ARACHNIDA (_CONTINUED_)—DELOBRANCHIATA = MEROSTOMATA (_CONTINUED_)—
EURYPTERIDA
=Order II. Eurypterida.=
The Eurypterida or Gigantostraca are found only in the Palaeozoic
formations. Some species of _Pterygotus_, _Slimonia_, and _Stylonurus_
have a length of from five to six feet, and are not only the largest
Invertebrates which have been found fossil but do not seem to be
surpassed in size at the present day except by some of the Dibranchiate
Cephalopods. All the Eurypterids were aquatic, and, with the possible
exception of forms found in the Coal Measures, all were marine. The
earliest examples occur in the Cambrian deposits, and the latest in the
Permian; but although the Eurypterids have thus a considerable
geological range, yet it is mainly in the Silurian and the Old Red
Sandstone that they are found, the principal genera represented in those
deposits being _Eurypterus_, _Stylonurus_, _Slimonia_, _Pterygotus_,
_Hughmilleria_, _Dolichopterus_, and _Eusarcus_. From the Cambrian rocks
the only form recorded is _Strabops_;[227] in the Ordovician the
imperfectly known _Echinognathus_[228] and some indeterminable fragments
have alone been found. In the Carboniferous deposits _Eurypterus_ and
_Glyptoscorpius_ occur, and the former survived into the Permian.[229]
[Illustration:
FIG. 161.—_Eurypterus fischeri_, Eichw. Upper Silurian, Rootziküll,
Oesel. Dorsal surface. _a_, Ocellus; _b_, lateral eye; 2–6,
appendages of prosoma; 7–12, segments of mesosoma; 13–18, segments
of metasoma; 19, tail-spine. (After Holm.)
]
The Eurypterid which is best known is _Eurypterus fischeri_ (Figs. 161,
162), which is found in the Upper Silurian rocks at Rootziküll in the
Island of Oesel (Gulf of Riga). In the Eurypterids from other deposits
the chitinous exoskeleton has been altered into a carbonaceous
substance, but in the specimens from Oesel the chitin is perfectly
preserved in its original condition; and since these specimens are found
in a dolomitic rock which is soluble in acid, it has been possible to
separate the fossil completely from the rock in which it is embedded,
with the result that the structure can be studied more easily and more
thoroughly than in the case of specimens from other localities.
Consequently _Eurypterus fischeri_[230] may, with advantage, be taken as
a type of the Eurypterida.
The general form of the body (Fig. 161) is somewhat like that of a
Scorpion, but is relatively broader and shorter. On the surface of many
parts of the exoskeleton numerous scale-like markings are found (Figs.
162, 163).[231] The =prosoma= or cephalothorax consists of six fused
segments covered by a quadrate carapace with its front angles rounded.
This bears on its dorsal surface two pairs of eyes—large kidney-shaped
lateral eyes and median ocelli (Fig. 161, _b_, _a_). The margin of the
dorsal part of the carapace is bent underneath to form a rim which joins
the ventral part of the carapace.
On the ventral surface of the prosoma (Fig. 162) six pairs of appendages
are seen, of which only the first pair (the chelicerae) are in front of
the mouth. The chelicerae are small, and each consists of a basal joint
and a chela, the latter being found parallel to the axis of the body;
they closely resemble the chelicerae of _Limulus_. The remaining five
pairs of appendages are found at the sides of the elongate mouth, and in
all these the gnathobases of the coxae are provided with teeth at their
inner margins and were able to function in mastication, whilst the
distal part of each appendage served as an organ of locomotion. The
posterior part of each coxa is plate-like and is covered (except in the
case of the sixth appendage) by the coxa of the next appendage behind. A
small process or “epicoxite” is found at the posterior end of the
toothed part of the coxae of the second, third, fourth, and fifth pairs
of appendages. The second appendage consists of seven joints, whilst the
remaining four consist of eight joints; none of these appendages end in
chelae. The second, third,[232] and fourth pairs of appendages are
similar to one another in structure, but become successively larger from
before backwards. These three pairs are directed radially outwards; each
consists of short joints tapering to the end of the limb, and bearing
spines at the sides and on the under surface, and also a spine at the
end of the last joint.
