It appears that the Malpighian tubes of Scorpio
are developed from the mesenteron, viz. that portion of the gut which
is formed by the hypoblast, whereas in Hexapod insects the similar
caecal tubes are developed from the proctodaeum or in-pushed portion
of the gut which is formed from epiblast. In fact it is not possible
to maintain that the renal excretory tubes of the gut are of one
common origin in the Arthropoda. They have appeared independently in
connexion with a change in the excretion of nitrogenous waste in
Arachnids, Crustacea, and the other classes of Arthropoda when aerial,
as opposed to aquatic, respiration has been established--and they have
been formed in some cases from the mesenteron, in other cases from the
proctodaeum. Their appearance in the air-breathing Arachnids does not
separate those forms from the water-breathing Arachnids which are
devoid of them, any more than does their appearance in certain
Amphipoda separate those Crustaceans from the other members of the
class.

Further, it is pointed out by Korschelt and Heider that the hinder
portion of the gut frequently acts in Arthropoda as an organ of
nitrogenous excretion in the absence of any special excretory tubules,
and that the production of such caeca from its surface in separate
lines of descent does not involve any elaborate or unlikely process of
growth. In other words, the Malpighian tubes of the terrestrial
Arachnida are _homoplastic_ with those of Hexapoda and Myriapoda, and
not _homogenetic_ with them. We are compelled to take a similar view
of the agreement between the tracheal air-tubes of Arachnida and other
tracheate Arthropods. They are homoplasts (see 18) one of another, and
do not owe their existence in the various classes compared to a common
inheritance of an ancestral tracheal system.

[Illustration: FIG. 30.--View from below of a scorpion (_Buthus
occitanus_) opened and dissected so as to show the pericardium with its
muscles, the lateral arteries, and the tergo-sternal muscles.

PRO, Prosoma.
dpm, Dorso-plastral muscle.
art, Lateral artery.
tsm^1, Tergo-sternal muscle (labelled dv in fig. 31) of the second
(pectiniferous) mesosomatic somite; this is the most anterior pair
of the series of six, none are present in the genital somite.
tsm^4, Tergo-sternal muscle of the fifth mesosomatic somite.
tsm^6, Tergo-sternal muscle of the enl