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figs. 17 and 18). They appear at first as outstanding processes on the surface of the body.
1911 Britannica-Arachnida-mygalomorphous spiders2.png
Fig. 6.—Dorsal surface of the same
entosternum as that drawn in fig. 5. Ph.N.,
pharyngeal notch.

(After Lankester, loc. cit.)

The exact mode in which the in-sinking of superficial outstanding limbs, carrying gill-lamellae, has historically taken place has been a matter of much speculation. It was to be hoped that the specimen of the Silurian scorpion (Palaeophonus) from Scotland, showing the ventral surface of the mesosoma (fig. 49), would throw light on this matter; but the specimen recently carefully studied by the writer and 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 as to the exact equivalence of margins, axis, &c., obtaining between the lung-book of Scorpio and the gill-book of Limulus. Zoologists are familiar with many instances (fishes, crustaceans) in which the protective walls of a water-breathing organ or gill-apparatus become converted into an air-breathing organ or lung, but there is no other case known of the conversion of gill processes themselves into air-breathing plates.

1911 Britannica-Arachnida-Limulus polyphemus3.png

Fig. 7.—Diagram of the dorsal surface of Limulus polyphemus.

oc, Lateral compound eyes. VIII to XIII, The six somites of the
oc′, Central monomeniscous eyes. mesosoma, each with a movable
PA, Post-anal spine. pleural spine and a pair of dorsal
I to VI, The six appendage-bearing entopophysis or muscle-attaching
somites of the prosoma. ingrowths.
VII, Usually considered to be the tergum                      XIV to XVIII, The confluent or
of the genital somite, but suggested by unexpressed six somites of the
Pocock to be that of the otherwise metasoma.
suppressed praegenital somite.  
    [According to the system of numbering explained in the text, if VII is the tergum of the
praegenital somite (as is probable) it should be labelled Prg without any number, and the
somites VIII to XIII should be lettered 1 to 6, indicating that they are the six normal somites
of the mesosoma; whilst XV to XVIII should be replaced by the numbers 7 to 12—an
additional suppressed segment (making up the typical six) being reckoned to the
metasomatic fusion.]

(From Lankester, Q. J. Micr. Sci. vol. xxi., 1881.)

1911 Britannica-Arachnida-scorpion.png
Fig. 8.—Diagram of the dorsal surface
of a scorpion to compare with
fig. 7. Letters and Roman numerals as
in fig. 7, excepting that VII is here
certainly the tergum of the first somite
of the mesosoma—the genital somite—
and is not a survival of the embryonic
praegenital somite. The anus (not seen)
is on the sternal surface.

(From Lankester, loc. cit.)

The identification of the lung-books of Scorpio with the gill-books of Limulus is practically settled by the existence of the pectens in Scorpio (fig. 14, VIII) on the second mesosomatic somite. There is no doubt that these are parapodial or limb appendages, carrying numerous imbricated secondary processes, and therefore comparable in essential structure to the leaf-bearing plates of the second mesosomatic somite of Limulus. They have remained unenclosed and projecting on the surface of the body, as once were the appendages of the four following somites. But they have lost their respiratory function. In non-aquatic life such an unprotected organ cannot subserve respiration. The “pectens” have become more firmly chitinized and probably somewhat altered in shape as compared with their condition in the aquatic ancestral scorpions. Their present function in scorpions is not ascertained. They are not specially sensitive under ordinary conditions, and may be touched or even pinched without causing any discomfort to the scorpion. It is probable that they acquire special 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 (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 the broad plates of Limulus.
It is important to note that the series of lamellae of the lung-book and the gill-book correspond exactly in structure, the narrow, flat blood-space in the lamellae being interrupted by pillar-like junctions of the two surfaces in both cases (see Lankester (4)), and the free surfaces of the adjacent lamellae being covered with a very delicate chitinous cuticle which is drawn out into delicate hairs and processes. The elongated axis which opens at the stigma in Scorpio and which can be cleared of soft, surrounding tissues and coagulated blood so as to present the appearance of a limb axis carrying the book-like leaves of the lung is not really, as it would seem to be at first sight, the limb axis. That is necessarily a blood-holding structure and is obliterated and fused with 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 in-grown cuticle give rise to the false appearance of a limb axis carrying the lamellae. The margins of the lamellae of the scorpion’s lung-book, which are lowermost in the figures (fig. 15) and appear to be free, are really those which are attached to the blood-holding axis. The true free ends are those nearest the stigma.
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 prae-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 somites are only beginning to make their appearance here. The balance of various considerations is against the latter hypothesis. Following the metasoma in Limulus, we have as in Scorpio the post-anal spine—in this case not a sting, but a powerful and important organ of locomotion, serving to turn the animal over when it has fallen upon its back. The nature of the post-anal spine has been strangely misinterpreted by some writers. Owen (7) maintained that it represented a number of coalesced somites, regardless of its post-anal position and mode of development. The agreement of the grouping of the somites, of the form of the parapodia (appendages, limbs) in each region, of the position of the genital aperture and operculum, of the position and character of the eyes, and of the powerful post-anal spines not seen in other Arthropods, is very convincing as to the affinity