are sacs on the anterior abdominal somites opening by narrow slits
(fig. 461) to the exterior. The anterior wall of each lung sac is
made up of thin plates arranged like the leaves of a book, and em-
bryology shows that these lung books are gill books drawn into the
ventral surface of the abdomen. The tracheae in development
pass through a gill-stage and a lung-stage, the tracheal tubes being
outgrowths of the spaces between the lung leaves which penetrate
all parts of the body.
The reproductive openings are on the basal somite of the abdo-
men. The spermatozoa are motile. The development is direct,
there being no metamorphosis.
Sub Class I. Gigantostraca.
Marine forms with gills on the 2-6 abdominal appendages;
bases of five pairs of cephalothoracic feet masticatory ; a pair of
medinn ocelli and a pair of compound eyes on the cephalothorax.
Order I. Xiphosura.
Cephalothorax large ; abdomen terminated by a long spiniform telson.
Limulus polyphemus of our east coast, commonly known as king crab
FIG. 457. FIG. 458.
Fio. 457. Limulus polyphemus.* horseshoe crab (orig.).
FIG. 458. Ventral surface of Limulus moluccanus. (From Ludwig-Leunis.)
chelicerse ; 2-5, walking feet ; 6', pushing foot ; 6 a , flabellum ; 7, genital" operculum
8, gills (there should be five) ; P, base ' '
or horseshoe crab. Other species on eastern shore of eastern continent.
They burrow beneath the sand and mud of the bottom and feed on worms.
In the spring they come to the shore to lay eggs.
Order II. Eurypterida.
Extinct Silurian and Devonian forms with small cephalothorax and
large twelve-jointed abdomen. The animals are intermediate between the
xiphosures and the scorpions. Eurypterus; Pterygotus. some species
seven feet long.
Sub Class II. Arachnida.
Under this name are included a number of orders of greater or
less extent which can be arranged around the spiders, or Aranea,
as a centre. There is considerable modification of form, and the
following account applies only to the more typical groups. In
these the cephalothorax and abdomen are separated by a distinct
line, and since the abdominal appendages almost entirely disappear
in the adult, the number of somites can only be ascertained where
their boundaries are evident. The number varies between six in
the phalangids and thirteen in the scorpions.
The cephalothorax is, except in the Solpugidae, a single piece-
//. ACEEATA: ARACHNIDA. 445
which bears six pairs of appendages; the four posterior pairs, con-
sisting typically of seven joints, are locomotor, so that the posses-
sion of eight legs is as characteristic for
an arachnid as ten for a decapod or six
for a hexapod. The first pair of append-
ages, the chelicerce (fig. 459), are preoral,
the second, m pedipaipi, beside that open-
ing. The chelicerae are short and con-
sist of two or three joints, the terminal
joint either folding back upon the other
or, pincer-like, meeting an opposable
thumb. In the spiders the last ioint or
n ... , . . . . * , FIG. 459. Mouth parts of Epeira.
claw is forced into the prey, introducing i, cheiicera; *, pedipaipi; p
, , , -, . . ! palpus; J, basal plate.
poison irom a sac in the basal joint.
The pedipaipi are elongate, leg-like, their basal joints often form-
ing a lip, the other joints forming the palpus, which may end with
a claw or a pincer.
The question has often been discussed as to whether the chelicerae are
the homologues of the antennas of other arthropods. The embryological
evidence, which cannot be detailed here, is in favor of their equivalence to
the second antenna of the Crustacea, and to the mandibles of insects.
Since the Arachnida usually suck their food, the oesophagus is
frequently widened to a sucking stomach, behind which comes the
true stomach, with which, as well as with the intestine, a number
of so-called liver tubes may arise (fig. 455, da, dt). These may
be restricted to the abdomen alone, as in the scorpions. The
hinder part of the intestine is often enlarged into a rectal vesicle
(stercoral pocket), just in front of which the excretory tubules (so-
called Malpighian tubules) empty. These resemble the true Mal-
pighian tubes of insects in function, but differ in being entodermal
in origin. Besides there also occur, coxal glands (modified ne-
phridia), of which only one pair comes to development, and this
may lose its external opening on the base of the appendage.
The oesophagus is always closely surrounded by a nerve ring
composed of brain above and of part of the ventral chain on the
sides and below, the thoracic and more or fewer of the abdominal
ganglia entering into its composition (fig. 405, D). Of sense organs,
besides tactile hairs, only the eyes (fig. 406), 2-12 in number, are
well known. Hearing is well developed, but it is uncertain whether
certain hairs on the legs and palpi are the seats of the recognition
of sound. The function of the ' lyriform organs/ which occur in
the skin of body and legs in several groups, is unknown.
