[1] Lord Cromer in _Egypt_, No. 1, 1905, p. 2.
ARABISTAN (formerly KHUZISTAN), a province of Persia, bounded on the S.
by the Persian Gulf, on the W. by Turkish territory, on the N. by
Luristan and on the E. by the Bakhtiari district and Fars. It has its
modern name, signifying "land of the Arabs," from the Arabs who form the
bulk of the population, and is subdivided into the districts of
Muhamrah, Fellahiyeh (the old Dorak), Ram Hormuz (popularly known as
Ramiz), Havizeh, Shushter and Dizful. It has a population of about
200,000 and pays a yearly revenue of about L30,000. The soil is very
fertile, but since the dam over the Karun at Ahvaz was swept away and
the numerous canals which diverted the waters of the river for
irrigation became useless, a great part of the province is uncultivated,
and most of the crops and produce depend for water on rainfall and
wells. The climate is hot, and in the low-lying, swampy districts very
unhealthy; the prevailing winds are north-west and south-east, the
former hot and dry from the arid districts west of Mesopotamia, the
latter bearing much moisture from the Persian Gulf and the Indian Ocean.
The principal Arab tribes are the Kab (generally known as Chaab) and
Beni Lam, the former mostly settled in towns and villages and by
religion Shi'ites, the latter nomads and Sunnites. The staples of food
are dates and fish in the south, elsewhere the produce of the herds and
flocks and rice, wheat and barley. Other products are maize, cotton,
silk and indigo, and the manufactures include carpets without pile,
coarse woollens, cottons and silk nettings. Dyeing is extensively
carried on in Dizful where most of the indigo is grown.
Khuzistan (meaning "the land of the Khuz") was a part of the Biblical
Elam, the classical Susiana, and appears in the great inscription of
Darius as Uvaja.
ARABS, the name given to that branch of the Semitic race which from the
earliest historic times inhabited the south-western portion of the
Arabian peninsula. The name, to-day the collective term for the
overwhelming majority of the surviving Semitic peoples, was originally
restricted to the nomad tribes who ranged the north of the peninsula
east of Palestine and the Syro-Arabian desert. In this narrow sense
"Arab" is used in the Assyrian inscriptions, in the Old Testament and in
the Minaean inscriptions. Before the Christian era it had come to
include all the inhabitants of the peninsula. This, it is suggested, may
have been due to the fact that the "Arabs" were the chief people near
the Greek and Roman colonies in Syria and Mesopotamia. Classical writers
use the term both in its local and general sense. The Arabs to-day
occupy, besides Arabia, a part of Mesopotamia, the western shores of the
Red Sea, the eastern coast of the Persian Gulf and the north of Africa.
The finest type of the race is found in south Arabia among the Ariba
Arabs, among the mountaineers of Hadramut and Yemen and among the
Bedouin tribes roaming over the interior of central and northern Arabia.
The Arabs of the coasts and those of Mesopotamia are hybrids, showing
Turkish, Negroid and Hamitic crossings. The people of Syria and
Palestine are hybrids of Arab, Phoenician and Jewish descent. The theory
that early Arab settlements were made on the east coast of Africa as far
as Sofala south of the Zambezi, is without foundation; the earliest Arab
settlement on the east coast of Africa that can be proved is Magadoxo
(Mukdishu) in the 10th century, and the ruined cities of Mashonaland,
once supposed to be the remains of Arab settlements, are now known to be
of medieval African origin. On the East African coast-lands Arab
influence is still considerable. Traces of the Arab type are met with in
Asia Minor, the Caucasus, western Persia and India, while the influence
of the Arab language and civilization is found in Europe (Malta and
Spain), China and Central Asia.
Ethnology.
The Arabs are at once the most ancient as they in many ways are the
purest surviving type of the true Semite. Certainly the inhabitants of
Yemen are not, and in historic times never were, pure Semites. Somali
and other elements, generally described under the collective racial name
of Hamitic, are clearly traceable; but the inland Arabs still present
the nearest approach to the primitive Semitic type. The origin of the
Arab race can only be a matter of conjecture. From the remotest historic
times it has been divided into two branches, which from their
geographical position it is simplest to call the North Arabians and the
South Arabians. Arabic and Jewish tradition trace the descent of the
latter from Joktan (Arabic _Kahtan_) son of Heber, of the former from
Ishmael. The South Arabians--the older branch--were settled in the
south-western part of the peninsula centuries before the uprise of the
Ishmaelites. These latter include not only Ishmael's direct descendants
through the twelve princes (Gen. xxv. 16), but the Edomites, Moabites,
Ammonites, Midianites and other tribes. This ancient and undoubted
division of the Arab race --roughly represented to-day by the
universally adopted classification into Arabs proper and Bedouin Arabs
(see BEDOUINS)-has caused much dispute among ethnologists. All
authorities agree in declaring the race to be Semitic in the broadest
ethnological signification of that term, but some thought they saw in
this division of the race an indication of a dual origin. They asserted
that the purer branch of the Arab family was represented by the
sedentary Arabs who were of Hamitic (Biblical Cushite), i.e. African
ancestry, and that the nomad Arabs were Arabs only by adoption, and were
nearer akin to the true Semite as sons of Ishmael. Many arguments were
adduced in support of this theory, (1) The unquestioned division in
remote historic times of the Arab race, and the immemorial hostility
between the two branches. (2) The concurrence of pre-Islamitic
literature and records in representing the first settlement of the
"pure" Arab as made in the extreme south-western part of the peninsula,
near Aden. (3) The use of Himyar, "dusky" or "red" (suggesting African
affinities), as the name sometimes for the ruling class, sometimes for
the entire people. (4) The African affinities of the Himyaritic
language. (5) The resemblance of the grammar of the Arabic now spoken by
the "pure" Arabs, where it differs from that of the North, to the
Abyssinian grammar. (6) The marked resemblance of the pre-Islamitic
institutions of Yemen and its allied provinces-its monarchies, courts,
armies and serfs--to the historical Africo-Egyptian type and even to
modern Abyssinia. (7) The physique of the "pure" Arab, the shape and
size of the head, the slenderness of the lower limbs, all suggesting an
African rather than an Asiatic origin. (8) The habits of the people,
viz. their sedentary rather than nomad occupations, their fondness for
village life, for dancing, music and society, their cultivation of the
soil, having more in common with African life than with that of the
western Asiatic continent. (9) The extreme facility of marriage which
exists in all classes of the southern Arabs with the African races, the
fecundity of such unions and the slightness or even total absence of any
caste feeling between the dusky "pure" Arab and the still darker
African, pointing to a community of origin. And further arguments were
found in the characteristics of the Bedouins, their pastoral and nomad
tendencies; the peculiarities of their idiom allied to the Hebrew; their
strong clan feeling, their continued resistance to anything like regal
power or centralized organization.
Such, briefly, were the more important arguments; but latterly
ethnologists are inclined to agree that there is little really to be
said for the African ancestry theory and that the Arab race had its
beginning in the deserts of south Arabia, that in short the true Arabs
are aborigines.
Mahommedans call the centuries before the Prophet's birth waqt-el
jahiliya, "the time of ignorance," but the fact is that the Arab world
has in some respects never since reached so high a level as it had in
those days which it suits Moslems to paint in dreary colours. Writing
was a fine art and poetry flourished. Eloquence was an accomplishment
all strove to acquire, and each year there were assemblies, lasting
sometimes a month, which were devoted to contests of skill among the
orators and poets, to listen to whose friendly rivalry tribesmen
journeyed long distances. Last, that surest index of a people's
civilization--the treatment of women--contrasted very favourably with
their position under the Koran. Women had rights and were respected. The
veil and the harem system were unknown before Mahomet. According to
Noldeke the Nabataean inscriptions and coins show that women held a high
social position in northern Arabia, owning large estates and trading
independently. Polyandry and polygamy, it is true, were practised, but
the right of divorce belonged to the woman as well as the man. Two kinds
of marriage were celebrated. One was a purely personal contract, with no
witnesses, the wife not leaving her home or passing under marital
authority. The other was a formal marriage, the woman becoming subject
to her husband by purchase or capture. Even captive women were not kept
in slavery. Arabic wealth and culture had indeed thus early reached a
stage which justified Professor Robertson Smith in writing, "In this
period the name of Arab was associated to Western writers with ideas of
effeminate indolence and peaceful opulence ... the golden age of Yemen."
But long before Mahomet's time this early Arab predominance was at an
end, possibly due in great measure to the loss of the caravan trade
through the increase of shipping. The abandonment of great cities and
the ruin of many tribes contributed to the apparent nationalization of
the Arab peoples. Though the traditional jealousy and hostility of the
two branches, the Yemenites and Maadites or Ishmaelites, remained, the
Arab world had attained by the levelling process of common misfortune
the superficial unity it presents to-day. The nation thus formed, never
a nation in the strict sense of the word, was distinctively and
thoroughly Semitic in character and language, and has remained unchanged
to the present day. The sporadic brilliancy of the ancient Arab kingdoms
gave place to a social and political lethargy, the continuation of which
for many centuries made the uprise of Saracenic empires seem a miracle
to a world ignorant of the Arab past. The Arab race up to Mahomet's day
had been in the main pagan. Monotheism, if it ever prevailed, early gave
place to sun and star worship, or simple idolatry. Professor Robertson
Smith suggests that totemism was the earliest form of Arabian idolatry,
and that each tribe had its sacred animal. This he supports by the fact
that some tribal names were derived from those of animals, and that
animal-worship was not unknown in Arabia. What seems certain is that
Arab religion was of a complex hybrid nature, not much to be wondered at
when one remembers that Arabia was the asylum of many religious
refugees, Zoroastrians, Jews, Christians. In the later pre-Islamitic
times spirits, or jinns, as they were called, of which each tribe or
family had its own, were worshipped, and there was but a vague idea of a
Supreme Being. Images of the jinns to the number of 360, one for each
day of the lunar year, were collected in the temple at Mecca, the chief
seat of their worship. That worship was of a sanguinary nature. Human
sacrifice was fairly frequent. Under the guise of religion female
infanticide was a common practice. At Mecca the great object of worship
was a plain black stone, and to it pilgrimages were made from every part
of Arabia. This stone was so sacred to the Arabs that even Mahomet dared
not dispense with it, and it remains the central object of sanctity in
the Ka'ba to-day. The temples of the Sabaeans and the Minaeans were
built east of their cities, a fact suggesting sun-worship, yet this is
not believed to have been the cult of the Minaeans. Common to both was
the worship of Attar, the male Ashtoreth.
With the appearance of Mahomet the Arabs took anew a place in the
world's history.
Physique.
Physically the Arabs are one of the strongest and noblest races of the
world. Baron de Larrey, surgeon-general to Napoleon on his expedition to
Egypt and Syria, writes: "Their physical structure is in all respects
more perfect than that of Europeans; their organs of sense exquisitely
acute, their size above the average of men in general, their figure
robust and elegant, their colour brown; their intelligence proportionate
to their physical perfection and without doubt superior, other things
being equal, to that of other nations." The typical Arab face is of an
oval form, lean-featured; the eyes a brilliant black, deep-set under
bushy eyebrows; nose aquiline, forehead straight but not high. In body
the Arab is muscular and long-limbed, but lean. Deformed individuals or
dwarfs are rare among Arabs; nor, except leprosy, which is common, does
any disease seem to be hereditary among them. They often suffer from
ophthalmia, though not in the virulent Egyptian form. They are
scrupulously clean in their persons, and take special care of their
teeth, which are generally white and even. Simple and abstemious in
their habits, they often reach an extreme yet healthy old age; nor is it
common among them for the faculties of the mind to give way sooner than
those of the body.
Character.
