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 ARACHNIDA

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species (L. polyphemus) is common on the coasts of the United ledge as to such appendages, and further evidence with regard to States, and is known as the King-Crab or Horse-shoe Crab. A single them is much to be desired. (For literature see Zittel, 22*.) specimen was found in the harbour of Copenhagen in the 18th century, having presumably been carried over by a ship to which it clung. A species of Limulus is found in the Buntersandstein of the Vosges ; L. Walchi is abundant in the Oolitic lithographic slates of Bavaria. The genera Belinurus, Aglaspis, Prestwichia, Hemiaspis, and Bunodes consist of small forms which occur in Palaeozoic rocks. In none of them are the appendages known, but in the form of the two carapaces and the presence of free somites they are distinctly intermediate between Limulus and the Trilobitse. The young form of Limulus itself (Fig. 40) is also similar to a Trilobite so far as its segmentation and trilobation are concerned. The lateral eyes of Limulus appear to be identical in structure and position with those of certain Trilobitae. Order 2. Gigantostraca (Figs. 46, 47).—Free-swimming forms, with the appendages of the 6th or 5th and 6th pairs flattened or

Fio. 47. —Pterygotus osiliensis, Schmidt. Silurian of Rootzikil. Restoration of the ventral surface, one-third the natural size, after Schmidt, a, camerostome or epistoma; m, chilarium or metasternite of the prosoma (so-called metastoma); oc, the compound eyes; 1 to 8, segments of the sixth prosomatic appendage; I' to V', first five opisthosomatic 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. (From Zittel’s Palceontology.) Grade b (of the Eu-arachnida). embolobranchia (Aeropneustea). In primitive forms the respiratory lamellae of the appendages of the 3rd, 4th, 5th, and 6th, 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

Pig. 46.—Eurypterus Fischeri, Eichwald. Silurian of Rootzikil. Restoration after Schmidt, half the size of nature. The dorsal aspect is presented showing the prosomatic shield with paired compound eyes and the prosomatic appendages II to VI. The small first pair of appendages is concealed from view by the carapace. 1 to 12 are the somites of the opisthosoma ; 13, the post-anal spine. (From ZitteTs Text-book of Palceontology. Macmillans, New York, 1896.) lengthened to act as oars ; segments of mesosoma and metasoma (=opisthosoma), twelve in number. Appendages of anterior pair very large and chelate. Sub-order Pterygotomorpha, Pterygotidse (Pterygotus). Appendages of anterior pair minute and chelate. f Stylonuridaa '{Stylonurus). Sub-order Eurypteromorpha-J Eurypteridse {Eurypterus, f Slimonia). Remarks.—Hhe 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 welldeveloped 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 know-

Fig. 48.—Dorsal view of a restoration of Paloeophonus nuncius, Thorell. The Silurian Scorpion from Gothland. (Restored after Thorell’s indications by Mr. R. I. Pocock.) 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 S. I. — 68