The respiratory organs already alluded to (p. 443) have their
spiracles, always few in number, on the anterior ventral part of
the abdomen and, it is stated, sometimes on the cephalothorax.
The internal organs are the lungs and the tracheae. A lung is a
rounded sac just inside the spiracle and consists of numerous leaves
on the anterior wall of the lung sac. Each leaf is covered on each
side by a thin layer of chitin and contains a blood space in its in-
terior, while between the leaves are flattened spaces into which the
air enters (fig. 456). The tracheae, on the
other hand, are branched tubes arising from
the abdominal spiracles and penetrating
the abdomen (fig. 460). These are lined
with chitin, and to strengthen them with-
out undue thickness this lining is thrown
into folds, usually arranged in a spiral.
In the scorpions and tetrapneumonous
Araneina only lungs occur. In other
spiders one pair of lungs is replaced by
tracheae, while in most other arachnids only
tracheae occur. (The smaller mites and
FIG. 460. Beginning of paired parasites lack specialized respiratory or-
trachese of Anyphozna uccen- , . , , ,, .
taatn. (After Bertkau.) st, gans and circulatory organs as well.)
These facts, aside from embryological con-
ditions, show that lungs and tracheae are morphologically equiva-
lent. The localization of respiration in the abdomen has resulted
in having the heart in the same region. It is noticeable that, as
the tracheae are developed, the circulatory vessels are reduced. In
the scorpions, which have only lungs, the circulation is most
In development the arachnidan tracheae arise from the abdominal
appendages, as do the lungs. (In the Solpugidse and some mites cepha-
lothoracic tracheae occur, but nothing is known of their development.)
This fact shows that they are entirely different in origin from the tracheae
of insects, while numberless details show that these structures are only to
be compared with the gills of Limulus.
The gonads (only the Tardigrades are hermaphroditic) are
abdominal in position and open by paired ducts (sometimes with a
single mouth) on the first abdominal somite. In most cases the
animals are oviparous, but the scorpions and many mites bear liv-
ing young. In many instances the mothers care for their eggs and
young, the scorpions carrying their families on their bodies. Only
rarely is there a metamorphosis, and then in the aberrant forms
//. ACERATA: SCORPIONIDA.
like the Linguatulida and Acarina, where the young have but two
or three pairs of appendages, acquiring the others later.
Legion I. Arthrogastrida.
Arachnida in which the abdominal somites are distinct.
Order I. Scorpionida.
The scorpions bear a superficial resemblance to crayfish and for
a long time were associated with them, since (fig. 402) they have
four pairs of walking feet (3-6), while the pedipalpi (2) are large
and bear pincers. The chelicerae are also chelate. The pedipalpi
and the two anterior pairs of legs have the basal joint expanded
for chewing. The peculiarities of the abdomen mark the group
off from all other arachnids. It consists of seven broader somites
attached by their whole width to the cephalothorax and behind
six narrower somites, forming a tail or postabdomen. The last
somite is bent ventrally in a sharp spine and contains two large
poison glands. It is the sting ' of the animal, which, in the case
of the small species, causes painful wounds in man ; and in the
large tropical species is, perhaps, fatal. Usually scorpions feed
upon insects, which they seize with the pincers, and, arching the
FIG. 461. Under surface of scorpion, showing the combs and the outlines of the lung
sacs with their spiracles (orig.).
tail over the back, kill with the sting. On the ventral surface of
the second abdominal somite (fig. 461) are a pair of appendages,
the combs or pectines; rods with teeth on one side of uncertain
function. They are clearly appendages with modified gill leaves,
and from their nearness to the sexual opening and their rich nerve
supply are supposed to be stimulating organs in copulation. The
next four segments bear spiracles which lead to four pairs of lung
sacs. The heart is abdominal and the ' liver 9 diverticula are con-
fined to the same region. The large number of abdominal ganglia
distinct from the oesophageal ring is also characteristic. From
three to six pairs of eyes occur.
The scorpions are inhabitants of warm regions, ranging north with us
to the Carolinas and Nebraska. Buthus* Centrums.*
Order II. Phrynoidea (Pedipalpi, Thelyphonida).
The thoracic segments are fused, and of the appendages only
the last three are walking feet, the third pair having the last
joint (tarsus) developed into a long many- jointed tactile flagel-
FlO. 462. Phrynus (Phrynichus) reniformis. (From Schmarda.)
him. The chelicerae are strong and spined, but end in a claw, not
in a pincer. The chelicerae are also clawed and are possibly poison
organs, since the bite of these animals is feared. The abdomen
consists of eleven or twelve somites and contains two pairs of lungs.
