at one time suggested that there was a genetic
affinity between the two groups--through _Galeodes_, or extinct forms
similar to it. The segmentation of the prosoma and the form of the
appendages bear a homoplastic similarity to the head, pro-, meso-, and
meta-thorax of a Hexapod with mandibles, maxillary palps and three
pairs of walking legs; while the opisthosoma agrees in form and number
of somites with the abdomen of a Hexapod, and the tracheal stigmata
present certain agreements in the two cases. Reference to literature
(36).

I to VI, The six prosomatic limbs cut short.
o, The eyes.
b, c, Demarcated areae of the cephalic or first prosomatic plate
corresponding respectively to appendages I, II, III, and to
appendage IV (see fig. 68).
d, Second plate of the prosoma-carrying appendage V.
e, Third plate of the prosoma-carrying appendage VI. The prae-genital
somite is absent.
1, First somite of the opisthosoma.
2, Second do.
S, Prosomatic tracheal aperture between legs IV and V.
S' and S", Opisthosomatic tracheal apertures.
10, Tenth opisthosomatic somite.
an, Anus.

(Original.)]

I to VI, Prosomatic appendages.
o, Sterno-coxal process of the basal segment of the second appendage.
1, Sternite of the genital or first opisthosomatic somite; the
prae-genital somite, though represented by a tergum, has no
separate ternal plate.
2 and 3, Sternites of the second and third somites of the
opisthosoma, each showing a tracheal stigma.
10 and 11, Sternites of the tenth and eleventh somites of the
opisthosoma.
an, Anus.

(Original by Pocock and Pickard-Cambridge.)]

I to VI, The prosomatic appendages.
o, Eyes.
prae-gen, Prae-genital somite.
1, Tergite of the genital or first opisthosomatic somite.
10, Tergite of the tenthsomite of the opisthosoma.
11, The evanescent eleventh somite of the opisthosoma.
an, Anus.

(Original.)]

Order 6. Pseudoscorpiones = Chelonethi, also called Chernetidia (see
figs. 70, 71, 72).--Prosoma covered by a single dorsal shield, at most
furnished with one or two diplostichous lateral eyes; sternal elements
obliterated or almost obliterated. Appendages of the 1st pair
bisegmented completely chelate, furnished with peculiar organs, the
_serrula_ and the _lamina_. Appendages of 2nd pair very large and
completely chelate, their basal segments meeting in the middle line,
as in the Uropygi, and provided in front with membranous lip-like
processes underlying the proboscis. Appendages of the 3rd, 4th, 5th
and 6th pairs similar in form and function, tipped with two claws,
their basal segments in contact in the median ventral line. The
prae-genital somite wide, not constricted, with large tergal plate,
but with its sternal plate small or inconspicuous. Opisthosoma
composed, at least in many cases, of eleven somites, the 11th somite
very small, often hidden within the both. Respiratory organs in the
form of tracheal tubes opening by a pair of stigmata in the 2nd and
3rd somites of the opisthosoma. Intromittent organ of male beneath
sternum of the 1st somite of the opisthosoma.

Sub-order a. Panctenodactyli.--Dorsal plate of prosoma (carapace)
narrowed in front; the appendages of the 1st pair small, much
narrower, taken together, than the posterior border of the carapace.
Serrula on movable digit of appendages of 1st pair fixed throughout
its length, and broader at its proximal than at its distal end; the
immovable digit with an external process.

Family--Cheliferidae (_Chelifer_ (figs. 70, 71, 72), _Chiridium_).
" Garypidae (_Garypus_).

I to VI, of the six prosomatic appendages.
o, Eyes.
prae-gen, Tergite of the prae-genital somite.
1, Genital or first opisthosomatic somite.
2, 3, 10, The second, third and tenth somites of the opisthosoma.
11, The minute eleventh somite;
an, the anus.

(Original.)]

Sub-order b. Hemictenodactyli.--Dorsal plate of prosoma scarcely
narrowed in front; the appendages of the 1st pair large, not much
narrower, taken together, than the posterior border of the carapace.
The serrula or the movable digit free at its distal end, narrowed at
the base; no external lamina on the immovable digit.

Family--Obisiidae (_Obisium, Pseudobisium_).
" Chthoniidae (_Chthonius, Tridenchthonius_).