[Illustration:
FIG. 162.—_Eurypterus fischeri_, Eichw. Upper Silurian, Rootziküll,
Oesel. Restoration of ventral surface; 1–6, appendages of prosoma;
_m_, metastoma. Immediately posterior to the metastoma is the
“median process” of the genital operculum. (After Holm.)
]
The fifth appendage is longer than the fourth and is directed backwards;
its second and third joints are short and ring-like; the others (fourth
to eighth) are long and similar to one another, each being of uniform
width throughout; the last joint is produced into a spine on each side,
and between these two is the movable end-spine; the other joints do not
bear long spines as is the case in the three preceding pairs of
appendages.
The sixth appendage is much larger and stronger than the others, and
like the fifth, is without long spines. The coxa is large and quadrate;
the second and third joints are short, like those of the fifth
appendage; the fourth, fifth, and sixth joints are longer and more or
less bell-shaped; the seventh and eighth joints are much larger than the
others and are flattened.
The metastoma (Fig. 162, _m_) is an oval plate immediately behind the
mouth; it covers the inner parts of the coxae of the sixth pair of
appendages, and represents the chilaria of _Limulus_. But, unlike the
latter, it is not a paired structure; nevertheless the presence of a
longitudinal groove on its anterior part renders probable the view that
it is derived from a paired organ.[233] The front margin of the
metastoma is indented and toothed. On its inner side in front is a
transverse plate, the endostoma, which is not seen from the exterior,
since the front margin of the metastoma extends a little beyond it.
Behind the prosoma are twelve free segments, of which the first six form
the =mesosoma= (Fig. 161, 7–12). The tergum on the dorsal surface of
each segment is broad and short, the middle part being slightly convex
and the lateral parts slightly concave; the external margin is bent
under, thus forming a narrow rim on the ventral surface. The tergum of
each segment overlaps the one next behind. The segments increase in
breadth slightly up to the fourth segment, posterior to which they
gradually become narrower.
On the ventral surface the segments of the mesosoma bear pairs of
plate-like appendages, each of which overlaps the one behind like the
tiles on a roof. On the posterior (or inner) surfaces of these
appendages are found the lamellar branchiae, which are oval in outline
(Fig. 165, _d_). Between the two appendages of the first pair is a
median process which is genital in function; this pair are larger than
the other appendages, and cover both first and second segments, the
latter being without any appendages, and they represent the genital
operculum of _Limulus_ (Fig. 153, 10). The form of the operculum, more
particularly of the median process, differs in the male and female. In
that which is believed to be the female (Fig. 162) the median process is
long, and extends beyond the posterior margin of the operculum; it is
formed of two small five-sided parts at the base which are united at the
sides to the two plates of the operculum; behind this is a long,
unpaired part, which is pointed in front; this, together with the
remaining parts, is not joined to the side-plates of the operculum, so
that the latter are here separated from one another. The third part of
the median process is shorter than the second, and bears at its end a
pair of small pointed and diverging plates, the tips of which reach to
the middle of the third plate-like appendages. On the inner side of the
operculum there are, in the female, a pair of curved, tubular organs,
attached to the anterior end of the median process, where they open, the
free ends being closed; the function of these organs is not known, but
was probably sexual.
In the male (Fig. 163, A, _a_) the median process is formed of two parts
only, and is very short, so that the two plates of the operculum unite
behind the process.
In the female a median process (Fig. 163, B) is also present between the
second pair of appendages (belonging to the third segment of the
mesosoma); it consists of a basal unpaired part, and of a pair of long
pointed pieces which project on to the next segment. Just as in the case
of the genital operculum the basal part is united in front to the
appendages, the remainder being free, and separating the greater part of
the two plate-like appendages. In the complete animal the median process
of this segment is covered by the median process of the genital
operculum. The remaining appendages of the female, and all the
appendages behind the operculum in the male, are without any median
process, and the plates of each pair unite by a suture in the middle
line.
[Illustration:
FIG. 163.—_Eurypterus fischeri_, Eichw. Upper Silurian. (After Holm.)
=A=, Genital operculum of male; _a_, median process. =B=, Middle
part of second appendage of the mesosoma in the female, showing the
median process.