There are eight eyes two large ones in the middle of the cephalo-
thorax, and three small ones on either side.
The species are tropical. Phrynus (fig. 462) has a simple abdomen ;
Thelyphonus* (fig. 405, D) has a short postabdomen which bears a long,
many-jointed thread. One species in the southwestern United States.
Order III. Microthelyphonida.
Small animals as yet known only from Texas, Sicily, Paraguay,
and Siam. They have a general resemblance to a scorpion, the
chelicerae are three-jointed and chelate, the pedipalpi simple, neither
these nor any of the legs having chewing lamellae. The head is
distinct from two * thoracic segments/ the abdomen is eleven'
jointed and is terminated by a long many-jointed caudal flagellum.
//. ACES ATA: SOLPUGIDA.
Lung sacs, which are true appendages without lung leaves, occur
on abdominal segments four to six, and are eversible. The ovary
FIG. 463. Koenenia wheeler i.* (From Wheeler.)
is unpaired, the testes paired. There is a circumcesophageal nerve
ring and a single abdominal ganglion. No Malpighian tubes
Order IV. Solpugida (Solifugae).
In these the cephalothorax is broken up into a head bearing the
chelicerae, pedipalpi, and the first pair of legs; and three posterior
free somites, each bearing a pair of legs, thus giving these forms a
certain resemblance to the Hexapoda (infra). The chelicerae are
strong and chelate, the pedipalpi are simple and are used in walk-
ing, while the first pair of legs are tactile. Respiration occurs by
four pairs of tracheae, the first of which opens between the first and
second ( thoracic ' somites, a condition which deserves embryologi-
cal investigation. The abdomen consists of nine or ten somites,
and the head bears two ocelli.
As the name implies, the Solpugidae are nocturnal, living by day in
holes in the sand and searching for their prey at night. In the Old World
they are reputed as poisonous, but no poison glands occur. "Warmer parts
of U. S. Solpuga,* Galeodes* Datames * (fig. 464).
FIG. 464. FIG. 465.
FIG. 464. Datames formidibilis* (After Putnam.)
FIG. 465. Chelifer bravaisi. (From Schmarda.) 1, cheliceree; 8, pedipalpi.
Order V. Pseudoscorpii.
These small forms resemble the true scorpions in the chelate
cheliceraB and pedipalpi (fig. 465), and in the abdomen joined by
its whole breadth to the thorax. They differ in the lack of post-
abdomen and sting. They breathe by tracheae; have from two to
four ocelli, and spinning glands opening on the second abdominal
These animals, 2-3 mm. long, live in moss, etc., and among old and
dusty books, where they feed on mites and minute insects. Their bodies
are flattened and they run side wise. Chelifer,* Obisium,* Chernes.*
Order VI. Phalangida.
The abdomen in the harvestman, or 'daddy long legs/ is less
evidently segmented than in the forms already mentioned, nor is
it sharply distinct from the cephalothorax. The small body bears
four pairs of exceedingly long legs; the cheliceraB are drawn out
//. ACERATA: AEANEINA. 451
in long horny processes; the pedipalpi are tactile organs as in the
true spiders. The males possess a long penis, and the females a
FIG. 466. A phalangid laying eggs. (After Henking.)
long ovipositor (fig. 466). They have two or four ocelli and
breathe by tracheae.
These largely nocturnal animals are predaceous, feeding upon small
mites. In structure they fqrm in some ways an approach to the
Acarina. Phalangium* Liobunum*
Legion II. Splicer ogastrida.
Arachnida with the abdominal somites fused so that no traces
of segmentation remain.
Order I. Araneina.
In the spiders the soft-skinned
body is divided by a deep con-
striction into cephalothorax and
abdomen (fig. 467). The four pairs
of legs are adapted for springing
or for walking, the hinder pair
being also accessory to the spin-
ning. It bears a comb-like claw
with which several threads are
combined into a stronger cable.
The chelicera bears a sharp claw
(fig. 459), traversed by the duct
of the poison gland with which the
prey is killed, although but few
(species Of LatrodecteS, fig. 468, FlO. 467. Epeira tnsularis* round- web
the tarantula, and the bird spiders,
Mygalidae) can injure man. The pedipalpi are used as feeling
organs and with the basal maxillary process to comminute the
food. In the male the pedipalpi have the terminal joint swollen
to a pear-shaped structure (fig. 469) by which the sexes are easily
FIG. 468. FIG. 469 FIG. 470.
FIG. 468. Latrodectes macfmis,* poison spider. (After Marx.)
