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Appendages of anterior pair very large and chelate.

Sub-order Pterygotomorpha, Pterygotidae (Pterygotus).      

Appendages of anterior pair minute and chelate.

Sub-order Eurypteromorpha Stylonuridae (Stylonurus).
Eurypteridae (Eurypterus, Slimonia).

1911 Britannica-Arachnida-Pterygotus osiliensis.png

From Zittel’s Palaeontology.

Fig. 47.—Pterygotus osiliensis, Schmidt. Silurian of Rootzikil. Restoration of the ventral
surface, about a third natural size, after Schmidt.

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

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

Remarks.—The Gigantostraca are frequently spoken of as “the Eurypterines.” Not more than thirty species are known. They became extinct in Palaeozoic times, and are chiefly found in the Upper Silurian, though extending upwards as far as the Carboniferous. They may be regarded as “macrourous” Xiphosura; that is to say, Xiphosura in which the nomomeristic number of eighteen well-developed somites is present and the posterior ones form a long tail-like region of the body. There still appears to be some doubt whether in the sub-order Eurypteromorpha the first pair of prosomatic appendages (fig. 46) is atrophied, or whether, if present, it has the form of a pair of tactile palps or of minute chelae. Though there are indications of lamelliform respiratory appendages on mesosomatic somites following that bearing the genital operculum, we cannot be said to have any proper knowledge as to such appendages, and further evidence with regard to them is much to be desired. (For literature see Zittel, 22*.)
Grade b (of the Eu-arachnida). embolobranchia (Aeropneustea).
In primitive forms the respiratory lamellae of the appendages of the 3rd, 4th, 5th and eth, or of the 1st and 2nd mesosomatic somites are sunk beneath the surface of the body, and become adapted to breathe atmospheric oxygen, forming the leaves of the so-called lung-books. In specialized forms these pulmonary sacs are wholly or partly replaced by tracheal tubes. The appendages of the mesosoma generally suppressed; in the more primitive forms one or two pairs may be retained as organs subservient to reproduction or silk-spinning. Mouth situated more forwards than in Delobranchia, no share in mastication being taken by the basal segments of the 5th and 6th pairs of prosomatic appendages. Lateral eyes, when present, represented by separate ocelli.
The prae-genital somite, after appearing in the embryo, either is obliterated (Scorpio, Galeodes, Opilio and others) or is retained as a reduced narrow region of the body, the “waist,” between prosoma and mesosoma. It is represented by a full-sized tergal plate in the Pseudo-scorpiones.
1911 Britannica-Arachnida-Palaeophonus nuncius.png
Restored after Thorell’s indications by R. I. Pocock.
Fig. 48.—Dorsal view of a restoration of
Palaeophonus nuncius, Thorell. The Silurian
scorpion from Gothland.
Section α. Pectinifera.—The primitive distinction between the mesosoma and the metasoma retained, the latter consisting of six somites and the former of six somites in the adult, each of which is furnished during growth with a pair of appendages. Including the prae-genital somite (fig. 16), which is suppressed in the adult, there are thirteen somites behind the prosoma. The appendages of the 1st and 2nd mesosomatic somites persisting as the genital operculum and pectones respectively, those of the 3rd, 4th, 5th and 6th somites (? in Palaeophonus) sinking below the surface during growth in connexion with the formation of the four pairs of pulmonary sacs (see fig. 17). Lateral eyes monostichous.
Order 1. Scorpiones.—Prosoma covered by a single dorsal shield, bearing typically median and lateral eyes; its sternal elements reduced to a single plate lodged between or behind the basal segments of the 5th and 6th pairs of appendages. Appendages of 1st pair tri-segmented, chelate; of 2nd pair chelate, with their basal segments subserving mastication; of 3rd, 4th, 5th and 6th pairs similar in form and function, except that in recent and Carboniferous forms the basal segments of the 3rd and 4th are provided with sterno-coxal (maxillary) lobes, those of the 4th pair meeting in the middle line and underlying the mouth. The five posterior somites of the metasoma constricted to form a “tail,” the post-anal sclerite persisting as a weapon of offence and provided with a pair of poison glands (see figs. 8, 10, 12, 13, 14, 15, 21 and 22).
Sub-order Apoxypoda.—The 3rd, 4th, 5th and 6th pairs of appendages short, stout, tapering, the segments about as wide as long, except the apical, which is distally slender, pointed, slightly curved, and without distinct movable claws.
Family—Palaeophonidae, Palaeophonus (figs. 48 and 49).
Sub-order Dionychopoda.—The 3rd, 4th, 5th and 6th pairs of appendages slender, not evenly tapering, the segments longer than wide; the apical segment short, distally truncate, and provided with a pair of movable claws. Basal segments of the 5th and 6th pairs of appendages abutting against the sternum of the prosoma (see fig. 10 and figs. 51, 52 and 53).

Family—Pandinidae (Pandinus, Opisthophthalmus, Urodacus).
 ”      Vejovidae (Vaejovis, Jurus, Euscorpius, Broteas).
 ”      Bothriuridae (Bothriurus, Cercophonius).
 ”      Buthidae (Buthus, Centrurus).

 ”   *Cyclophthalmidae (Cyclophthalmus)
 ”   *Eoscorpiidae (Eoscorpius, Centromachus)

1911 Britannica-Arachnida-Palaeophonus Hunteri.png
Fig. 49.—Ventral view of a restoration of Palaeophonus Hunteri, Pocock, the Silurian scorpion from Lesmahagow,
Scotland. Restored by R. I. Pocock. The meeting of the coxae of all the prosomatic limbs in front of the pentagonal
sternum; the space for a genital operculum; the pair of pectens, and the absence of any evidence of pulmonary
stigmata are noticeable in this specimen.

(See Pocock, Quart. Jour. Micr. Sci., 1901.)

Remarks on the Order Scorpiones.—The Scorpion is one of the great animals of ancient lore and tradition. It and the crab are the only two invertebrates which had impressed the minds of early men sufficiently to be raised to the dignity of astronomical representation. It is all the more remarkable that the scorpion proves to be the oldest animal form of high elaboration which has persisted to the present day. In the Upper Silurian two specimens of a scorpion have been found (figs. 48, 49), one in Gothland and one in Scotland,