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ARACHNIDA

close genetic relationship. The view that Limulus, the king-crab, is an Arachnid was maintained as long ago as 1829 by Straus-Durkheim (1), on the ground of its possession of an internal cartilaginous sternum also possessed by the Arachnida (see Figs. 1, 2, 3, 4, 5, and 6) and of the similarity of the disposition of the six leg-like appendages around the mouth in the two cases (see i igs. 4o and 63). The evidence of the exact equivalence of the segmentation and appendages of Limulus and Scorpio, and of a number of remarkable points of agreement in their structure, was

Fio. 1.—Entosternum, entosternite or plastron of Limulus polyphemus, Latr. Dorsal surface. LAP, left anterior process ; RAP, right anterior process ; PhN, pharyngeal notch; ALR, anterior lateral rod or tendon; PLR, posterior lateral rod or tendon ; PLP, posterior lateral process. Natural size. (From Lankester, Q. J. Mic. Sci. N.S. vol. xxiv. 1884.) , furnished by Lankester in an article published in 1881 (“ Limulus an Arachnid,” Quart. Journ. Micr. Sci. vol. xxi. N.S.), and in a series of subsequent memoirs, in which the structure of the entosternum, of the coxal glands, of the eyes, of the veno-pericardiac muscles, of the respiratory lamellae, and of other parts, was for the first time described, and in which the new facts discovered were shown uniformly to support the hypothesis that Limulus is an Arachnid. A list of these memoirs is given at the close of this article (2, 3, 4, 5, and 13). The Eurypterines

Edwards in his beautiful memoir (6) on the structure of that animal), and secondly by the erroneous identification of the double sternal plates of Limulus, called “ chilaria,” by Owen, with a pair of appendages (7). Once the identity of the chilaria with the pentagonal sternal plate of the scorpion is recognized — an identification first insisted on by Lankester — the whole series of segments and appendages in the two animals, Limulus and Scorpio, are seen to correspond most closely, segment for segment, with one another (see A5- pIG 3_—Entosternum of Scorpion (Palamnasus Indus, 7 and 8). Ihe De Geer); dorsal surface, asp, paired anterior process of the sub-neural arch; snp, sub - neural structure of the arch; ap, anterior lateral process (same as RAP prosomatic ap- and LAP in Fig. 1); Imp, lateral median process (same as ALR and PLR of Fig. 1); pp, posterior pendages or legs process (same as PLP in Fig. l); pf, posterior flap or diaphragm of Newport; ml and m2, perforations is also seen to of the diaphragm for the passage of muscles ; DR, present many the paired dorsal ridges ; GC, gastric canal or foramen AC, arterial canal or foramen. Magnified significant points five times linear. (After Lankester, loc. dt.) of agreement (see figures), but a curious discrepancy existed in the six-jointed structure of the limb in Limulus, which differed from the seven-jointed limb of Scorpio by the defect of one joint. Mr R. I. Pocock of the British Museum has lately observed that in Limulus a marking exists on the fourth joint, which apparently indicates a previous division of this segment into two, and thus establishes the agreement of Limulus and Scorpio in this small feature of the number of segments in the legs (see Fig. 11). It is not desirable to occupy the limited space of this article by a full description of the limbs and segments of Limulus and Scorpio. The reader is referred to the complete series of figures here given, with their explanatory legends (Figs. 12, 13, 14, 15). Certain matters, however, require comment and explanation to render the comparison intelligible.1 The tergites, or chitinized dorsal halves of the body rings, are fused to form a “ prosomatic carapace,” or carapace of Fig. 4.—Ventral surface of the same ento- the prosoma, in both sternum as that drawn in Fig. 3. Letters as Limulus and Scorpio Fig. 3 with the addition of NC, neural i in canal or foramen. (After Lankester, loo. oil.) (see Figs. 7 and 8). This region corresponds in both cases to six somites, as indicated by the presence of six pairs of limbs. On the surface of the carapace there are in both animals a pair of central eyes with simple lens and a pair of lateral eye-tracts, which in Limulus consist of closely-aggregated simple eyes, forming a “ compound ” eye, whilst in Scorpio they present

PMP. Fig. 2.—Ventral surface of the entosternum of Limulus polyphemus, Latr. Letters as in Fig. 1 with the addition of NF, neural fossa protecting the aggregated ganglia of the central nervous system; PVP, left posterior ventral process ; PMP, posterior median process. Natural size. (From Lankester.) (Gigantostraca) were included in the identification, although at that time they were supposed to possess only five pairs of anterior or prosomatic appendages. They have now been shown to possess six pairs (Fig. 47), as do Limulus and Scorpio. The various comparisons previously made between the structure of Limulus and the Eurypterines on the one hand, and that of a typical Arachnid, such as Scorpio, on the other, had been vitiated by erroneous notions as to the 1 The discussion of the segmentation or metamerism of the origin of the nerves supplying the anterior appendages of Araehnida in this article should he read after a perusal of the article Limulus (which were finally removed by Alphonse Milne- Artheopoda by the same author.