somites of the
metasoma.

[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 Set_. vol. xxi., 1881)]

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.

[Illustration: 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 V