]
The =metasoma= (Fig. 161, 13–18) consists of six segments which become
longer and narrower from before backwards. Each segment is covered by a
ring-like sheath or sclerite, and bears no appendages. The posterior end
of the last segment is produced into a lobe on each side, and between
these lobes the long, narrow tail-spine arises (Fig. 161, 19).
The other genera of the Eurypterida do not differ in any important
morphological respects from the form just described, All the genera, of
which about thirteen have been recognised, are placed in one family.
[Illustration:
FIG. 164.—_Pterygotus osiliensis_, Schmidt, Upper Silurian,
Rootziküll, Oesel. Ventral surface. Reduced. (After Schmidt.) 1–6,
Appendages of the prosoma; 7–12, mesosoma; 7, 8, genital operculum;
13–18, metasoma; 19, tail-plate; _a_, epistome; _b_, metastoma; _c_,
coxae of sixth pair of appendages.
]
=Fam. Eurypteridae.=—The carapace varies somewhat in outline; in
_Slimonia_ it is more distinctly quadrate than in _Eurypterus_, whilst
in _Pterygotus_ (Fig. 164) and _Hughmilleria_[234] it is semi-ovoid. The
lateral eyes are at the margin of the carapace in _Pterygotus_,
_Slimonia_ (Fig. 165, _a_), and _Hughmilleria_, but in the other genera,
including the earliest form, _Strabops_,[235] they are on the dorsal
surface at a greater or less distance from the margin.
The pre-oral appendages of _Pterygotus_ (Fig. 164, 1) differ from those
of other genera in their much greater length and in the large size of
the chelae; they probably consist of a proximal joint and chelae only,
although, commonly, they are represented as having a larger number of
joints. Unlike _Eurypterus_ and _Pterygotus_, the second pair of
appendages in _Slimonia_ (Fig. 165, 2) differ from the third, fourth,
and fifth pairs in being distinctly smaller and more slender, and it is
probable that they were tactile. Whilst in _Eurypterus_ the fifth pair
of appendages are larger than the three preceding pairs, and also differ
from them in structure, in the genus _Pterygotus_ (Fig. 164, 5) they
agree closely with the second, third, and fourth pairs, and in
_Slimonia_ (Fig. 165, 5) they are nearly the same as the third and
fourth pairs. The sixth pair of appendages are much larger and more
powerful than the fifth pair in nearly all genera; in _Stylonurus_ (Fig.
166), however, the sixth pair are similar to the fifth, both being
greatly elongated and slender; also in _Eusarcus_ (_Drepanopterus_) the
sixth pair differ less from the preceding pair of appendages than is
usually the case.
[Illustration:
FIG. 165.—_Slimonia acuminata_, Salter. Upper Silurian. Restoration of
ventral surface, × ⅑. 1–6, Appendages of prosoma; 7, 8, genital
operculum; 7–12, mesosoma; 13–18, segments of metasoma; 19,
tail-spine; _a_, lateral eye; _b_, metastoma, covering the inner
parts of the coxae of the last pair of appendages; _c_, median
process of genital operculum; _d_, branchial lamellae seen through
the plate-like appendages. (After Laurie.)
]
In _Pterygotus_ there is a well-developed epistome (Fig. 164, _a_)
between the mouth and the front margin of the carapace, thus occupying
the same position as the hypostome of Trilobites (p. 233). The metastoma
is always well developed and forms one of the distinguishing features of
the Eurypterids; in form it varies from oval in _Eurypterus_, to cordate
in _Slimonia_, and lyrate in _Dolichopterus_.
The principal modifications seen in the genital operculum are in the
form of its median process; in _Slimonia_ this either ends in three
sharp points posteriorly (Fig. 165, _c_), or has the form of a truncated
cone; its form in _Eurypterus_ has already been described.
_Glyptoscorpius_ differs from other Eurypterids in the possession of
comb-like organs closely resembling the pectines of Scorpions.