Thus, physically, they yield to few races, if any, of mankind; mentally,
they surpass most, and are only kept back in the march of progress by
the remarkable defect of organizing power and incapacity for combined
action. Lax and imperfect as are their forms of government, it is with
impatience that even these are borne; of the four caliphs who alone
reigned--if reign theirs could be called--in Arabia proper, three died a
violent death; and of the Wahhabi princes, the most genuine
representatives in later times of pure Arab rule, almost all have met
the same fate. The Arab face, which is not unkindly, but never smiling,
expresses that dignity and gravity which are typical of the race. While
the Arab is always polite, good-natured, manly and brave, he is also
revengeful, cruel, untruthful and superstitious. Of the Arab nature
Burckhardt (other authorities, e.g. Barth and Rohlfs, are far less
complimentary) wrote: "The Arab displays his manly character when he
defends his guest at the peril of his own life, and submits to the
reverses of fortune, to disappointment and distress, with the most
patient resignation. He is distinguished from a Turk by the virtues of
pity and gratitude. The Turk is cruel, the Arab of a more kind temper;
he pities and supports the wretched, and never forgets the generosity
shown to him even by an enemy." The Arab will lie and cheat and swear
false oaths, but once his word is pledged he may be trusted to the last.
There are some oaths such as _Wallah_ (by Allah) which mean nothing, but
such an oath as the threefold one with _wa, bi_ and _ta_ as particles of
swearing the meanest thief will not break. In temper, or at least in the
manifestation of it, the Arab is studiously calm; and he rarely so much
as raises his voice in a dispute. But this outward tranquillity covers
feelings alike keen and permanent; and the remembrance of a rash jest or
injurious word, uttered years before, leads only too often to that
blood-revenge which is a sacred duty everywhere in Arabia.
There exist, however, marked tribal or almost semi-national diversities
of character among the Arabs. Thus, the inhabitants of Hejaz are noted
for courtesy and blamed for fickleness; those of Nejd are distinguished
by their stern tenacity and dignity of deportment; the nations of Yemen
are gentle and pliant, but revengeful; those of Hasa and Oman cheerful
and fond of sport, though at the same time turbulent and unsteady.
Anything approaching to a game is rare in Nejd, and in the Hejaz
religion and the yearly occurrence of the pilgrim ceremonies almost
exclude all public diversions; but in Yemen the well-known game of the
"jerid," or palm-stick, with dances and music is not rare. In Oman such
amusements are still more frequent. Again in Yemen and Oman,
coffee-houses, where people resort for conversation, and where public
recitals, songs and other amusements are indulged in, stand open all
day; while nothing of the sort is tolerated in Nejd. So too the
ceremonies of circumcision or marriage are occasions of gaiety and
pastime on the coast, but not in the central provinces.
Manners and customs.
An Arab town, or even village, except it be the merest hamlet, is
invariably walled round; but seldom is a stronger material than dried
earth used; the walls are occasionally flanked by towers of like
construction. A dry ditch often surrounds the whole. The streets are
irregular and seldom parallel. The Arab, indeed, lacks an eye for the
straight. The Arab carpenter cannot form a right angle; an Arab servant
cannot place a cloth square on a table. The Ka'ba at Mecca has none of
its sides or angles equal. The houses are of one or two storeys, rarely
of three, with flat mud roofs, little windows and no external ornament.
If the town be large, the expansion of one or two streets becomes a
market-place, where are ranged a few shops of eatables, drugs, coffee,
cottons or other goods. Many of these shops are kept by women. The chief
mosque is always near the market-place; so is also the governor's
residence, which, except in size and in being more or less fortified
Arab fashion, does not differ from a private house. Drainage is
unthought of; but the extreme dryness of the air obviates the
inconvenience and disease that under other skies could not fail to
ensue, and which in the damper climates of the coast make themselves
seriously felt. But the streets are roughly swept every day, each
householder taking care of the roadway that lies before his own door.
Whitewash and colour are occasionally used in Yemen, Hejaz and Oman;
elsewhere a light ochre tint, the colour of the sun-dried bricks,
predominates, and gives an Arab town the appearance at a distance of a
large dust-heap in the centre of the bright green ring of gardens and
palm-groves. Baked bricks are unknown in Arabia, and stone buildings are
rare, especially in Nejd. Palm branches and the like, woven in wattles,
form the dwellings, of the poorer classes in the southern districts.
Many Arab towns possess watch-towers, like huge round factory chimneys
in appearance, built of sun-dried bricks, and varying in height from 50
to 100 ft. or even more. Indeed, two of these constructions at the town
of Birkat-el-Mauj, in Oman, are said to be each of 170 ft. in height,
and that of Nezwah, in the same province, is reckoned at 140; but these
are of stone.
The principal feature in the interior of an Arab house is the "kahwah"
or coffee-room. It is a large apartment spread with mats, and sometimes
furnished with carpets and a few cushions. At one end is a small furnace
or fireplace for preparing coffee. In this room the men congregate; here
guests are received, and even lodged; women rarely enter it, except at
times when strangers are unlikely to be present. Some of these
apartments are very spacious and supported by pillars; one wall is
usually built transversely to the compass direction of the Ka'ba; it
serves to facilitate the performance of prayer by those who may happen
to be in the kahwah at the appointed times. The other rooms are
ordinarily small.
The Arabs are proverbially hospitable. A stranger's arrival is often the
occasion of an amicable dispute among the wealthier inhabitants as to
who shall have the privilege of receiving him. Arab cookery is of the
simplest. Roughly-ground wheat cooked with butter; bread in thin cakes,
prepared on a heated iron plate or against the walls of an open oven; a
few vegetables, generally of the leguminous kinds; boiled mutton or
camel's flesh, among the wealthy; dates and fruits--this is the _menu_
of an ordinary meal. Rice is eaten by the rich and fish is common on the
coasts. Tea, introduced only a few decades back, is now largely drunk. A
food of which the Arabs are fond is locusts boiled in salt and water and
then dried in the sun. They taste like stale shrimps, but there is a
great sale for them. Spices are freely employed; butter much too largely
for a European taste.
After eating, the hands are always washed, soap or the ashes of an
alkaline plant being used. A covered censer with burning incense is then
passed round, and each guest perfumes his hands, face, and sometimes his
clothes; this censer serves also on first receptions and whenever
special honour is intended. In Yemen and Oman scented water often does
duty for it. Coffee, without milk or sugar, but flavoured with an
aromatic seed brought from India, is served to all. This, too, is done
on the occasion of a first welcome, when the cups often make two or
three successive rounds; but, in fact, coffee is made and drunk at any
time, as frequently as the desire for it may suggest itself; and each
time fresh grains are sifted, roasted, pounded and boiled--a very
laborious process, and one that requires in the better sort of
establishments a special servant or slave for the work. Arabs generally
make but one solid meal a day--that of supper, soon after sunset. Even
then they do not eat much, gluttony being rare among them, and even
daintiness esteemed disgraceful. Wine, like other fermented drinks, is
prohibited by the Koran, and is, in fact, very rarely taken, though the
inhabitants of the mountains of Oman are said to indulge in it. On the
coast spirits of the worst quality are sometimes procured; opium and
hashish are sparingly indulged in. On the other hand, wherever
Wahhabiism has left freedom of action, tobacco-smoking prevails; short
pipes of clay, long pipes with large open bowls, or most frequently the
water-pipe or "nar-ghileh," being used. The tobacco smoked is generally
strong and is either brought from the neighbourhood of Bagdad or grown
in the country itself. The strongest quality is that of Oman; the leaf
is broad and coarse, and retains its green colour even when dried; a few
whiffs have been known to produce absolute stupor. The aversion of the
Wahhabis to tobacco is well known; they entitle it "mukhzi" or "the
shameful," and its use is punished with blows, as the public use of wine
would be elsewhere.
Dress.
In dress much variety prevails. The loose cotton drawers girded at the
waist, which in hot climates do duty for trousers, are not often worn,
even by the upper classes, in Nejd or Yemama, where a kind of silk
dressing-gown is thrown over the long shirt; frequently, too, a brown or
black cloak distinguishes the wealthier citizen; his head-dress is a
handkerchief fastened round the head by a band. But in Hejaz, Yemen and
Oman, turbans are by no means uncommon; the ordinary colour is white;
they are worn over one or more skullcaps. Trousers also form part of the
dress in the two former of these districts; and a voluminous sash, in
which a dagger or an inkstand is stuck, is wrapped round the waist. The
poorer folk, however, and the villagers often content themselves with a
broad piece of cloth round the loins, and another across the shoulders.
In Oman trousers are rare, but over the shirt a long gown, of peculiar
and somewhat close-fitting cut, dyed yellow, is often worn. The women in
these provinces commonly put on loose drawers and some add veils to
their head-dresses; they are over-fond of ornaments (gold and silver);
their hair is generally arranged in a long plait hanging down behind.
All men allow their beards and moustaches full growth, though this is
usually scanty. Most Arabs shave their heads, and indeed all, strictly
speaking, ought by Mahommedan custom to do so. An Arab seldom or never
dyes his hair. Sandals are worn more often than shoes; none but the very
poorest go barefoot.
Slavery.
Slavery is still, as of old times, a recognized institution throughout
Arabia; and an illicit traffic in blacks is carried on along the coasts
of the Persian Gulf and the Red Sea. The slaves themselves were obtained
chiefly from the east African coast districts down as far as Zanzibar,
but this source of supply was practically closed by the end of the 19th
century. Slaves are usually employed in Arabia as herdsmen or as
domestic servants, rarely in agricultural work; they also form a
considerable portion of the bodyguards with which Eastern greatness
loves to surround itself. Like their countrymen elsewhere, they readily
embrace the religion of their masters and become zealous Mahommedans.
Arab custom enfranchises a slave who has accepted Islam at the end of
seven years of bondage, and when that period has arrived, the master,
instead of exacting from his slave the price of freedom, generally, on
giving him his liberty, adds the requisite means for supporting himself
and a family in comfort. Further, on every important occasion, such as a
birth, circumcision, a marriage or a death, one or more of the household
slaves are sure of acquiring their freedom. Hence Arabia has a
considerable free black population; and these again, by inter-marriage
with the whites around, have filled the land with a mulatto breed of
every shade, till, in the eastern and southern provinces especially, a
white skin is almost an exception. In Arabia no prejudice exists against
negro alliances; no social or political line separates the African from
the Arab. A negro may become a sheik, a kadi, an amir, or whatever his
industry and his talents may render him capable of being. This is
particularly so in Nejd, Yemen and Hadramut; in the Hejaz and the north
a faint line of demarcation may be observed between the races.
Military qualities.
The Arabs are good soldiers but poor generals. Personal courage,
wonderful endurance of privation, fixity of purpose, and a contempt of
death are qualities common to almost every race, tribe and clan that
compose the Arab nation. In skirmishing and harassing they have few
equals, while at close quarters they have often shown themselves capable
of maintaining, armed with swords and spears alone, a desperate struggle
against guns and bayonets, neither giving nor receiving quarter. Nor are
they wholly ignorant of tactics, their armies, when engaged in regular
war, being divided into centre and wings, with skirmishers in front and
a reserve behind, often screened at the outset of the engagement by the
camels of the expedition. These animals, kneeling and ranged in long
parallel rows, form a sort of entrenchment, from behind which the
soldiers of the main body fire their matchlocks, while the front
divisions, opening out, act on either flank of the enemy. This
arrangement of troops may be traced in Arab records as far back as the
5th century, and was often exemplified during the Wahhabi wars.
Arab women are scarcely less distinguished for their bravery than the
men. Records of armed heroines occur frequently in the chronicles or
myths of the pre-Islamitic time; and in authentic history the Battle of
the Camel, 656 A.D., where Ayesha, the wife of Mahomet, headed the
charge, is only the first of a number of instances in which Arab amazons
have taken, sword in hand, no inconsiderable share in the wars and
victories of Islam. Even now it is the custom for an Arab force to be
always accompanied by some courageous maiden, who, mounted on a
blackened camel, leads the onslaught, singing verses of encouragement
for her own, of insult for the opposing tribe. Round her litter the
fiercest of the battle rages, and her capture or death is the signal of
utter rout; it is hers also to head the triumph after the victory of her
clan.
Education.
There is little education, in the European sense of the word, in Arabia.
Among the Bedouins there are no schools, and few, even of the most
elementary character, in the towns or villages. Where they exist, little
beyond the mechanical reading of the Koran, and the equally mechanical
learning of it by rote, is taught. On the other hand, Arab
male-children, brought up from early years among the grown-up men of the
house or tent, learn more from their own parents and at home than is
common in other countries; reading and writing are in most instances
thus acquired, or rather transmitted; besides such general principles
of grammar and eloquence, often of poetry and history, as the elders
themselves may be able to impart. To this family schooling too are due
the good manners, politeness, and self-restraint that early distinguish
Arab children. In the very few instances where a public school of a
higher class exists, writing, grammar and rhetoric sum up its teachings.