FIG. 469.- Pedipalp of Pardosa uncnta. (After Emerton.)
FIG. 470. Spinnerets of Epeira diadema. (After War burton.) 1, 2, 3, first, second,
and third spinnerets; /, threads.
distinguished. This is used to convey the spermatozoa to the
female, a rather dangerous process, as the male is apt to be killed
by the much stronger mate.
At the hinder end of the abdomen, just in front of the anus,
are the spinnerets, which are reduced appendages, as is shown by
their paired arrangement and their jointing (fig. 470), as well as
by development. They are truncate and have at the tip a ' spin-
ning field' from which numerous minute, two-jointed spinning
tubes, resembling hairs, arise, each of which is the end of a duct
of a silk gland. Different kinds of glands, producing silk for differ-
ent purposes, occur. The number of spinnerets varies between two
and three pairs, and in front of these may be an unpaired spinning
region, the cribrellum, so that hundreds or even thousands (Epei-
ridae) of glands may be present.
The secretion of the glands hardens in contact with the air, and the
single threads are united by the combs of the hinder feet, into a larger cord
which can be regulated in size according to the number of glands which
are active. Yet the largest cord is finer than the finest silkworm silk,
hence it is often used for the cross-hairs of telescopes. The spider silk has
many uses; it is used to line the nests, to form cocoons for the eggs, as a
means of descent from high places, and to form the well-known webs.
The nervous system consists of a brain and a circumoesophageal ring,
and, in the Mygalidae, a single abdominal ganglion. The arrangement of
the six or eight ocelli and the relative lengths of the legs are matters of
systematic importance. Two pairs of respiratory organs occur. In the
Tetrapneumones there are two pair of lungs, but in the Dipneumones the
//. ACERATA: ACARINA. 453
hinder pair are replaced by tracheae, which may open by separate spiracles
(Tetrasticta) or by a common opening (Tristicta, fig. 460).
Sub Order I. TETRAPNEUMONES. Four lungs, four spinnents and
eight eyes in two rows. The MYGALnxsare the most important group, large-
forms which spring upon their prey, capturing even small birds and mice.
To the genus Mygale* belong the spiders (commonly but erroneously called
tarantulas) which occur in banana bunches. Here also belong the trap-
door spiders, Cteniza,* of the southwest, which excavate burrows in the
FIG. 471. Cteniza ccementaria in its tube, closing the lid. o, eyes ; b, inside of lid!
with places for the claws ; c, egg cocoon.
soil, line them with silk, and close them with a hinged lid (fig. 471).
Sub Order II. DIPNEUMONES. One pair of lungs, one of trachea;
six spinnerets. Here belong most of the native and numerous tropical
species. Some (VAGABUND^E) use their webs only to line the nests and
enclose the eggs, which are either hidden away or carried about attached to
the body, while they spring upon or chase their prey. SEDENTARIA are-
the web builders, their webs varying widely in structure. Of the first
group the SALTIGRADA include forms which jump upon their prey (Attus,*
PhidippuS)* Habrocentrum*), and the CITIGRADA (Lycosa,* Dolomedes,*
Trochosa *), which run their prey down. Among these is the true Taran-
tula, T. apulice of Italy, whose bite was once believed to cause a frenzy only
to be cured by peculiar music (' Tarantello '). The Sedentaria are divided,
according to the web-building habits- The ORBITELARLE or orb weavers
(Epeira* Argiope*) form vertical webs which in many instances are com-
plete circles. The RETITELARLE (Theridium* Erigone *) build irregular
webs. The species of Latrodectes * are reputed poisonous to man (fig.
468). The TUBITELARI.E build horizontal webs with a tube to the mar-
gin in which they lay in wait for insects.
Order II. Acarina.
The mites, partly from parasitism, partly from other conditions
of life, have become, in some instances, considerably modified.
With the fusion of cephalothorax and abdomen the last traces of
segmentation in the body are lost. Yet they retain the six pairs
of appendages four pairs of legs which at once distinguish them
from the parasitic hexapods; and two pairs of mouth parts, modi-
fied into a sucking beak. This consists of a tube formed by the
basal joints of the pedipaJpi, in which the chelicerae, either chelate,
clawed, or stylet-like, play.
Since the mites are small and half or wholly parasitic, they are much
simplified in structure. Frequently heart and tracheae are lacking. The
larva as it escapes from the egg lacks the last pair of legs and then closely
resembles certain imperfectly segmented parasitic insects like the lice.
