522
ARACHNIDA
mesosomatic somite. There is no doubt that these are besides the gill-book and its base; it is fused with its imfellow of the opposite side. On the other hand, in Scorpio, parapodial or limb appendages, carrying numerous bricated secondary processes, and therefore comparable in the gill-book-bearing appendage has sunk below the suressential structure to the leaf-bearing plates of the second face* forming a recess or chamber for itself, which com- mesosomatic somite of Limulus. They have remained unmunicates with the exterior by an oval enclosed and projecting on the surface of the body, as once or circular “stigma” were the appendages of the four following somites. But (Fig. 10, stg). That this in-sinking has taken place, and that the lung-books or in-sunken gillbooks of Scorpio really represent appendages (that is to say, limbs or parapodia) is proved by their developmental history (see Figs. 17 and 18). They appear at first as outstanding processes on the surface of the body. The exact mode in which the insinking of superficial outstanding limbs, carrying gilllamellse, has historically taken place Fia. 10.—Ventral view of a Scorpion, Palanmceus has been a matter indus, De Geer, to show the arrangement of the coxae of the limbs, the sternal elements, genital of much speculaplate and pectens. M, mouth behind the oval tion. It was to be median camerostome ; I, the chelicerae; II, the chelae ; III to VI, the four pairs of walking legs; hoped that the VII30, the genital somite or first somite of the mesosoma with the genital operculum (a fused specimen of the pair of limbs); VII Ip, the pectiniferous somite ; Silurian scorpion IXsJp to Xllslfif, the four pulmonary somites; met, the pentagonal metasternite of the prosoma (Palaeophonus) from behind all the cox®; x, the sternum of the ' . A. , ' • pectiniferous somite; y, the broad first somite bCOtiana, snowing of the metasoma. the ventral surface of the mesosoma (Fig. 49), would throw light on this matter; but the specimen recently carefully studied by the writer and Mr Pocock reveals neither gill-bearing limbs nor stigmata. The probability appears to be against an actual introversion of the appendage and its lamellae, as was at one time suggested by Lankester. It is probable that such an in-sinking as is shown in the accompanying diagram has taken place (Fig. 15); but we are yet in need of evidence Fig 12 —The prosomatic appendages of Limulus polyphemus (right) and Scorpio(left), Palamncms indus compared. The corresponding appendages are as to the exact marked with the same Roman numeral. The Arabic numerals indicate the equivalence of segments of the legs, cox, coxa or basal segment of the leg; stc, the sterno-coxal process or jaw-like up-growth of the coxa ; epc, the articulated margins, axis, &c., movable outgrowth of the coxa, called the epi-coxite (present only in III obtaining between of the Scorpion and III, IV, and V of Limulus); exi, the exopodite of the sixth limb of Limulus; a, b, c, d, movable processes on the same leg (see the lung-book of Fig. 11.—Third leg of Limulus polyphemns, showing for some suggestions on the morphology of this leg, Pocock in Quart. Journ. ,,„ ] fu„ the division of the fourth segment of the leg by a f Micr. Sci. March 1901; see also Fig. 50 below and explanation.) (From ocuipru d u. . mrc groove S into two, thus giving seven segments to Lankester, loc. cit.) gill-book of Limu- the leg as in Scorpion. (From a drawing by Mr. Pocock lus. Zoologists are d they have lost their respiratory function. In non-aquatic familiar with many instances (fishes, crustaceans) in which life such an unprotected organ cannot subserve respiration. the protective walls of a water-breathing organ or gill- The “pectens” have become more firmly chitinized and apparatus become converted into an air-breathing organ or, probably somewhat altered in shape as compared with their lung, but there is no other case known of the conversion condition in the aquatic ancestral scorpions. Their present function in scorpions is not ascertained. They are of gill processes themselves into air-breathing plates. The identification of the lung-books of Scorpio with the not specially sensitive under ordinary conditions, and may gill-books of Limulus is practically settled by the existence be touched or even pinched without causing any discomof the pectens in Scorpio (Fig. 14, VIII.) on the second fort to the scorpion. It is probable that they acquire special