Law and theology, in the narrow sense that both these words have in the
Islamitic system, are explained in afternoon lectures given in most
mosques; and some verses of the Koran, with one of the accepted
commentaries, that of Baidawi for example, form the basis of the
instruction. Great attention is paid to accuracy of grammar and purity
of diction throughout Arabia; yet something of a dialectic difference
may be observed in the various districts. The purest Arabic, that which
is as nearly as possible identical in the choice of words and in its
inflections with the language of the Koran, is spoken in Nejd, and the
best again of that in the province of Suder. Next in purity comes the
Arabic of Shammar. Throughout the Hejaz in general, the language, though
extremely elegant, is not equally correct; in el-Hasa, Bahrein and Oman
it is decidedly influenced by the foreign element called Nabataean. In
Yemen, as in other southern districts of the peninsula, Arabic merges
insensibly into the Himyaritic or African dialect of Hadramut and Mahra.
(See SEMITIC LANGUAGES.)
BIBLIOGRAPHY.--Lieutenant Wellsted, _Travels in Arabia_ (Lond., 1838);
"Narrative of a Journey to the Ruins of Nakeb el Hajar" (_Jour. R.
Geog. Soc._ vii. 20); Carsten Niebuhr. _Travels through Arabia_
(transl. into English by Robert Heron, 2 vols., Edin., 1792); John
Lewis Burckhardt, _Travels in Arabia_ (2 vols., Lond., 1829); _Notes
on the Bedouins and Wahabis_, (2 vols., Lond., 1830; in German,
Weimar, 1831); C.J. Cruttenden, _Journal of an Excursion to Sana'a,
the Capital of Yemen_ (Bombay, 1838); A. Sprenger, _Die alte
Geographie Arabiens als Grundlage der Entwicklungsgeschichte des
Semitismus_ (Berne, 1875); Sir Richard F. Burton, _Personal Narrative
of a Pilgrimage to El Medinah and Meccah_ (Lond., 1855); W. Robertson
Smith, _Kinship and Marriage in Early Arabia_ (Cambridge); E. Reclus,
_Les Arabes_ (Brussels, 1898); Lady Anne Blunt, _A Pilgrimage to Nejd_
(2 vols., Lond., 1881); C.M. Doughty, _Arabia Deserta_ (2 vols.,
1888); Rev. S.M. Zwemer, _Arabia: the Cradle of Islam_ (1900);
Albrecht Zehme, _Arabien und die Araber, seit hundert Jahren_ (1875).
ARACAJU, a city and seaport of Brazil, capital of the state of Sergipe,
170 m. N.N.E. of Bahia, on the river Cotinguiba, or Cotindiba, 6 m. from
the coast. The municipality, of which it forms a part, had a population
in 1890 of 16,336, about two-thirds of whom lived in the city itself.
Aracaju is a badly built town on the right bank of the river at the base
of a ridge of low sand-hills and has the usual features of an
unprogressive provincial capital. Good limestone is quarried in its
vicinity, and the country tributary to this port produces large
quantities of sugar. Cotton is also grown, and the back country sends
down hides and skins for shipment. The anchorage is good, but a
dangerous bar at the mouth of the river prevents the entrance of vessels
drawing more than 12 ft. The port is visited, therefore, only by the
smaller steamers of the coastwise lines. The river is navigable as far
as the town of Maroim, about 10 m. beyond Aracaju. The city was founded
in 1855.
ARACATY, or ARACATI, a city and port of Brazil, in the state of Ceara,
75 m. S.E. of Fortaleza, on the river Jaguaribe, 8 m. from the sea. Pop.
of the municipality (1890) 20,182, of whom about 12,000 belonged to the
city. A dangerous bar at the mouth of the river permits the entrance
only of the smaller coasting steamers, but the port is an important
commercial centre, and exports considerable quantities of cotton, hides,
manicoba, rubber, fruit, and palm wax.
ARACHNE, in Greek mythology, the daughter of Idmon of Colophon in Lydia,
a dyer in purple. She had acquired such skill in the art of weaving that
she ventured to challenge Athena. While the goddess took as subjects her
quarrel with Poseidon as to the naming and possession of Attica, and the
warning examples of those who ventured to pit themselves against the
immortals, Arachne depicted the metamorphoses of the gods and their
amorous adventures. Her work was so perfect that Athena, enraged at
being unable to find any blemish in it, tore it to pieces. Arachne
hanged herself in despair; but the goddess out of pity loosened the
rope, which became a cobweb, while Arachne herself was changed into a
spider (Ovid, _Metam_. vi. 5-145). The story probably indicates the
superiority of Asia over Greece in the textile arts.
ARACHNIDA, the zoological name given in 1815 by Lamarck (Gr. [Greek:
harachnae], a spider) to a class which he instituted for the reception
of the spiders, scorpions and mites, previously classified by Linnaeus
in the order Aptera of his great group Insecta. Lamarck at the same time
founded the class Crustacea for the lobsters, crabs and water-fleas,
also until then included in the order Aptera of Linnaeus. Lamarck
included the Thysanura and the Myriapoda in his class Arachnida. The
Insecta of Linnaeus was a group exactly equivalent to the Arthropoda
founded a hundred years later by Siebold and Stannius. It was thus
reduced by Lamarck in area, and made to comprise only the six-legged,
wing-bearing "Insecta." For these Lamarck proposed the name Hexapoda;
but that name has been little used, and they have retained to this day
the title of the much larger Linnaean group, viz. Insecta. The position
of the Arachnida in the great sub-phylum Arthropoda, according to recent
anatomical and embryological researches, is explained in the article
ARTHROPODA. The Arachnida form a distinct class or line of descent in
the grade Euarthropoda, diverging (perhaps in common at the start with
the Crustacea) from primitive Euarthropods, which gave rise also to the
separate lines of descent known as the classes Diplopoda, Crustacea,
Chilopoda and Hexapoda.
[Illustration: FIG. 1.--Entosternum, entosternite or plastron of
_Limulus polyphemus_, Latr. Dorsal surface.
LAP, Left anterior process. PLR, Posterior lateral rod or
RAP, Right anterior process. tendon.
PhN, Pharyngeal notch. PLP, Posterior lateral process.
ALR, Anterior lateial rod or tendon. Natural size.
(From Lankester, _Q J. Mic. Sci._, N S vol. xxiv, 1884)]
_Limulus an Arachnid._--Modern views as to the classification and
affinities of the Arachnida have been determined by the demonstration
that _Limulus_ and the extinct Eurypterines (_Pterygotus_, &c.) are
Arachnida; that is to say, are identical in the structure and relation
of so many important parts with _Scorpio_, whilst differing in those
respects from other Arthropoda, that it is impossible to suppose that
the identity is due to homoplasy or convergence, and the conclusion must
be accepted that the resemblances arise from close genetic relationship.
The view that Limulus, the king-crab, is an Arachnid was maintained as
long ago as 1829 by Strauss-Durckheim (1), on the ground of its
possession of an internal cartilaginous sternum--also possessed by the
Arachnida (see figs. 1, 2, 3, 4, 5 and 6)--and of the similarity of the
disposition of the six leg-like appendages around the mouth in the two
cases (see figs. 45 and 63). The evidence of the exact equivalence of
the segmentation and appendages of Limulus and Scorpio, and of a number
of remarkable points of agreement in structure, was furnished by Ray
Lankester in an article published in 1881 ("Limulus an Arachnid,"
_Quart. Journ. Micr. Sci._ vol. xxi. N.S.), and in a series of
subsequent memoirs, in which the structure of the entosternum, of the
coxal glands, of the eyes, of the veno-pericardiac muscles, of the
respiratory lamellae, and of other parts, was for the first time
described, and in which the new facts discovered were shown uniformly to
support the hypothesis that Limulus is an Arachnid. A list of these
memoirs is given at the close of this article (2, 3, 4, 5 and 13). The
Eurypterines (Gigantostraca) were included in the identification,
although at that time they were supposed to possess only five pairs of
anterior or prosomatic appendages. They have now been shown to possess
six pairs (fig. 47), as do Limulus and Scorpio.
[Illustration: FIG. 2.--Ventral surface of the entosternum of _Limulus
polyphemus_, Latr. Letters as in fig. 1 with the addition of NF, neural
fossa protecting the aggregated ganglia of the central nervous system;
PVP, left posterior ventral process; PMP, posterior median process.
Natural size.
(From Lankester.)]
[Illustration: FIG. 3.--Entosternum of scorpion (_Palamnaeus Indus_, de
Geer); dorsal surface.
asp, Paired anterior process of the sub-neural arch.
snp, Sub-neural arch.
ap, Anterior lateral process (same as RAP and LAP in fig. 1).
lmp, Lateral median process (same as ALR and PLR of fig. 1).
pp, Posterior process (same as PLP in fig. 1).
pf, Posterior flap or diaphragm of Newport.
m^1 and m^2, Perforations of the diaphragm for the passage of
muscles.
DR, The paired dorsal ridges.
GC, Gastric canal or foramen.
AC, Arterial canal or foramen.
(After Lankester, _loc. cit._)]
The various comparisons previously made between the structure of Limulus
and the Eurypterines on the one hand, and that of a typical Arachnid,
such as Scorpio, on the other, had been vitiated by erroneous notions as
to the origin of the nerves supplying the anterior appendages of Limulus
(which were finally removed by Alphonse Milne-Edwards in his beautiful
memoir (6) on the structure of that animal), and secondly by the
erroneous identification of the double sternal plates of Limulus, called
"chilaria," by Owen, with a pair of appendages (7). Once the identity of
the chilaria with the pentagonal sternal plate of the scorpion is
recognized--an identification first insisted on by Lankester--the whole
series of segments and appendages in the two animals, Limulus and
Scorpio, are seen to correspond most closely, segment for segment, with
one another (see figs. 7 and 8). The structure of the prosomatic
appendages or legs is also seen to present many significant points of
agreement (see figures), but a curious discrepancy existed in the
six-jointed structure of the limb in Limulus, which differed from the
seven-jointed limb of Scorpio by the defect of one joint. R.I. Pocock of
the British Museum has observed that in Limulus a marking exists on the
fourth joint, which apparently indicates a previous division of this
segment into two, and thus establishes the agreement of Limulus and
Scorpio in this small feature of the number of segments in the legs (see
fig. 11).
[Illustration: FIG. 4.--Ventral surface of the same entosternum as that
drawn in fig. 3. Letters as in fig. 3 with the addition of NC, neural
canal or foramen.
(After Lankester, _loc. cit._)]
It is not desirable to occupy the limited space of this article by a
full description of the limbs and segments of Limulus and Scorpio. The
reader is referred to the complete series of figures here given, with
their explanatory legends (figs 12, 13, 14, 15). Certain matters,
however, require comment and explanation to render the comparison
intelligible. The tergites, or chitinized dorsal halves of the body
rings, are fused to form a "prosomatic carapace," or carapace of the
prosoma, in both Limulus and Scorpio (see figs. 7 and 8). This region
corresponds in both cases to six somites, as indicated by the presence
of six pairs of limbs. On the surface of the carapace there are in
both animals a pair of central eyes with simple lens and a pair of
lateral eye-tracts, which in Limulus consist of closely-aggregated
simple eyes, forming a "compound" eye, whilst in Scorpio they present
several separate small eyes. The microscopic structure of the central
and the lateral eyes has been shown by Lankester and A.G. Bourne (5)
to differ; but the lateral eyes of Scorpio were shown by them to be
similar in structure to the lateral eyes of Limulus, and the central
eyes of Scorpio to be identical in structure with the central eyes of
Limulus (see below).
[Illustration: FIG. 5.--Entosternum of one of the mygalomorphous
spiders; ventral surface. Ph.N., pharyngeal notch. The posterior
median process with its repetition of triangular segments closely
resembles the same process in Limulus.
(From Lankester, _loc. cit._)]
[Illustration: FIG. 6.--Dorsal surface of the same entosternum as that
drawn in fig. 5. Ph.N., pharyngeal notch.
