CHAPTER XI

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; in the Ordovician the imperfectly known Echinognathus and some indeterminable fragments have alone been found. In the Carboniferous deposits Eurypterus and Glyptoscorpius occur, and the former survived into the Permian.

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 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). 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, 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.

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. 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.

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.

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 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, 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.

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.

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, 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, 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.)

CECIL WARBURTON, M.A.

Christ’s College, Cambridge; Zoologist to the Royal Agricultural Society