coxae.

(From Lankester, _loc. cit_, after Packard.)]

_Differences between Limulus and Scorpio._--We have now passed in
review the principal structural features in which Limulus agrees with
Scorpio and differs from other Arthropoda. There remains for
consideration the one important structural difference between the two
animals. Limulus agrees with the majority of the Crustacea in being
destitute of renal excretory caeca or tubes opening into the hinder
part of the gut. Scorpio, on the other hand, in common with all
air-breathing Arthropoda except Peripatus, possesses these tubules,
which are often called Malpighian tubes. A great deal has been made of
this difference by some writers. It has been considered by them as
proving that Limulus, in spite of all its special agreements with
Scorpio (which, however, have scarcely been appreciated by the writers
in question), really belongs to the Crustacean line of descent, whilst
Scorpio, by possessing Malpighian tubes, is declared to be
unmistakably tied together with the other Arachnida to the tracheate
Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess
Malpighian tubes.

[Illustration: FIG. 29.--Diagram of the arterial system of A, Scorpio,
and B, Limulus. The Roman numerals indicate the body somites and the
two figures are adjusted for comparison. ce, Cerebral arteries; sp,
supra-spinal or medullary artery; c, caudal artery; l, lateral
anastomotic artery of Limulus. The figure B also shows the peculiar
neural investiture formed by the cerebral arteries in Limulus and the
derivation from this of the arteries to the limbs, III, IV, VI,
whereas in Scorpio the latter have a separate origin from the anterior
aorta.

(From Lankester, "Limulus an Arachnid.")]

It must be pointed out that the presence or absence of such renal
excretory tubes opening into the intestine appears to be a question of
adaptation to the changed physiological conditions of respiration, and
not of morphological significance, since a pair of renal excretory
tubes of this nature is found in certain Amphipod Crustacea
(Talorchestia, &c.) which have abandoned a purely aquatic life. This
view has been accepted and supported by Professors Korschelt and
Heider (16). An important fact in its favour was discovered by Laurie
(17), who investigated the embryology of two species of Scorpio under
Lankester's direction.