(After Lankester, _loc. cit._)]
Following the prosoma is a region consisting of six segments (figs. 14
and 15), each carrying a pair of plate-like appendages in both Limulus
and Scorpio. This region is called the mesosoma. The tergites of this
region and those of the following region, the metasoma, are fused to
form a second or posterior carapace in Limulus, whilst remaining free
in Scorpio. The first pair of foliaceous appendages in each animal is
the genital operculum; beneath it are found the openings of the
genital ducts. The second pair of mesosomatic appendages in Scorpio
are known as the "pectens." Each consists of an axis, bearing numerous
blunt tooth-like processes arranged in a series. This is represented
in Limulus by the first gill-bearing appendage. The leaves (some 150
in number) of the gill-book (see figure) correspond to the tooth-like
processes of the pectens of Scorpio. The next four pairs of appendages
(completing the mesosomatic series of six) consist, in both Scorpio
and Limulus, of a base carrying each 130 to 150 blood-holding,
leaf-like plates, lying on one another like the leaves of a book.
Their minute structure is closely similar in the two cases; the
leaf-like plates receive blood from the great sternal sinus, and serve
as respiratory organs. The difference between the gill-books of
Limulus and the lung-books of Scorpio depends on the fact that the
latter are adapted to aerial respiration, while the former serve for
aquatic respiration. The appendage carrying the gill-book stands out
on the surface of the body in Limulus, and has other portions
developed besides the gill-book and its base; it is fused with its
fellow of the opposite side. On the other hand, in Scorpio, the
gill-book-bearing appendage has sunk below the surface, forming a
recess or chamber for itself, which communicates with the exterior by
an oval or circular "stigma" (fig. 10, stg). That this in-sinking
has taken place, and that the lung-books or in-sunken gill-books of
Scorpio really represent appendages (that is to say, limbs or
parapodia) is proved by their developmental history (see figs. 17 and
18). They appear at first as outstanding processes on the surface of
the body.
The exact mode in which the in-sinking of superficial outstanding
limbs, carrying gill-lamellae, has historically taken place has been a
matter of much speculation. It was to be hoped that the specimen of
the Silurian scorpion (_Palaeophonus_) from Scotland, showing the
ventral surface of the mesosoma (fig. 49), would throw light on this
matter; but the specimen recently carefully studied by the writer and
Pocock reveals neither gill-bearing limbs nor stigmata. The
probability appears to be against an actual introversion of the
appendage and its lamellae, as was at one time suggested by Lankester.
It is probable that such an in-sinking as is shown in the accompanying
diagram has taken place (fig. 15); but we are yet in need of evidence
as to the exact equivalence of margins, axis, &c., obtaining between
the lung-book of Scorpio and the gill-book of Limulus. Zoologists are
familiar with many instances (fishes, crustaceans) in which the
protective walls of a water-breathing organ or gill-apparatus become
converted into an air-breathing organ or lung, but there is no other
case known of the conversion of gill processes themselves into
air-breathing plates.
[Illustration: FIG. 7.--Diagram of the dorsal surface of _Limulus
polyphemus_.
oc, Lateral compound eyes.
oc', Central monomeniscous eyes.
PA, Post-anal spine.
I to VI, The six appendage-bearing somites of the prosoma.
VII, Usually considered to be the tergum of the genital somite, but
suggested by Pocock to be that of the otherwise suppressed
praegenital somite.
VIII to XIII, The six somites of the mesosoma, each with a movable
pleural spine and a pair of dorsal entopophysis or muscle-attaching
ingrowths.
XIV to XVIII, The confluent or unexpressed six somites of the
metasoma.
[According to the system of numbering explained in the text, if VII
is the tergum of the praegenital somite (as is probable) it should
be labelled _Prg_ without any number, and the somites VIII to XIII
should be lettered 1 to 6, indicating that they are the six normal
somites of the mesosoma; whilst XV to XVIII should be replaced by
the numbers 7 to 12--an additional suppressed segment (making up the
typical six) being reckoned to the metasomatic fusion.]
(From Lankester, _Q J. Micr Set_. vol. xxi., 1881)]
The identification of the lung-books of Scorpio with the gill-books of
Limulus is practically settled by the existence of the pectens in
Scorpio (fig. 14, VIII) on the second mesosomatic somite. There is no
doubt that _these_ are parapodial or limb appendages, carrying
numerous imbricated secondary processes, and therefore comparable in
essential structure to the leaf-bearing plates of the second
mesosomatic somite of Limulus. They have remained unenclosed and
projecting on the surface of the body, as once were the appendages of
the four following somites. But they have lost their respiratory
function. In non-aquatic life such an unprotected organ cannot
subserve respiration. The "pectens" have become more firmly chitinized
and probably somewhat altered in shape as compared with their
condition in the aquatic ancestral scorpions. Their present function
in scorpions is not ascertained. They are not specially sensitive
under ordinary conditions, and may be touched or even pinched without
causing any discomfort to the scorpion. It is probable that they
acquire special sensibility at the breeding season and serve as
"guides" in copulation. The shape of the legs and the absence of
paired terminal claws in the Silurian _Palaeophonus_ (see figs. 48 and
49) as compared with living scorpions (see fig. 10) show that the
early scorpions were aquatic, and we may hope some day in
better-preserved specimens than the two as yet discovered, to find the
respiratory organs of those creatures in the condition of projecting
appendages serving aquatic respiration somewhat as in Limulus, though
not necessarily repeating the exact form of the broad plates of
Limulus.
[Illustration: FIG. 8.--Diagram of the dorsal surface of a scorpion to
compare with fig. 7. Letters and Roman numerals as in fig. 7,
excepting that VII is here certainly the tergum of the first somite of
the mesosoma--the genital somite--and is _not_ a survival of the
embryonic praegenital somite. The anus (not seen) is on the sternal
surface.
(From Lankester, _loc. cit._)]
It is important to note that the series of lamellae of the lung-book
and the gill-book correspond _exactly_ in structure, the narrow, flat
blood-space in the lamellae being interrupted by pillar-like junctions
of the two surfaces in both cases (see Lankester (4)), and the free
surfaces of the adjacent lamellae being covered with a very delicate
chitinous cuticle which is drawn out into delicate hairs and
processes. The elongated axis which opens at the stigma in Scorpio and
which can be cleared of soft, surrounding tissues and coagulated blood
so as to present the appearance of a limb axis carrying the book-like
leaves of the lung is not really, as it would seem to be at first
sight, the limb axis. That is necessarily a blood-holding structure
and is obliterated and fused with soft tissues of the sternal region
so that the lamellae cannot be detached and presented as standing out
from it. The apparent axis or basal support of the scorpion's
lung-books shown in the figures, is a false or secondary axis and
merely a part of the infolded surface which forms the air-chamber. The
maceration of the soft parts of a scorpion preserved in weak spirit
and the cleaning of the chitinized in-grown cuticle give rise to the
false appearance of a limb axis carrying the lamellae. The margins of
the lamellae of the scorpion's lung-book, which are _lowermost_ in the
figures (fig. 15) and appear to be free, are really those which are
attached to the blood-holding axis. The true free ends are those
nearest the stigma.
Passing on now from the mesosoma we come in Scorpio to the metasoma of
six segments, the first of which is broad whilst the rest are
cylindrical. The last is perforated by the anus and carries the
post-anal spine or sting. The somites of the metasoma carry no
parapodia. In Limulus the metasoma is practically suppressed. In the
allied extinct Eurypterines it is well developed, and resembles that
of Scorpio. In the embryo Limulus (fig. 42) the six somites of the
mesosoma are not fused to form a carapace at an early stage, and they
are followed by three separately marked metasomatic somites; the other
three somites of the metasoma have disappeared in Limulus, but are
represented by the unsegmented prae-anal region. It is probable that
we have in the metasoma of Limulus a case of the disappearance of once
clearly demarcated somites. It would be possible to suppose, on the
other hand, that new somites are only beginning to make their
appearance here. The balance of various considerations is against the
latter hypothesis. Following the metasoma in Limulus, we have as in
Scorpio the post-anal spine--in this case not a sting, but a powerful
and important organ of locomotion, serving to turn the animal over
when it has fallen upon its back. The nature of the post-anal spine
has been strangely misinterpreted by some writers. Owen (7) maintained
that it represented a number of coalesced somites, regardless of its
post-anal position and mode of development. The agreement of the
grouping of the somites, of the form of the parapodia (appendages,
limbs) in each region, of the position of the genital aperture and
operculum, of the position and character of the eyes, and of the
powerful post-anal spines not seen in other Arthropods, is very
convincing as to the affinity of Limulus and Scorpio. Perhaps the
most important general agreement of Scorpio compared with Limulus and
the Eurypterines is the division of the body into the three regions
(or tagmata)--prosoma, mesosoma and metasoma--each consisting of six
segments, the prosoma having leg-like appendages, the mesosoma having
foliaceous appendages, and the metasoma being destitute of appendages.
[Illustration: FIG. 9.--Ventral view of the posterior carapace or
meso-metasomatic (opisthospmatic) fusion of _Limulus polyphemus_. The
soft integument and limbs of the mesosoma have been removed as well as
all the viscera and muscles, so that the inner surface of the terga of
these somites with their entopophyses are seen. The unsegmented dense
chitinous sternal plate of the metasoma (XIII to XVIII) is not
removed. Letters as in fig. 7.
(After Lankester, _loc. cit._)]
In 1893, some years after the identification of the somites of Limulus
with those of Scorpio, thus indicated, had been published, zoologists
were startled by the discovery by a Japanese zoologist, Kishinouye
(8), of a seventh prosomatic somite in the embryo of Limulus
longispina. This was seen in longitudinal sections, as shown in fig.
19. The simple identification of somite with somite in Limulus and
Scorpio seemed to be threatened by this discovery. But in 1896 Dr
August Brauer of Marburg (9) discovered in the embryo of Scorpio a
seventh prosomatic somite (see VII PrG, figs. 17 and 18), or, if we
please so to term it, a _praegenital_ somite, hitherto unrecognized.
In the case of Scorpio this segment is indicated in the embryo by the
presence of a pair of rudimentary appendages, carried by a well-marked
somite. As in Limulus, so in Scorpio, this unexpected somite and its
appendages disappear in the course of development. In fact, more or
less complete "excalation" of the somite takes place. Owing to its
position it is convenient to term the somite which is excalated in
Limulus and Scorpio "the praegenital somite." It appears not
improbable that the sternal plates wedged in between the last pair of
legs in both Scorpio and Limulus, viz. the pentagonal sternite of
Scorpio (fig. 10) and the chilaria of Limulus (see figs. 13 and 20),
may in part represent in the adult the sternum of the excalated
praegenital somite. This has not been demonstrated by an actual
following out of the development, but the position of these pieces and
the fact that they are (in Limulus) supplied by an independent
segmental nerve, favours the view that they may comprise the sternal
area of the vanished praegenital somite. This interpretation, however,
of the "metasternites" of Limulus and Scorpio is opposed by the
coexistence in Thelyphonus (figs. 55, 57 and 58) of a similar
metasternite with a complete praegenital somite. H.J. Hansen (10) has
recognized that the "praegenital somite" persists in a rudimentary
condition, forming a "waist" to the series of somites in the Pedipalpi
and Araneae. The present writer is of opinion that it will be found
most convenient to treat this evanescent somite as something special,
and not to attempt to reckon it to either the prosoma or the mesosoma.
These will then remain as typically composed each of six
appendage-bearing somites-the prosoma comprising in addition the
ocular prosthomere.[1] When the praegenital somite or traces of it are
present it should not be called "the seventh prosomatic" or the "first
mesosomatic," but simply the "praegenital somite." The first segment
of the mesosoma of Scorpio and Limulus thus remains the first segment,
and can be identified as such throughout the Eu-arachnida, carrying as
it always does the genital apertures. But it is necessary to remember,
in the light of recent discoveries, that the sixth prosomatic pair of
appendages is carried on the seventh somite of the whole series, there
being two prosthomeres or somites in front of the mouth, the first
carrying the eyes, the second the chelicerae; also that the first
mesosomatic or genital somite is not the seventh or even the eighth of
the whole senes of somites which have been historically present, but
is the ninth, owing to the presence or to the excalation of a
praegenital somite. It seems that confusion and trouble will be best
avoided by abstaining from the introduction of the non-evident
somites, the ocular and the praegenital, into the numerical
nomenclature of the component somites of the three great body regions.
