Chapter XXI: Act 1737: , and the Apportionment Act 1834, and is now allowed generally (12)

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.

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.

(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.

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.

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).

(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.

(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.

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.

(_From Korschelt and Heider, after Laurie_.)]

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.)]

(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.

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.

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.

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.)]

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.

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.

(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.

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.

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).]

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.

From Lankester, "Limulua an Arachnid."

ps, Muscular suctorial enlargement of the pharynx.
sal, Prosomatic pair of gastric caeca in Scorpio, called salivary
glands by some writers.
c^1, and c^2, The anterior two pairs of gastric caeca and ducts of
the mesosomatic region.
c^3, c^4 and c^5. Caeca and ducts of Scorpio not represented in
Limulus.
M, The Malpighian or renal caecal diverticula of Scorpio.
pro, The proctodaeum or portion of gut leading to anus and formed
embryologically by an inversion of the epiblast at that orifice.]

When it is admitted--as seems to be reasonable--that the primitive Arachnida would, like the primitive Crustacea, be anomomeristic and anomotagmic, we shall not demand of claimants for the rank of primitive Arachnids agreement with Limulus and Scorpio in respect of the exact number of their somites and the exact grouping of those somites; and when we see how diverse are the modifications of the branches of the appendages both in Arachnida and in other classes of Arthropoda (q.v.), we shall not over-estimate a difference in the form of this or that appendage exhibited by the claimant as compared with the higher Arachnids. With those considerations in mind, the claim of the extinct group of the trilobites to be considered as representatives of the lower and more primitive steps in the Arachnidan genealogy must, it seems, receive a favourable judgment. They differ from the Crustacea in that they have only a single pair of prae-oral appendages, the second pair being definitely developed as mandibles. This fact renders their association with the Crustacea impossible, if classification is to be the expression of genetic affinity inferred from structural coincidence. On the contrary, this particular point is one in which they agree with the higher Arachnida. But little is known of the structure of these extinct animals; we are therefore compelled to deal with such special points of resemblance and difference as their remains still exhibit. They had lateral eyes[5] which resemble no known eyes so closely as the lateral eyes of Limulus. The general form and structure of their prosomatic carapace are in many striking features identical with that of Limulus. The trilobation of the head and body--due to the expansion and flattening of the sides or "pleura" of the tegumentary skeleton--is so closely repeated in the young of Limulus that the latter has been called "the trilobite stage" of Limulus (fig. 42 compared with fig. 41). No Crustacean exhibits this trilobite form. But most important of the evidences presented by the trilobites of affinity with Limulus, and therefore with the Arachnida, is the tendency less marked in some, strongly carried out in others, to form a pygidial or telsonic shield--a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of Limulus. When to this is added the fact that a post-anal spine is developed to a large size in some trilobites (fig. 38), like that of Limulus and Scorpio, and that lateral spines on the pleura of the somites are frequent as in Limulus, and that neither metasomatic fusion of somites nor post-anal spine, nor lateral pleural spines are found in any Crustacean, nor all three together in any Arthropod besides the trilobites and Limulus--the claim of the trilobites to be considered as representing one order of a lower grade of Arachnida, comparable to the grade Entomostraca of the Crustacea, seems to be established.

The fact that the single pair of prae-oral appendages of trilobites, known only as yet in one genus, is in that particular case a pair of uni-ramose antennae--does not render the association of trilobites and Arachnids improbable. Although the prae-oral pair of appendages in the higher Arachnida is usually chelate, it is not always so; in spiders it is not so; nor in many Acari. The bi-ramose structure of the post-oral limbs, demonstrated by Beecher in the trilobite Triarthrus, is no more inconsistent with its claim to be a primitive Arachnid than is the foliaceous modification of the limbs in Phyllopods inconsistent with their relationship to the Arthrostracous Crustaceans such as Gammarus and Oniscus.

Thus, then, it seems that we have in the trilobites the representatives of the lower phases of the Arachnidan pedigree. The simple anomomeristic trilobite, with its equi-formal somites and equi-formal appendages, is one term of the series which ends in the even more simple but degenerate Acari. Between the two and at the highest point of the arc, so far as morphological differentiation is concerned, stands the scorpion; near to it in the trilobite's direction (that is, on the ascending side) are Limulus and the Eurypterines--with a long gap, due to obliteration of the record, separating them from the trilobite. On the other side--tending downwards from the scorpion towards the Acari--are the Pedipalpi, the spiders, the book-scorpions, the harvest-men and the water-mites.

The strange nobody-crabs or Pycnogonids occupy a place on the ascending half of the arc below the Eurypterines and Limulus. They are strangely modified and degenerate, but seem to be (as explained in the systematic review) the remnant of an Arachnidan group holding the same relation to the scorpions which the Laemodipoda hold to the Podophthalmate Crustacea.

* * * * *

We have now to offer a classification of the Arachnida and to pass in review the larger groups, with a brief statement of their structural characteristics.

In the bibliography at the close of this article (referred to by leaded arabic numerals in brackets throughout these pages), the titles of works are given which contain detailed information as to the genera and species of each order or sub-order, their geographical distribution and their habits and economy so far as they have been ascertained. The limits of space do not permit of a fuller treatment of those matters here.

TABULAR CLASSIFICATION[6] OF THE ARACHNIDA.

CLASS. ARACHNIDA.