_Slimonia_ apparently differs from other genera in that the plate-like
appendages on the posterior three segments of the mesosoma do not meet
in the middle line (Fig. 165, 10–12). In some forms, such as
_Pterygotus_ (Fig. 164), there is a nearly gradual decrease in the width
of the segments in passing from the mesosoma to the metasoma; but in
some others, which in this respect are less primitive, such as
_Slimonia_ (Fig. 165), the posterior five segments of the body (like
those of Scorpions) are distinctly narrower and longer than the
preceding segments. The long tail-spine of _Eurypterus_ is represented
in _Slimonia_ by an oval plate produced into a spine at the end (Fig.
165, 19); whilst in some species of _Pterygotus_ the plate is bi-lobed
at the posterior end (Fig. 164, 19). In _Hughmilleria_ the tail-spine is
lanceolate.
The Eurypterids resemble the Xiphosura in many respects. In both groups
the prosoma consists of at least six fused segments, and bears two pairs
of eyes—one pair simple, the other grouped eyes—on the dorsal surface of
the carapace. The number and position of the appendages of the prosoma
in Eurypterids agree with those of _Limulus_. The chelicerae are closely
similar in both cases. The coxae of all five pairs of legs in
Eurypterids are toothed and function in mastication; similarly in
_Limulus_ all are spiny except the coxae of the last pair of legs. In
both a similar epicoxite is present on the coxae. The number of joints
in the legs is somewhat greater in the Eurypterids than in _Limulus_,
and in the former none of the legs end in chelae, whereas in the latter
all the walking legs, except the last, and also the first in the male,
may be chelate. The metastoma of Eurypterids differs in being a large
unpaired plate, but is represented in _Limulus_ by the pair of
relatively small chilaria. On the mesosoma the genital operculum and
plate-like appendages with branchial lamellae are similar in both
groups, but in the Eurypterids the genital operculum shows a greater
development and covers the second segment, which is without plate-like
appendages. A striking difference between the two groups is seen in the
segments of the mesosoma and metasoma; in Eurypterids these are all
free, whilst in _Limulus_ they are fused together, but this difference
is bridged over by some of the Palaeozoic Xiphosura (Fig. 159, A) in
which those segments are free.
[Illustration:
FIG. 166.—_Stylonurus lacoanus_, Claypole. Upper Devonian,
Pennsylvania. Restoration of dorsal surface. Length nearly five
feet. (After Beecher.)
]
The Eurypterids present a striking resemblance to Scorpions. In both
groups the segments in the three regions of the body are the same in
number, and the appendages of the prosoma also agree in number and
position. The pre-oral appendages are chelate in both, but the second
pair of appendages are chelate in the Scorpions only. In Eurypterids the
coxae of the five pairs of legs are toothed and meet in the middle line,
but in the Scorpions the coxae of the last two pairs do not meet; this
difference, however, appears to be bridged over in the earliest known
Scorpion—_Palaeophonus_,[236] from the Silurian rocks. The Eurypterids
are distinguished from the Scorpions by the much greater development of
the last pair of legs. The large metastoma of the former is homologous
with the sternum of the Scorpion. The genital operculum is much smaller
in Scorpions than in Eurypterids, and in this respect the latter agree
with _Thelyphonus_ (one of the Pedipalpi) more than with the Scorpions.
The pectines are absent in the Eurypterids except in _Glyptoscorpius_.
Instead of the lung-books of the Scorpions the Eurypterids possess
branchial lamellae on the plate-like appendages; but this difference
between the two groups appears to be bridged over by _Palaeophonus_,
which was marine, and may have possessed branchial lamellae since
stigmata seem to be absent.
_Glyptoscorpius_,[237] which is found in the Lower Carboniferous of the
south of Scotland, is a form of considerable interest. It is about a
foot in length, and agrees in many respects with Eurypterida, but it may
be necessary to separate it from that group since it possesses pectines,
and the legs end in a double claw; it cannot, however, be regarded as a
link between Eurypterids and Scorpions, but must rather be considered as
an offshoot from the former, since the latter group was already in
existence at a much earlier period.
ARACHNIDA EMBOLOBRANCHIATA
(SCORPIONS, SPIDERS, MITES, ETC.)
BY
CECIL WARBURTON, M.A.
Christ’s College, Cambridge; Zoologist to the Royal Agricultural Society