We shall, therefore, ignoring the ocular somite, speak of the first,
second, third, fourth, fifth and sixth leg-bearing somites of the
prosoma, and indicate the appendages by the Roman numerals, I, II,
III, IV, V, VI, and whilst ignoring the praegenital somite we shall
speak of the first, second, third, &c., somite of the mesosoma or
opisthosoma (united mesosoma and metasoma) and indicate them by the
Arabic numerals.
[Illustration: FIG. 10.--Ventral view of a scorpion, _Palamnaeus
indus_, de Geer, to show the arrangement of the coxae of the limbs,
the sternal elements, genital plate and pectens.
M, Mouth behind the oval median camerostome.
I, The chelicerae.
II, The chelae.
III to VI, the four pairs of walking legs.
VIIgo, The genital somite or first somite of the mesosoma with the
genital operculum (a fused pair of limbs).
VIIIp, The pectiniferous somite.
IXstg to XIIstg, the four pulmonary somites.
met, The pentagonal metasternite of the prosoma behind all the
coxae.
x, The sternum of the pectiniferous somite.
y, The broad first somite of the metasoma.]
There are a number of other important points of structure besides
those referring to the somites and appendages in which Limulus agrees
with Scorpio or other Arachnida and differs from other Arthropoda. The
chief of these are as follows:--
[Illustration: FIG. 11.--Third leg of _Limulus polyphemus_, showing
the division of the fourth segment of the leg by a groove S into two,
thus giving seven segments to the leg as in scorpion.
(From a drawing by Pocock.)]
1. _The Composition of the Head_ (that is to say, of the anterior part
of the prosoma) _with especial Reference to the Region in Front of the
Mouth._--It appears (see ARTHROPODA) that there is embryological
evidence of the existence of two somites in Arachnida which were
originally post-oral, but have become prae-oral by adaptational
shifting of the oral aperture. These forwardly-slipped somites are
called "prosthomeres." The first of these has, in Arachnids as in
other Arthropods, its pair of appendages represented by the eyes. The
second has for its pair of appendages the small pair of limbs which in
all living Arachnids is either chelate or retrovert (as in spiders),
and is known as the chelicerae. It is possible, as maintained by some
writers (Patten and others), that the lobes of the cerebral nervous
mass in Arachnids indicate a larger number of prosthomeres as having
fused in this region, but there is no _embryological_ evidence at
present which justifies us in assuming the existence in Arachnids of
more than two prosthomeres. The position of the chelicerae of Limulus
and of the ganglionic nerve-masses from which they receive their
nerve-supply, is closely similar to that of the same structures in
Scorpio. The cerebral mass is in Limulus more easily separated by
dissection as a median lobe distinct from the laterally-placed ganglia
of the chelceral somite than is the case in Scorpio, but the relations
are practically the same in the two forms. Formerly it was supposed
that in Limulus both the chelicerae and the next following pair of
appendages were prosthomerous, as in Crustacea, but the dissections of
Alphonse Milne-Edwards (6) demonstrated the true limitations of the
cerebrum, whilst embryological researches have done as much for
Scorpio. Limulus thus agrees with Scorpio and differs from the
Crustacea, in which there are three prosthomeres--one ocular and two
carrying palpiform appendages. It is true that in the lower Crustacea
(Apus, &c.) we have evidence of the gradual movement forward of the
nerve-ganglia belonging to these palpiform appendages. But although in
such lower Crustacea the nerve-ganglia of the third prosthomere have
not fused with the anterior nerve-mass, there is no question as to the
prae-oral position of two appendage-bearing somites in addition to the
ocular prosthomere. The Crustacea have, in fact, three prosthomeres in
the head and the Arachnida only two, and Limulus agrees with the
Arachnida in this respect and differs from the Crustacea. The central
nervous systems of Limulus and of Scorpio present closer agreement in
structure than can be found when a Crustacean is compared with either.
The wide divarication of the lateral cords in the prosoma and their
connexion by transverse commissures, together with the "attraction" of
ganglia to the prosomatic ganglion group which properly belong to
hinder segments, are very nearly identical in the two animals. The
form and disposition of the ganglion cells are also peculiar and
closely similar in the two. (See Patten (42) for important
observations on the neuromeres, &c., of Limulus and Scorpio.)
[Illustration: FIG. 12.--The prosomatic appendages of _Limulus
polyphemus_ (right) and Scorpio (left), _Palamnaeus indus_ compared.
The corresponding appendages are marked with the same Roman numeral.
The Arabic numerals indicate the segments of the legs.
cox, Coxa or basal segment of the leg.
stc, The sterno-coxal process or jaw-like up-growth of the coxa.
epc, The articulated movable outgrowth of the coxa, called the
epi-coxite (present only in III of the scorpion and III, IV and V
of Limulus).
ex^1, The exopodite of the sixth limb of Limulus.
a, b, c, d, Movable processes on the same leg (see for some
suggestions on the morphology of this leg, Pocock in _Quart. Journ.
Micr. Sci._ March 1901; see also fig. 50 below and explanation).
(From Lankester, _loc. cit._)]
[Illustration: FIG. 13.--Diagrams of the metasternite st, with genital
operculum op, and the first lamelligerous pair of appendages ga, with
uniting sternal element st of Scorpio (left) and Limulus (right).
(From Lankester, _loc. cit._)]
2. _The Minute Structure of the Central Eyes and of the Lateral
Eyes._--Limulus agrees with Scorpio not only in having a pair of
central eyes and also lateral eyes, but in the microscopic structure
of those organs, which differs in the central and lateral eyes
respectively. The central eyes are "simple eyes," that is to say, have
a single lens, and are hence called "monomeniscous." The lateral eyes
are in Limulus "compound eyes," that is to say, consist of many lenses
placed close together; beneath each lens is a complex of protoplasmic
cells, in which the optic nerve terminates. Each such unit is termed
an "ommatidium." The lateral eyes of Scorpio consist of groups of
separate small lenses each with its ommatidium, but they do not form a
continuous compound eye as in Limulus. The ommatidium (soft structure
beneath the lens-unit of a compound eye) is very simple in both
Scorpio and Limulus. It consists of a single layer of cells,
continuous with those which secrete the general chitinous covering of
the prosoma. The cells of the ommatidium are a good deal larger than
the neighbouring common cells of the epidermis. They secrete the
knob-like lens (fig. 22). But they also receive the nerve fibres of
the optic nerve. They are at the same time both optic nerve-end cells,
that is to say, retina cells, and corneagen cells or secretors of the
chitinous lens-like cornea. In Limulus (fig. 23) each ommatidium has a
peculiar ganglion cell developed in a central position, whilst the
ommatidium of the lateral eyelets of Scorpio shows small intermediate
cells between the larger nerve-end cells. The structure of the lateral
eye of Limulus was first described by Grenacher, and further and more
accurately by Lankester and Bourne (5) and by Watase; that of Scorpio
by Lankester and Bourne, who showed that the statements of von Graber
were erroneous, and that the lateral eyes of Scorpio have a single
cell-layered or "monostichous" ommatidium like that of Limulus. Watase
has shown, in a very convincing way, how by deepening the pit-like set
of cells beneath a simple lens the more complex ommatidia of the
compound eyes of Crustacea and Hexapoda may be derived from such a
condition as that presented in the lateral eyes of Limulus and
Scorpio. (For details the reader is referred to Watase (11) and to
Lankester and Bourne (5).) The structure of the central eyes of
Scorpio and spiders and also of Limulus differs essentially from that
of the lateral eyes in having two layers of cells (hence called
diplostichous) beneath the lens, separated from one another by a
membrane (figs. 24 and 25). The upper layer is the corneagen and
secretes the lens, the lower is the retinal layer. The mass of soft
cell-structures beneath a large lens of a central eye is called an
"ommatoeum." It shows in Scorpio and Limulus a tendency to segregate
into minor groups or "ommatidia." It is found that in embryological
growth the retinal layer of the central eyes forms as a separate
pouch, which is pushed in laterally beneath the corneagen layer from
the epidermic cell layer. Hence it is in origin double, and consists
of a true retinal layer and a post-retinal layer (fig. 24, B), though
these are not separated by a membrane. Accordingly the diplostichous
ommatoeum or soft tissue of the Arachnid's central eye should strictly
be called "triplostichous," since the deep layer is itself doubled or
folded. The retinal cells of both the lateral and central eyes of
Limulus and Scorpio produce cuticular structures on their sides; each
such piece is a rhabdomere and a number (five or ten) uniting form a
rhabdom (fig. 26). In the specialized ommatidia of the compound eyes
of Crustacea and Hexapods the rhabdom is an important structure.[2] It
is a very significant fact that the lateral and central eyes of
Limulus and Scorpio not only agree each with each in regard to their
monostichous and diplostichous structure, but also in the formation in
both classes of eyes of rhabdomeres and rhabdoms in which the
component pieces are five or a multiple of five (fig. 26). Whilst each
unit of the lateral eye of Limulus has a rhabdom of ten[3] pieces
forming a star-like chitinous centre in section, each lateral eye of
Scorpio has several rhabdoms of five or less rhabdomeres, indicating
that the Limulus lateral eye-unit is more specialized than the
detached lateral eyelet of Scorpio, so as to present a coincidence of
one lens with one rhabdom. Numerous rhabdomeres (grouped as rhabdoms
in Limulus) are found in the retinal layer of the central eyes also.
[Illustration: FIG. 14.--The first three pairs of mesosomatic
appendages of Scorpio and Limulus compared.
VII, The genital operculum.
VIII, The pectens of Scorpio and the first branchial plate of
Limulus.
IX, The first pair of lung-books of Scorpio and the second branchial
plate of Limulus.
gp, Genital pore.
epst, Epistigmatic sclerite.
stg, Stigma or orifice of the hollow tendons of the branchial plates
of Limulus.
(After Lankester, _loc. cit._)]
Whilst Limulus agrees thus closely with Scorpio in regard to the eyes,
it is to be noted that no Crustacean has structures corresponding to
the peculiar diplostichous central eyes, though these occur again
(with differences in detail) in _Hexapoda_. Possibly, however, an
investigation of the development of the median eyes of some Crustacea
(Apus, Palaemon) may prove them to be diplostichous in origin.
3. _The so-called_ "_Coxal Glands_."--In 1882 (_Proc. Roy. Soc_. No.
221) Lankester described under the name "coxal glands" a pair of
brilliantly white oviform bodies lying in the Scorpion's prosoma
immediately above the coxae of the fifth and sixth pairs of legs (fig.
27). These bodies had been erroneously supposed by Newport (12) and
other observers to be glandular outgrowths of the alimentary canal.
They are really excretory glands, and communicate with the exterior by
a very minute aperture on the posterior face of the coxa of the fifth
limb on each side. When examined with the microscope, by means of the
usual section method, they are seen to consist of a labyrinthine tube
lined with peculiar cells, each cell having a deep vertically striated
border on the surface farthest from the lumen, as is seen in the cells
of some renal organs. The coils and branches of the tube are packed by
connective tissue and blood spaces. A similar pair of coxal glands,
lobate instead of ovoid in shape, was described by Lankester in
Mygale, and it was also shown by him that the structures in Limulus
called "brick-red glands" by Packard have the same structure and
position as the coxal glands of Scorpio and Mygale. In Limulus these
organs consist each of four horizontal lobes lying on the coxal margin
of the second, third, fourth, and fifth prosomatic limbs, the four
lobes being connected to one another by a transverse piece or stem
(fig. 28). Microscopically their structure is the same in essentials
as that of the coxal glands of Scorpio (13). Coxal glands have since
been recognized and described in other Arachnida. In 1900 it was shown
that the coxal gland of Limulus is provided with a very delicate
thin-walled coiled duct which opens, even in the adult condition, by a
minute pore on the coxa of the fifth leg (Patten and Hazen, 13A).