_Remarks._--The book-scorpions--so called because they were, in old
times, found not unfrequently in libraries--are found in rotten wood
and under stones. The similarity of the form of their appendages to
those of the scorpions suggests that they are a degenerate group
derived from the latter, but the large size of the prae-genital somite
in them would indicate a connexion with forms preceding the scorpions.
Reference to literature (37).

III to VI, The third, fourth, fifth and sixth appendages of the
prosoma.
a, Movable (hinged) sclerite (so-called hood) overhanging the first
pair of appendages.
b, Fused terga of the prosoma followed by the opisthosoma of four
visible somites.
an, Orifice within which the caudal segments are withdrawn.
E, Extremity of the fifth appendage of the male modified to subserve
copulation.

(Original drawing by Pocock and Pickard-Cambridge.)]

Order 7. Podogona = Ricinulei (see figs. 73 to 76).--Dorsal area of
prosoma furnished with two shields, a larger behind representing,
probably, the tergal elements of the somites, and a smaller in front,
which is freely articulated to the former and folds over the
appendages of the 1st pair. Ventral area without distinct sternal
plates. Appendages of 1st pair, bisegmented, completely chelate.
Appendages of 2nd pair, with their basal segments uniting in the
middle line below the mouth, weakly chelate at apex. Appendages of
3rd, 4th, 5th and 6th pairs similar in form; their basal segments in
contact in the middle line and immovably welded, except those of the
3rd pair, which have been pushed aside so that the bases of the 2nd
and 4th pairs are in contact with each other. A movable membranous
joint between the prosoma and the opisthosoma, the generative aperture
opening upon the ventral side of the membrane. Prae-genital somite
suppressed; the opisthosma consisting of nine segments, whereof the
first and second are almost suppressed and concealed within the joint
between the prosoma and the opisthosoma; the following four large and
manifest, and the remaining three minute and forming a slender
generally-retracted tail like that of _Thelyphonus_. Respiratory
organs tracheal, opening by a pair of spiracles in the prosoma above
the base of the fifth appendage on each side. Intromittent organ of
male placed at the distal end of the appendage of the 5th pair.

(Original drawing by Pocock and Pickard-Cambridge.)]

Family--Cryptostemmidae (_Cryptostemma, Poliochera_), Carboniferous.

_Remarks on the Podogona._--The name given to this small but
remarkable group has reference to the position of the male
intromittent organ (fig. 73, E). They are small degenerate animals
with a relatively firm integument. Not more than four species and
twice that number of specimens are known. They have been found in West
Africa and South America. A fact of special interest in regard to them
is that the genus Poliochera, from the Coal Measures, appears to be a
member of the same group. The name Cryptostemma, given to the
first-known genus of the order, described by Guerin-Meneville, refers
to the supposed concealment of the eyes by the movable cephalic
sclerite. Reference to literature (38).

I to VI, The six pairs of appendages of the prosoma, the last three
cut short.
1, 2, 3, 4, The four somites of the opisthosoma.
a, Visible hood overhanging the first pair of appendages.
b, Position of the genital orifice.
c, Part of 3rd appendage.
d, Fourth segment of 2nd appendage. Observe that the basal segment
of appendage III does _not_ meet its fellow in the middle line.

(Original drawing by Pocock and Pickard-Cambridge.)]

Order 8. Opilione (see fig. 77).--Dorsal area of prosoma covered by a
single shield usually bearing a pair of eyes. Sternal elements much
reduced. Appendages of 1st pair large, three segmented and completely
chelate; of 2nd pair either simple and pediform, or prehensile and
subchelate; of remaining four pairs, similar in form, ambulatory in
function; the basal segment of the 2nd, 3rd and sometimes of the 4th
pairs of appendages furnished with sterno-coxal (maxillary) lobe.
Opisthosoma confluent throughout its breadth with the prosoma, with
the dorsal plate of which its anterior tergal plates are more or less
fused; at most ten opisthosomatic somites traceable; the generative
aperture thrust far forwards between the basal segments of the 6th
appendages. Prae-genital somite suppressed. Respiratory organs
tracheal, opening by a pair of stigmata situated immediately behind
the basal segments of the 6th pair of appendages on what is probably
the sternum of the 2nd opisthosomatic somite and also in some cases
upon the 5th segment of the legs.

Intromittent organ of male lying within the genital orifice.