ARAOHNIDA sensibility at the breeding season and serve as “ guides ” in copulation. The shape of the legs and the absence of paired terminal claws in the Silurian Palaeophonus (see Figs. 48 and 49) as compared with living scorpions
523
soft tissues of the sternal region so that the lamellae cannot be detached and presented as standing out from it. The apparent axis or basal support of the scorpion’s lung-books shown in the figures, is a false or secondary axis and merely a part of the infolded surface which forms the air-chamber. The maceration of the soft parts of a scorpion preserved in weak spirit and the cleaning of the chitinized ingrown cuticle give rise to the false appearance of a limb axis
Fig. 13.—Diagrams of the meta-stemite st, with genital operculum op, and the first lamelligerous pair of appendages c/a, with uniting sternal element st of Scorpio (left) and Limulus (right.) (From Lankester, loo. tit.) (see Fig. 10) show that the early scorpions were aquatic, and we may hope some day in better-preserved specimens than the two as yet discovered, to find the respiratory — organs of those creatures in the condition of projecting appendages serving aquatic respiration somewhat as in Limulus, though not necessarily repeating the exact form of ttip Virnnrl 15.—The remaining threethat pairs of mesosomatic appendages Scorpio andlung-book, Limulus. whilst Lettersas . . fnlofp<5 nf l,imnln<! ' . Fig. in Fig. 14. 1130 indicates there are 130 lamellae in theofScorpion’s 1150 It IS important to note that the series indicates that 150 similar lamellae are counted in the gill of Limulus. (After Lankester, loc. Clt of lamellae of the lung-book and the gill'^ book correspond exactly in structure, the narrow, flat, blood- carrying the lamellae. The margins of the lamellae of the space in the lamellae being interrupted by pillar-like junc- scorpion’s lung-book which are lowermost in the figures tions of the two surfaces in both cases (see Lankester (4)), (Fig. 15) and appear to be free are really those which are and the free surfaces of the adjacent lamellae being covered attached to the blood-holding axis. The true free ends with a very delicate chitinous cuticle which is drawn out are those nearest the stigma. into delicate hairs and processes. The elongated axis which Passing on now from the mesosoma we come in Scorpio to the metasoma of six segments, the first of which is broad whilst the rest are cylindrical. The last is perforated by the anus and carries the post-anal spine or sting. The somites of the metasoma carry no parapodia. In Limulus the metasoma is practically suppressed. In the allied extinct Eurypterines it is well developed, and resembles that of Scorpio. In the embryo Limulus (Fig. 42) the six somites of the mesosoma are not fused to form a carapace at an early stage, and they are followed by three separately marked metasomatic somites; the other three somites of the metasoma have disappeared in Limulus, but are represented by the unsegmented prse-anal region. It is probable that we have in the metasoma of Limulus a case of the disappearance of once clearly demarcated somites. It would be possible to suppose, on the other hand, that new Fig. 14.—The first three pairs of mesosomatic appendages of Scorpio and Limulus compared VII, somites are only beginning to make their the genital operculum ; VIII, the pectens of Scorpio and the first branchial plate of Limulus ; IXj -i the first pair of lung-books of Scorpio and the second branchial plate of Limulus ; gp, genital pore; ^PPCiD'anee Uere. The balance of various opst, epistigmatic sclerite ; stg, stigma or orifice of the hollow tendons of the branchial plates of considerations is against the latter hvDOLimulus. (After Lankester, loo. tit.) . . . ® . /I thesis. lollowmg the metasoma in Limuopens at the stigma in Scorpio and which can be cleared lus, we have as in Scorpio the post-anal spine—in this of soft, surrounding tissues and coagulated blood so as to case not a sting, but a powerful and important organ of present the appearance of a limb axis carrying the book- locomotion, serving to turn the animal over when it has like leaves of the lung is not really, as it would seem to fallen upon its back. The nature of the post-anal spine be at first sight, the limb axis. That is necessarily a has been strangely misinterpreted by some writers. Owen blood-holding structure and is obliterated and fused with (7) maintained that it represented a number of coalesced