Previously to this, Lankester's pupil Gulland had shown (1885) that in
the embryo the coxal gland is a comparatively simple tube, which opens
to the exterior in this position and by its other extremity into a
coelomic space. Similar observations were made by Laurie (17) in
Lankester's laboratory (1890) with regard to the early condition of
the coxal gland of Scorpio, and by Bertkau (41) as to that of the
spider Atypus. H.M. Bernard (13B) showed that the opening remains in
the adult scorpion. In all the embryonic or permanent opening is on
the coxa of the fifth pair of prosomatic limbs. Thus an organ newly
discovered in Scorpio was found to have its counterpart in Limulus.
The name "coxal gland" needs to be carefully distinguished from
"crural gland," with which it is apt to be confused. The crural
glands, which occur in many terrestrial Arthropods, are epidermal in
origin and totally distinct from the coxal glands. The coxal glands of
the Arachnida are structures of the same nature as the green glands of
the higher Crustacea and the so-called "shell glands" of the
Entomostraca. The latter open at the base of the fifth pair of limbs
of the Crustacean, just as the coxal glands open on the coxal joint of
the fifth pair of limbs of the Arachnid. Both belong to the category
of "coelomoducts," namely, tubular or funnel-like portions of the
coelom opening to the exterior in pairs in each somite (potentially,)
and usually persisting in only a few somites as either "urocoels"
(renal organs) or "gonocoets" (genital tubes). In Peripatus they occur
in every somite of the body. They have till recently been very
generally identified with the nephridia of Chaetopod worms, but there
is good reason for considering the true nephridia (typified by the
nephridia of the earthworm) as a distinct class of organs (see
Lankester in vol. ii. chap. in. of _A Treatise on Zoology_, 1900). The
genital ducts of Arthropoda are, like the green glands, shell glands
and coxal glands, to be regarded as coelomoducts (gonocoels). The
coxal glands do not establish any special connexion between Limulus
and Scorpio, since they also occur in the same somite in the lower
Crustacea, but it is to be noted that the coxal glands of Limulus are
in minute structure and probably in function more like those of
Arachnids than those of Crustacea.
[Illustration: FIG. 15.--The remaining three pairs of mesosomatic
appendages of Scorpio and Limulus. Letters as in fig. 14. l130
indicates that there are 130 lamellae in the scorpion's lung-book,
whilst l150 indicates that 150 similar lamellae are counted in the
gill of Limulus.
(After Lankester, _loc. cit._)]
4. _The Entosternites and their Minute Structure._--Strauss-Durckheim
(1) was the first to insist on the affinity between Limulus and the
Arachnids, indicated by the presence of a free suspended entosternum
or plastron or entosternite in both. We have figured here (figs. 1 to
6) the entosternites of Limulus, Scorpio and Mygale. Lankester some
years ago made a special study of the histology (3) of these
entosternites for the purpose of comparison, and also ascertained the
relations of the very numerous muscles which are inserted into them
(4). The entosternites are cartilaginous in texture, but they have
neither the chemical character nor the microscopic structure of the
hyaline cartilage of Vertebrates. They yield chitin in place of
chondrin or gelatin--as does also the cartilage of the Cephalopod's
endoskeleton. In microscopic structure they all present the closest
agreement with one another. We find a firm, homogeneous or sparsely
fibrillated matrix in which are embedded nucleated cells (corpuscles
of protoplasm) arranged in rows of three, six or eight, parallel with
the adjacent lines of fibrillation.
[Illustration: FIG. 16.--Diagram to show the way in which an
outgrowing gill-process bearing blood-holding lamellae, may give rise,
if the sternal body wall sinks inwards, to a lung-chamber with
air-holding lamellae.
I is the embryonic condition.
bs, Blood sinus.
L is the condition of outgrowth with gl, gill lamellae.
A is the condition of in-sinking of the sternal surface and
consequent enclosure of the lamelligerous surface of the appendage
in a chamber with narrow orifice--the pulmonary air-holding
chamber.
pl, Pulmonary lamellae.
bs, Blood sinus.
(After Kingsley.)]
A minute entosternite having the above-described structure is found in
the Crustacean Apus between the bases of the mandibles, and also in
the Decapoda in a similar position, but in no Crustacean does it
attain to any size or importance. On the other hand, the entosternite
of the Arachnida is a very large and important feature in the
structure of the prosoma, and must play an important part in the
economy of these organisms. In Limulus (figs. 1 and 2) it has as many
as twenty-five pairs of muscles attached to it, coming to it from the
bases of the surrounding limbs and from the dorsal carapace and from
the pharynx. It consists of an oblong plate 2 in. in length and 1 in
breadth, with a pair of tendinous outgrowths standing out from it at
right angles on each side. It "floats" between the prosomatic nerve
centres and the alimentary canal. In each somite of the mesosoma is a
small, free entosternite having a similar position, but below or
ventral to the nerve cords, and having a smaller number of muscles
attached to it. The entosternite was probably in origin part of the
fibrous connective tissue lying close to the integument of the sternal
surface--giving attachment to muscles corresponding more or less to
those at present attached to it. It became isolated and detached, why
or with what advantage to the organism it is difficult to say, and at
that period of Arachnidan development the great ventral nerve cords
occupied a more lateral position than they do at present. We know that
such a lateral position of the nerve cords preceded the median
position in both Arthropoda and Chaetopoda. Subsequently to the
floating off of the entosternite the approximation of the nerve cords
took place in the prosoma, and thus they were able to take up a
position below the entosternite. In the mesosoma the approximation had
occurred before the entosternites were formed.
[Illustration: FIG. 17.--Embryo of scorpion, ventral view showing
somites and appendages.
sgc, Frontal groove.
sa, Rudiment of lateral eyes.
obl, Camerostome (upper lip).
so, Sense-organ of Patten.
PrGabp^1, Rudiment of the appendage of the praegenital somite which
disappears.
abp^2, Rudiment of the right half of the genital operculum.
abp^3, Rudiment of the right pecten.
abp^4 to abp^7. Rudiments of the four appendages which carry the
pulmonary lamellae.
I to VI, Rudiments of the six limbs of the prosoma.
VIIPrG, The evanescent praegenital somite.
VIII, The first mesosomatic somite or genital somite.
IX, The second mesosomatic somite or pectiniferous somite.
X to XIII, The four pulmoniferous somites.
XIV, The first metasomatic somite.
(After Brauer, _Zeitsch. wiss. Zool_., vol. lix., 1895.)]
In the scorpion (figs. 3 and 4) the entosternite has tough
membrane-like outgrowths which connect it with the body-wall, both
dorsally and ventrally forming an oblique diaphragm, cutting off the
cavity of the prosoma from that of the mesosoma. It was described by
Newport as "the diaphragm." Only the central and horizontal parts of
this structure correspond precisely to the entosternite of Limulus:
the right and left anterior processes (marked ap in figs. 3 and 4, and
RAP, LAP, in figs. 1 and 2) correspond in the two animals, and the
median lateral process _lmp_ of the scorpion represents the tendinous
outgrowths ALR, PLR of Limulus. The scorpion's entosternite gives rise
to outgrowths, besides the great posterior flaps, pf, which form the
diaphragm, unrepresented in Limulus. These are a ventral arch forming
a neural canal through which the great nerve cords pass (figs. 3 and
4, _snp_), and further a dorsal gastric canal and arterial canal which
transmit the alimentary tract and the dorsal artery respectively
(figs. 3 and 4, GC, DR).
[Illustration: FIG. 18.--Portion of a similar embryo at a later stage
of growth. The praegenital somite, VII PrG, is still present, but has
lost its rudimentary appendages; go, the genital operculum, left half;
Km, the left pecten; abp^4 to abp^7, the rudimentary appendages of
the lung-sacs.
(After Brauer, _loc. cit_.)]
In Limulus small entosternites are found in each somite of the
appendage-bearing mesosoma, and we find in Scorpio, in the only somite
of the mesosoma which has a well-developed pair of appendages, that of
the pectens, a small entosternite with ten pairs of muscles inserted
into it. The supra-pectinal entosternite lies ventral to the nerve
cords.
In Mygale (figs. 5 and 6) the form of the entosternite is more like
that of Limulus than is that of Scorpio. The anterior notch Ph.N. is
similar to that in Limulus, whilst the imbricate triangular pieces of
the posterior median region resemble the similarly-placed structures
of Limulus in a striking manner.
[Illustration: FIG. 19.--Section through an early embryo of _Limulus
longispina_, showing seven transverse divisions in the region of the
unsegmented anterior carapace. The seventh, VII, is anterior to the
genital operculum, op, and is the cavity of the praegenital somite
which is more or less completely suppressed in subsequent development,
possibly indicated by the area marked VII in fig. 7 and by the great
entopophyses of the prosomatic carapace.
(After Kishinouye, _Journ. Sci. Coll. Japan_, vol. v., 1892.)]
It must be confessed that we are singularly ignorant as to the
functional significance of these remarkable organs--the entosternites.
Their movement in an upward or downward direction in Limulus and
Mygale must exert a pumping action on the blood contained in the
dorsal arteries and the ventral veins respectively. In Scorpio the
completion of the horizontal plate by oblique naps, so as to form an
actual diaphragm shutting off the cavity of the prosoma from the rest
of the body, possibly gives to the organs contained in the anterior
chamber a physiological advantage in respect of the supply of arterial
blood and its separation from the venous blood of the mesosoma.
Possibly the movement of the diaphragm may determine the passage of
air into or out of the lung-sacs. Muscular fibres connected with the
suctorial pharynx are in Limulus inserted into the entosternite, and
the activity of the two organs may be correlated.
5. _The Blood and the Blood-vascular System._--The blood fluids of
Limulus and Scorpio are very similar. Not only are the blood
corpuscles of Limulus more like in form and granulation to those of
Scorpio than to those of any Crustacean, but the fluid is in both
animals strongly impregnated with the blue-coloured respiratory
proteid, haemocyanin. This body occurs also in the blood of Crustacea
and of Molluscs, but its abundance in both Limulus and Scorpio is very
marked, and gives to the freshly-shed blood a strong indigo-blue tint.
[Illustration: FIG. 20.--View of the ventral surface of the mid-line
of the prosomatic region of _Limulus polyphemus_. The coxae of the
five pairs of limbs following the chelicerae were arranged in a series
on each side between the mouth, M, and the metasternites, mets.
sf, The sub-frontal median sclerite.
Ch, The chelicerae.
cam, The camerostome or upper lip.
M, The mouth.
pmst, The promesosternal sclerite of chitinous plate, unpaired.
mets, The right and left metasternites (corresponding to the
similarly placed pentagonal sternite of Scorpio). Natural size.
(After Lankester.)]
The great dorsal contractile vessel or "heart" of Limulus is closely
similar to that of Scorpio; its ostia or incurrent orifices are placed
in the same somites as those of Scorpio, but there is one additional
posterior pair. The origin of the paired arteries from the heart
differs in Limulus from the arrangement obtaining in Scorpio, in that
a pair of lateral commissural arteries exist in Limulus (as described
by Alphonse Milne-Edwards (6)) leading to a suppression of the more
primitive direct connexion of the four pairs of posterior lateral
arteries and of the great median posterior arteries with the heart
itself (fig. 29). The arterial system is very completely developed in
both Limulus and Scorpio, branching repeatedly until minute arterioles
are formed, not to be distinguished from true capillaries; these open
into irregular swollen vessels which are the veins or venous sinuses.
A very remarkable feature in Limulus, first described by Owen, is the
close accompaniment of the prosomatic nerve centres and nerves by
arteries, so close indeed that the great ganglion mass and its
out-running nerves are actually sunk in or invested by arteries. The
connexion is not so intimate in Scorpio, but is nevertheless a very
close one, closer than we find in any other Arthropods in which the
arterial system is well developed, e.g. the Myriapoda and some of the
arthrostracous Crustacea. It seems that there is a primitive tendency
in the Arthropoda for the arteries to accompany the nerve cords, and a
"supra-spinal" artery--that is to say, an artery in close relation to
the ventral nerve cords--has been described in several cases. On the
other hand, in many Arthropods, especially those which possess
tracheae, the arteries do not have a long course, but soon open into
wide blood sinuses. Scorpio certainly comes nearer to Limulus in the
high development of its arterial system, and the intimate relation of
the anterior aorta and its branches to the nerve centres and great
nerves, than does any other Arthropod.