The red mites or TROMBIDIID^E and the water mites, HYDRACHNID.E (Hy-
draclma* Atax *), are free-living in the adult condition, but parasitic as
young. The IXODID^: or ticks (Ixodes*}, live in woods or on bushes, attack
man and other mammals, burrowing beneath the skin, sucking the blood un-
til they become enormously swollen and fall off. The much smaller males
FIG. 472. Sarcoptes scabei, female itch mite. (After Leuckart )
FIG. 473. Demodex folticulorum, follicle mite. (From Ludwig-Leunis.)
are attached to the females and take no food. Argas persicus, of eastern
lands, with habits like a bedbug, is poisonous. The GAMASID^E are para-
sitic, species of Gamasus * occurring on beetles and Dermanyss-us* on bats.
The ACARID.E include permanent parasites like Sarcoptes scabei*' (fig. 472),
the cause of the 'itch,' and the closely allied cheese mite. The follicle
mite, Demodex folliculorum,* lives in the sebaceous glands of various
mammals, including man (fig. 473).
Order III. Linguatulida.
Elongate mites like Demodex lead to the Linguatulida, which
as adults live in the frontal sinuses of carnivorous mammals, as en-
cysted young in the liver of herbivorous forms, especially rodents.
The body is long, flattened and ringed, and hence somewhat tape-
worm-like (fig. 112). The adults have the mouth at the base of
a chitinous capsule, and on either side are two hooks regarded as
the claws of the first and second legs. Inside the body is a spa-
cious cavity traversed by the alimentary canal which is without
appendages. The nervous system is largely a circumcesophageal
//. ACERATA: LINGUATULIDA, TARDIGRADA.
ring; the sexual organs are very complicated, the males having
the openings in front, the females at the hinder end.
The presence of these parasites in animals causes a profuse catarrh,
and the eggs pass out with the mucus. Falling on vegetation, these are
FIG. 474. FIG. 475.
FIG. 474. Larva of Pentastomum proboscideum. (After Stiles.) rf, stomach; c, gland
cells ; m, mouth ; st, stylet ; ?/, posterior larval hooks ; J, 2, legs.
FIG. 475. Macrobiotus hufelandi, water bear. (After drawings by Greef and Plate.)
I-IV^ legs ; d, accessory glands ; m, stomach ; mfc, mouth capsule ; ov, ovary ; sp,
salivary glands ; st, stylets ; vm, excretory tubules ; blood cells in the body.
liable to be eaten by various animals. The larvae (tig. 474) have a boring
apparatus in front and two pairs of legs, the latter being lost in the
metamorphosis except for the hooks. It is by no means certain that
these are degenerate arachnids. The points in favor of such a position
are about equally balanced by those against. Pentastomum.
Usually associated with the Arachnida are two other groups of very
doubtful position, which until more definite knowledge is obtained, may
remain near them.
These are minute fresh-water forms, known to microscopists as
4 water bears ' (fig. 475), which owe their name to their slow motions.
They have four pairs of short, hooked legs, their sole Arachnidan charac-
ter. The genital ducts empty into the rectum ; the nervous system has
four ventral ganglia ; heart and respiratory organs are lacking. In de-
velopment they are remarkable for the large ccelomic pouches. In the
feet are glands recalling nephridia in their history. It is possible that
these animals are to be placed among the Coelhelminthes. Macrobiotus*
These marine animals have a cylindrical body, with a tubular probos-
cis in front and an abdominal appendage behind, and four pairs of very
long legs. In front of the legs is a pair of small chelate appendages and
usually a pair more like pedipalpi. In the male there is an additional
pair of ' ovigerous ' legs to which the eggs are attached after being
deposited by the female, thus giving a total of seven appendages, a num-
FIG. 476. Nymphon stroemii * (orig.). c, chelicerse ; o, ovigerous legs ; p, pedipalpi ?
ber not reached in any arachnid. Diverticula of the stomach extend into-
the legs ; a heart is present, but respiratory organs are lacking. The-
Pycnogonids, which creep slowly over seaweeds and hydroids, may be (1)
a distinct group of arthroproda, or (2) modified arachnids, or (3), and less
probable, Crustacea. Nymphon* Phoxichilidium* Colossendeis.*
Class III. Malacopoda (Protracheata).
These forms, including only a single family PERIPATID^E, show
a strange mixture of annelid and arthropodan (or ' tracheate ')
Fio. 477. Peripatus capensis. (From Balfour, after Moseley.)
characters, so that they are usually regarded as representatives of
the stock, early separated from the annelids, from which the Insecta
have descended. They recall the annelids by the presence of
nephridia, so characteristic of that group, which begin by a closed
vesicle (reduced coelom), pursue a short course, and expand into a
urinary bladder before opening at the bases of the legs (fig. 478,