_Grade A. ANOMOMERISTICA._
Sub-Class. TRILOBITAE.
Orders. Not satisfactorily determined.

_Grade B. NOMOMERISTICA._
Sub-Class I. PANTOPODA.
Order 1. Nymphonomorpha.
" 2. Ascorhynchomorpha.
" 3. Pycnogonomorpha.
Sub-Class II. EU-ARACHNIDA.
Grade a. DELOBRANCHIA, Lankester (_vel_ HYDROPNEUSTEA, Pocock).
Order 1. Xiphosura.
" 2. Gigantostraca.
Grade b. EMBOLOBRANCHIA, Lankester (_vel_ AEROPNEUSTEA, Pocock).
_Section_ [alpha]. _Pectinifera._
Order 1. Scorpionidea.
Sub-order a. Apoxypoda.
" b. Dionychopoda.
_Section_ [beta]. _Epectinata._
Order 2. Pedipalpi.
Sub-order a. Uropygi.
Tribe 1. Urotricha.
" 2. Tartarides.
Sub-order b. Amblypygi.
Order 3. Araneae.
Sub-order a. Mesothelae.
" b. Opisthothelae.
Tribe 1. Mygalomorphae.
" 2. Arachnomorphae.
Order 4. Palpigradi (= Microthelyphonidae).
Order 5. Solifugae (= Mycetophorae).
Order 6. Pseudoscorpiones (= Chelonethi).
Sub-order a. Panctenodactyli.
" b. Hemirtenodactyli.
Order 7. Podogona (= Ricinulel).
Order 8. Opiliones.
Sub-order a. Laniatores.
" b. Palpatores.
" c. Anepignathi.
Order 9. Rhynchostomi (= Acari).
Sub-order a. Notostigmata.
" b. Cryptostigmata.
" c. Metastigmata.
" d. Prostigmata.
" e. Astigmata.
" f. Vermiformia.
" g. Tetrapoda.

CLASS. ARACHNIDA.--Euarthropoda having two prosthomeres (somites which
have passed from a post-oral to a prae-oral position), the appendages
of the first represented by eyes, of the second by solitary rami which
are rarely antenniform, more usually chelate. A tendency is exhibited
to the formation of a metasomatic as well as a prosomatic carapace by
fusion of the tergal surfaces of the somites. Intermediate somites
forming a mesosoma occur, but tend to fuse superficially with the
metasomatic carapace or to become co-ordinated with the somites of the
metasoma, whether fused or distinct to form one region, the
opisthosoma (abdomen of authors). In the most highly developed forms
the two anterior divisions (tagmata) of the body, prosoma and
mesosoma, each exhibit six pairs of limbs, pediform and plate-like
respectively, whilst the metasoma consists of six limbless somites and
a post-anal spine. The genital apertures are placed in the first
somite following the prosoma, excepting where a praegenital somite,
usually suppressed, is retained. Little is known of the form of the
appendages in the lowest archaic Arachnida, but the tendency of those
of the prosomatic somites has been (as in the Crustacea) to pass from
a generalized bi-ramose or multi-ramose form to that of uni-ramose
antennae, chelae and walking legs.

The Arachnida are divisible into two grades of structure--according to
the fixity or non-fixity of the number of somites building up the
body:--

_Grade A_ (_of the Arachnida_). _ANOMOMERISTICA._--Extinct archaic
Arachnida, in which (as in the Entomostracous Crustacea) the number of
well-developed somites may be more or less than eighteen and may be
grouped only as head (prosoma) and trunk or may be further
differentiated. A telsonic tergal shield of greater or less size is
always present, which may be imperfectly divided into well-marked but
immovable tergites indicating incompletely differentiated somites. The
single pair of palpiform appendages in front of the mouth has been
found in one instance to be antenniform, whilst the numerous post-oral
appendages in the same genus were bi-ramose. The position of the
genital apertures is not known. Compound lateral eyes present; median
eyes wanting. The body and head have the two pleural regions of each
somite flattened and expanded on either side of the true gut-holding
body-axis. Hence the name of the sub-class signifying tri-lobed, a
condition realized also in the Xiphosurous Arachnids. The members of
this group, whilst resembling the lower Crustacea (as all lower groups
of a branching genealogical tree must do), differ from them
essentially in that the head exhibits only one prosthomere (in
addition to the eye-bearing prosthomere) with palpiform appendages (as
in all Arachnida) instead of two. The Anomomeristic Arachnida form a
single sub-class, of which only imperfect fossil remains are known.

Sub-class (of the Anomomeristica). TRILOBITAE.--The single sub-class
Trilobitae constitutes the grade Anomomeristica. It has been variously
divided into orders by a number of writers. The greater or less
evolution and specialization of the metasomatic carapace appears to be
the most important basis for classification--but this has not been
made use of in the latest attempts at drawing up a system of the
Trilobites. The form of the middle and lateral regions of the
prosomatic shield has been used, and an excessive importance attached
to the demarcation of certain areas in that structure. Sutures are
stated to mark off some of these pieces, but in the proper sense of
that term as applied to the skeletal structures of the Vertebrata, no
sutures exist in the chitinous cuticle of Arthropoda. That any partial
fusion of originally distinct chitinous plates takes place in the
cephalic shield of Trilobites, comparable to the partial fusion of
bony pieces by suture in Vertebrata, is a suggestion contrary to fact.