[Illustration: FIG. 2l.--Development of the lateral eyes of a
scorpion. h, Epidermic cell-layer; _mes_, mesoblastic connective
tissue; n, nerves; II, III, IV, V, depressions of the epidermis in
each of which a cuticular lens will be formed.
(_From Korschelt and Heider, after Laurie_.)]
[Illustration: FIG. 22.--Section through the lateral eye of
_Euscorpius italicus_.
lens, Cuticular lens.
nerv c, Retinal cells (nerve-end cells).
rhabd, Rhabdomes.
nerv f, Nerve fibres of the optic nerve.
int, Intermediate cells (lying between the bases of the retinal
cells).
(After Lankester and Bourne from Parker and Habwell's _Text book of
Zoology_, Macmillan & Co.)]
[Illustration: FIG. 23.--Section through a portion of the lateral eye
of Limulus, showing three ommatidia--A, B and C. hyp, The epidermic
cell-layer (so-called hypodermis), the cells of which increase in
volume below each lens, l, and become nerve-end cells or
retinula-cells, rt; in A, the letters rh point to a rhabdomere
secreted by the cell rt; c, the peculiar central spherical cell; n,
nerve fibres; mes, mesoblastic skeletal tissue; ch, chitinous
cuticle.
(From Korschelt and Heider after Watase.)]
An arrangement of great functional importance in regard to the venous
system must now be described, which was shown in 1883 by Lankester to
be common to Limulus and Scorpio. This arrangement has not hitherto
been detected in any other class than the Arachnida, and if it should
ultimately prove to be peculiar to that group, would have considerable
weight as a proof of the close genetic affinity of Limulus and
Scorpio.
[Illustration: FIG. 24.--Diagrams of the development and adult
structure of one of the paired central eyes of a scorpion.
A, Early condition before the lens is deposited, showing the folding
of the epidermic cell-layer into three.
B, Diagram showing the nature of this infolding.
C, Section through the fully formed eye.
h, Epidermic cell-layer.
r, The retinal portion of the same which, owing to the infolding,
lies between gl, the corneagen or lens-forming portion, and pr, the
post-retinal or capsular portion or fold.
l, Cuticular lens.
g, Line separating lens from the lens-forming or corneagen cells of
the epidermis.
n, Nerve fibres.
rh, Rhabdomeres.
[How the inversion of the nerve-end-cells and their connexion with
the nerve-fibres is to be reconciled with the condition found in the
adult, or with that of the monostichous eye, has not hitherto been
explained.]
(From Korschelt and Heider.)]
The great pericardial sinus is strongly developed in both animals. Its
walls are fibrous and complete, and it holds a considerable volume of
blood when the heart itself is contracted. Opening in pairs in each
somite, right and left into the pericardial sinus are large veins,
which bring the blood respectively from the gill-books and the
lung-books to that chamber, whence it passes by the ostia into the
heart. The blood is brought to the respiratory organs in both cases by
a great venous collecting sinus having a ventral median position. In
both animals _the wall of the pericardial sinus is connected by
vertical muscular bands to the wall of the ventral venous sinus_ (its
lateral expansions around the lung-books in Scorpio) in each somite
through which the pericardium passes. There are seven pairs of these
_veno-pericardiac vertical muscles_ in Scorpio, and eight in Limulus
(see figs. 30, 31, 32). It is obvious that the contraction of these
muscles must cause a depression of the floor of the pericardium and a
rising of the roof of the ventral blood sinus, and a consequent
increase of volume and flow of blood to each. Whether the pericardium
and the ventral sinus are made to expand simultaneously or all the
movement is made by one only of the surfaces concerned, must depend on
conditions of tension. In any case it is clear that we have in these
muscles an apparatus for causing the blood to flow differentially in
increased volume into either the pericardium, through the veins
leading from the respiratory organs, or from the body generally into
the great sinuses which bring the blood to the respiratory organs.
These muscles act so as to pump the blood through the respiratory
organs.
[Illustration: FIG 25.--Section through one of the central eyes of a
young Limulus.
L, Cuticular or corneous lens.
hy, Epidermic cell-layer.
corn, Its corneagen portion immediately underlying the lens.
ret, Retinula cells.
nf, Nerve fibres.
con. tiss, Connective tissue (mesoblastic skeletal tissue).
(After Lankester and Bourne, _Q. J. Mic. Sci._, 1883.)]
It is not surprising that with so highly developed an arterial system
Limulus and Scorpio should have a highly developed mechanism for
determining the flow of blood to the respiratory organs. That this is,
so to speak, a need of animals with localized respiratory organs is
seen by the existence of provisions serving a similar purpose in other
animals, e.g. the branchial hearts of the Cephalopoda.
The veno-pericardiac muscles of Scorpio were seen and figured by
Newport but not described by him. Those of Limulus were described and
figured by Alphonse Milne-Edwards, but he called them merely
"transparent ligaments," and did not discover their muscular
structure. They are figured and their importance for the first time
recognized in the memoir on the muscular and skeletal systems of
Limulus and Scorpio by Lankester, Beck and Bourne (4).
6. _Alimentary Canal and Gastric Glands._--The alimentary canal in
Scorpio, as in Limulus, is provided with a powerful suctorial pharynx,
in the working of which extrinsic muscles take a part. The mouth is
relatively smaller in Scorpio than in Limulus--in fact is minute, as
it is in all the terrestrial Arachnida which suck the juices of either
animals or plants. In both, the alimentary canal takes a straight
course from the pharynx (which bends under it downwards and backwards
towards the mouth in Limulus) to the anus, and is a simple, narrow,
cylindrical tube (fig. 33). The only point in which the gut of Limulus
resembles that of Scorpio rather than that of any of the Crustacea, is
in possessing more than a single pair of ducts or lateral outgrowths
connected with ramified gastric glands or gastric caeca. Limulus has
two pairs of these, Scorpio as many as six pairs. The Crustacea never
have more than one pair. The minute microscopic structure of the
gastric glands in the two animals is practically identical. The
functions of these gastric diverticula have never been carefully
investigated. It is very probable that in Scorpio they do not serve
merely to secrete a digestive fluid (shown in other Arthropoda to
resemble the pancreatic fluid), but that they also become distended by
the juices of the prey sucked in by the scorpion--as certainly must
occur in the case of the simple unbranched gastric caeca of the
spiders.
The most important difference which exists between the structure of
Limulus and that of Scorpio is found in the hinder region of the
alimentary canal. Scorpio is here provided with a single or double
pair of renal excretory tubes, which have been identified by earlier
authors with the Malpighian tubes of the Hexapod and Myriapod insects.
Limulus is devoid of any such tubes. We shall revert to this subject
below.
[Illustration: FIG. 26.
A, Diagram of a retinula of the central eye of a scorpion consisting
of five retina-cells (ret), with adherent branched pigment cells
(pig).
B, Rhabdom of the same, consisting of five confluent rhabdomeres.
C, Transverse section of the rhabdom of a retinula of the scorpion's
central eye, showing its five constituent rhabdomeres as rays of a
star.
D, Transverse section of a retinula of the lateral eye of Limulus,
showing ten retinula cells (ret), each bearing a rhabdomere (rhab).
(After Lankester.)]
[Illustration: FIG. 27.--Diagram showing the position of the coxal
glands of a scorpion, _Buthus australis_, Lin., in relation to the
legs, diaphragm (entosternal flap), and the gastric caeca.
1 to 6, The bases of the six prosomatic limbs.
A, prosomatic gastric gland (sometimes called salivary).
B, Coxal gland.
C, Diaphragm of Newport = fibrous flap of the entosternum.
D, Mesosomatic gastric caeca (so-called liver).
E, Alimentary canal.
(From Lankester, _Q. J. Mic. Sci._, vol. xxiv. N.S. p. 152.)]
7. _Ovaries and Spermaries: Gonocoels and Gonoducts._--The scorpion is
remarkable for having the specialized portion of coelom from the walls
of which egg-cells or sperm-cells are developed according to sex, in
the form of a simple but extensive network. It is not a pair of simple
tubes, nor of dendriform tubes, but a closed network. The same fact is
true of Limulus, as was shown by Owen (7) in regard to the ovary, and
by Benham (14) in regard to the testis. This is a very definite and
remarkable agreement, since such a reticular gonocoel is not found in
Crustacea (except in the male Apus). Moreover, there is a significant
agreement in the character of the spermatozoa of Limulus and Scorpio.
The Crustacea are--with the exception of the Cirrhipedia--remarkable
for having stiff, motionless spermatozoids. In Limulus Lankester found
(15) the spermatozoa to possess active flagelliform "tails," and to
resemble very closely those of Scorpio which, as are those of most
terrestrial Arthropoda, are actively motile. This is a microscopic
point of agreement, but is none the less significant.
In regard to the important structures concerned with the fertilization
of the egg, Limulus and Scorpio differ entirely from one another. The
eggs of Limulus are fertilized in the sea after they have been laid.
Scorpio, being a terrestrial animal, fertilizes by copulation. The
male possesses elaborate copulatory structures of a chitinous nature,
and the eggs are fertilized in the female without even quitting the
place where they are formed on the wall of the reticular gonocoel. The
female scorpion is viviparous, and the young are produced in a highly
developed condition as fully formed scorpions.
[Illustration: FIG. 28.--The right coxal gland of _Limulus
polyphemus_, Latr.
a^2 to a^5, Posterior borders of the chitinous bases of the coxae of
the second, third, fourth and fifth prosomatic limbs.
b, Longitudinal lobe or stolon of the coxal gland.
c, Its four transverse lobes or outgrowths corresponding to the four
coxae.
(From Lankester, _loc. cit_, after Packard.)]
_Differences between Limulus and Scorpio._--We have now passed in
review the principal structural features in which Limulus agrees with
Scorpio and differs from other Arthropoda. There remains for
consideration the one important structural difference between the two
animals. Limulus agrees with the majority of the Crustacea in being
destitute of renal excretory caeca or tubes opening into the hinder
part of the gut. Scorpio, on the other hand, in common with all
air-breathing Arthropoda except Peripatus, possesses these tubules,
which are often called Malpighian tubes. A great deal has been made of
this difference by some writers. It has been considered by them as
proving that Limulus, in spite of all its special agreements with
Scorpio (which, however, have scarcely been appreciated by the writers
in question), really belongs to the Crustacean line of descent, whilst
Scorpio, by possessing Malpighian tubes, is declared to be
unmistakably tied together with the other Arachnida to the tracheate
Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess
Malpighian tubes.
[Illustration: FIG. 29.--Diagram of the arterial system of A, Scorpio,
and B, Limulus. The Roman numerals indicate the body somites and the
two figures are adjusted for comparison. ce, Cerebral arteries; sp,
supra-spinal or medullary artery; c, caudal artery; l, lateral
anastomotic artery of Limulus. The figure B also shows the peculiar
neural investiture formed by the cerebral arteries in Limulus and the
derivation from this of the arteries to the limbs, III, IV, VI,
whereas in Scorpio the latter have a separate origin from the anterior
aorta.
(From Lankester, "Limulus an Arachnid.")]
It must be pointed out that the presence or absence of such renal
excretory tubes opening into the intestine appears to be a question of
adaptation to the changed physiological conditions of respiration, and
not of morphological significance, since a pair of renal excretory
tubes of this nature is found in certain Amphipod Crustacea
(Talorchestia, &c.) which have abandoned a purely aquatic life. This
view has been accepted and supported by Professors Korschelt and
Heider (16). An important fact in its favour was discovered by Laurie
(17), who investigated the embryology of two species of Scorpio under
Lankester's direction. It appears that the Malpighian tubes of Scorpio
are developed from the mesenteron, viz. that portion of the gut which
is formed by the hypoblast, whereas in Hexapod insects the similar
caecal tubes are developed from the proctodaeum or in-pushed portion
of the gut which is formed from epiblast. In fact it is not possible
to maintain that the renal excretory tubes of the gut are of one
common origin in the Arthropoda. They have appeared independently in
connexion with a change in the excretion of nitrogenous waste in
Arachnids, Crustacea, and the other classes of Arthropoda when aerial,
as opposed to aquatic, respiration has been established--and they have
been formed in some cases from the mesenteron, in other cases from the
proctodaeum. Their appearance in the air-breathing Arachnids does not
separate those forms from the water-breathing Arachnids which are
devoid of them, any more than does their appearance in certain
Amphipoda separate those Crustaceans from the other members of the
class.
Further, it is pointed out by Korschelt and Heider that the hinder
portion of the gut frequently acts in Arthropoda as an organ of
nitrogenous excretion in the absence of any special excretory tubules,
and that the production of such caeca from its surface in separate
lines of descent does not involve any elaborate or unlikely process of
growth. In other words, the Malpighian tubes of the terrestrial
Arachnida are _homoplastic_ with those of Hexapoda and Myriapoda, and
not _homogenetic_ with them. We are compelled to take a similar view
of the agreement between the tracheal air-tubes of Arachnida and other
tracheate Arthropods. They are homoplasts (see 18) one of another, and
do not owe their existence in the various classes compared to a common
inheritance of an ancestral tracheal system.
[Illustration: FIG. 30.--View from below of a scorpion (_Buthus
occitanus_) opened and dissected so as to show the pericardium with its
muscles, the lateral arteries, and the tergo-sternal muscles.
PRO, Prosoma.
dpm, Dorso-plastral muscle.
art, Lateral artery.
tsm^1, Tergo-sternal muscle (labelled dv in fig. 31) of the second
(pectiniferous) mesosomatic somite; this is the most anterior pair
of the series of six, none are present in the genital somite.
tsm^4, Tergo-sternal muscle of the fifth mesosomatic somite.
tsm^6, Tergo-sternal muscle of the enlarged first metasomatic
somite.
Per, Pericardium.
VPM^1 to VPM^7, The series of seven pairs of veno-pericardiac
muscles (labelled pv in fig. 31).
There is some reason to admit the existence of another more anterior
pair of these muscles in Scorpio; this would make the number exactly
correspond with the number in Limulus.
(After Lankester, _Trans. Zool. Soc._ vol. xi, 1883.)]
_Conclusions arising from the Close Affinity of Limulus and
Scorpio._--When we consider the relationships of the various classes of
Arthropoda, having accepted and established the fact of the close
genetic affinity of Limulus and Scorpio, we are led to important
conclusions. In such a consideration we have to make use not only of the
fact just mentioned, but of three important generalizations which serve
as it were as implements for the proper estimation of the relationships
of any series of organic forms. First of all there is the generalization
that the relationships of the various forms of animals (or of plants) to
one another is that of the ultimate twigs of a much-branching
genealogical tree. Secondly, identity of structure in two organisms does
not necessarily indicate that the identical structure has been inherited
from an ancestor common to the two organisms compared (homogeny), but
may be due to independent development of a like structure in two
different lines of descent (homoplasy). Thirdly, those members of a
group which, whilst exhibiting undoubted structural characters
indicative of their proper assignment to that group, yet are simpler
than and inferior in elaboration of their organization to other members
of the group, are not necessarily representatives of the earlier and
primitive phases in the development of the group--but are very often
examples of retrogressive change or degeneration. The second and third
implements of analysis above cited are of the nature of cautions or
checks. Agreements are not _necessarily_ due to common inheritance;
simplicity is not _necessarily_ primitive and ancestral.
On the other hand, we must not rashly set down agreements as due to
"homoplasy" or "convergence of development" if we find two or three or
more concurrent agreements. The probability is against agreement being
due to homoplasy when the agreement involves a number of really separate
(not correlated) coincidences. Whilst the chances are in favour of some
_one_ homoplastic coincidence or structural agreement occurring between
some member or other of a large group a and some member or other of a
large group b, the matter is very different when by such an initial
coincidence the two members have been particularized. The chances
against these two selected members exhibiting _another_ really
independent homoplastic agreement are enormous: let us say 10,000 to 1.
The chances against yet another coincidence are a hundred million to
one, and against yet one more "coincidence" they are the square of a
hundred million to one. Homoplasy can only be assumed when the
coincidence is of a simple nature, and is such as may be reasonably
supposed to have arisen by the action of like selective conditions upon
like material in two separate lines of descent.[4]
So, too, degeneration is not to be lightly assumed as the explanation of
a simplicity of structure. There is a very definite criterion of the
simplicity due to degeneration, which can in most cases be applied.
Degenerative simplicity is never uniformly distributed over all the
structures of the organism. It affects many or nearly all the structures
of the body, but leaves some, it may be only one, at a high level of
elaboration and complexity. Ancestral simplicity is more uniform, and
does not co-exist with specialization and elaboration of a single organ.
Further: degeneration cannot be inferred safely by the examination of an
isolated case; usually we obtain a series of forms indicating the steps
of a change in structure--and what we have to decide is whether the
movement has been from the simple to the more complex, or from the more
complex to the simple. The feathers of a peacock afford a convenient
example of primitive and degenerative simplicity. The highest point of
elaboration in colour, pattern and form is shown by the great
eye-painted tail feathers. From these we can pass by gradual transitions
in two directions, viz. either to the simple lateral tail feathers with
a few rami only, developed only on one side of the shaft and of uniform
metallic coloration--or to the simple contour feathers of small size,
with the usual symmetrical series of numerous rami right and left of the
shaft and no remarkable colouring. The one-sided specialization and the
peculiar metallic colouring of the lateral tail feathers mark them as
the extreme terms of a degenerative series, whilst the symmetry,
likeness of constituent parts _inter se_, and absence of specialized
pigment, as well as the fact that they differ little from any average
feather of birds in general, mark the contour feather as primitively
simple, and as the starting-point from which the highly elaborated
eye-painted tail feather has gradually evolved.
Applying these principles to the consideration of the Arachnida, we
arrive at the conclusion that the smaller and simpler Arachnids are not
the more primitive, but that the Acari or mites are, in fact, a
degenerate group. This was maintained by Lankester in 1878 (19), again
in 1881 (20); it was subsequently announced as a novelty by Claus in
1885 (21). Though the aquatic members of a class of animals are in some
instances derived from terrestrial forms, the usual transition is from
an aquatic ancestry to more recent land-living forms. There is no doubt,
from a consideration of the facts of structure, that the aquatic
water-breathing Arachnids, represented in the past by the Eurypterines
and to-day by the sole survivor Limulus, have preceded the terrestrial
air-breathing forms of that group. Hence we see at once that the
better-known Arachnida form a series, leading from Limulus-like aquatic
creatures through scorpions, spiders and harvest-men, to the degenerate
Acari or mites. The spiders are specialized and reduced in apparent
complexity, as compared with the scorpions, but they cannot be regarded
as degenerate since the concentration of structure which occurs in them
results in greater efficiency and power than are exhibited by the
scorpion. The determination of the relative degree of perfection of
organization attained by two animals compared is difficult when we
introduce, as seems inevitable, the question of efficiency and power,
and do not confine the question to the perfection of morphological
development. We have no measure of the degree of power manifested by
various animals--though it would be possible to arrive at some
conclusions as to how that "power" should be estimated. It is not
possible here to discuss that matter further. We must be content to
point out that it seems that the spiders, the pedipalps, and other large
Arachnids have not been derived from the scorpions directly, but have
independently developed from aquatic ancestors, and from one of these
independent groups--probably through the harvest-men from the
spiders--the Acari have finally resulted.
[Illustration: FIG. 31.--Diagram of a lateral view of a longitudinal
section of a scorpion.
After Beck, _Trans. Zool. Soc._ VOL. xi., 1883.
d, Chelicera.
ch, Chela.
cam_, Camerostome.
m, Mouth.
ent_, Entosternum.
p, Pecten.
stig^1, First pulmonary aperture.
stig^4, Fourth pulmonary aperture.
dam, Muscle from carapace to a praeoral entosclerite.
ad, Muscle from carapace to entosternum.
md, Muscle from tergite of genital somite to entosternum (same as dpm
in fig. 30).
dv^1 to dv^6, Dorso-ventral muscles (same as the series labelled tsm
in fig. 30).
pv^1 to pv^7, The seven veno-pericardiac muscles of the right side
(labelled VPM in fig. 30).]
[Illustration: FIG. 32.--Diagram of a lateral view of a longitudinal
section of Limulus.
After Benham, _Trans Zool. Soc._ vol. xi, 1883.
Suc, Suctorial pharynx.
al, Alimentary canal.
Ph, Pharynx.
M, Mouth.
Est, Entosternum.
VS, Ventral venous sinus.
chi, Chilaria.
go, Genital operculum.
br^1 to br^5, Branchial appendages,
met, Unsegmented metasoma.
Entap^4, Fourth dorsal entapophysis of left side.
tsm, Tergo-sternal muscles, six pairs as in Scorpio (labelled dv in
fig. 31).
VPM^1 to VPM^8, The eight pairs of veno-pericardiac muscles (labelled
pv in fig. 31). VPM^1 is probably represented in Scorpio, though not
marked in figs. 30 and 31.]
Leaving that question for consideration in connexion with the systematic
statement of the characters of the various groups of Arachnida which
follows on p. 299, it is well now to consider the following question,
viz., seeing that Limulus and Scorpio are such highly developed and
specialized forms, and that they seem to constitute as it were the first
and second steps in the series of recognized Arachnida--what do we know,
or what are we led to suppose with regard to the more primitive
Arachnida from which the Eurypterines and Limulus and Scorpio have
sprung? Do we know in the recent or fossil condition any such primitive
Arachnids? Such a question is not only legitimate, but prompted by the
analogy of at least one other great class of Arthropods. The great
Arthropod class, the Crustacea, presents to the zoologist at the present
day an immense range of forms, comprising the primitive phyllopods, the
minute copepods, the parasitic cirrhipedes and the powerful crabs and
lobsters, and the highly elaborated sand-hoppers and slaters. It has
been insisted, by those who accepted Lankester's original doctrine of
the direct or genetic affinity of the Chaetopoda and Arthropoda, that
Apus and Branchipus really come very near to the ancestral forms which
connected those two great branches of Appendiculate (Parapodiate)
animals. On the other hand, the land crabs are at an immense distance
from these simple forms. The record of the Crustacean family-tree is, in
fact, a fairly complete one--the lower primitive members of the group
are still represented by living forms in great abundance. In the case of
the Arachnida, if we have to start their genealogical history with
Limulus and Scorpio, we are much in the same position as we should be in
dealing with the Crustacea, were the whole of the Entomostraca and the
whole of the Arthrostraca wiped out of existence and record. There is no
possibility of doubt that the series of forms corresponding in the
Arachnidan line of descent, to the forms distinguished in the Crustacean
line of descent as the lower grade--the Entomostraca--have ceased to
exist, and not only so, but have left little evidence in the form of
fossils as to their former existence and nature. It must, however, be
admitted as probable that we should find some evidence, in ancient rocks
or in the deep sea, of the early more primitive Arachnids. And it must
be remembered that such forms must be expected to exhibit, when found,
differences from Limulus and Scorpio as great as those which separate
Apus and Cancer. The existing Arachnida, like the higher Crustacea, are
"nomomeristic," that is to say, have a fixed typical number of somites
to the body. Further, they are like the higher Crustacea,
"somatotagmic," that is to say, they have this limited set of somites
grouped in three (or more) "tagmata" or regions of a fixed number of
similarly modified somites --each tagma differing in the modification of
its fixed number of somites from that characterizing a neighbouring
"tagma." The most primitive among the lower Crustacea, on the other
hand, for example, the Phyllopoda, have not a fixed number of somites,
some genera--even allied species--have more, some less, within wide
limits; they are "anomomeristic." They also, as is generally the case
with anomomeristic animals, do not exhibit any conformity to a fixed
plan of "tagmatism" or division of the somites of the body into regions
sharply marked off from one another; the head or prosomatic tagma is
followed by a trunk consisting of somites which either graduate in
character as we pass along the series or exhibit a large variety in
different genera, families and orders, of grouping of the somites. They
are anomotagmic, as well as anomomeristic.
[Illustration: FIG. 33.--The alimentary canal and gastric glands of a
scorpion (A) and of Limulus (B).