with two pairs of fissures, situated in the last segment of the thorax
and the first segment of the abdomen. Lastly, in a young Anilocra, I
find the heart (Figure 15) extending through the whole length of the
abdomen and furnished with four (or five?) fissures, which are not
placed in pairs but alternately to the right and left in successive
segments. In other animals of this order, which I have as yet only
cursorily examined, further differences will no doubt occur. But why, in
two orders so nearly allied to each other, should we find in the one
such a constancy, in the other such a variability, of the same highly
important organ? From the schoolmen we need expect no explanation, they
will either decline the discussion of the "wherefore" as foreign to
their province, as lying beyond the boundaries of Natural History, or
seek to put down the importunate question by means of a sounding
paraphrase of the facts, abundantly sprinkled with Greek words. As I
have unfortunately forgotten my Greek, the second way out of the
difficulty is closed to me; but as I luckily reckon myself not amongst
the incorporated masters, but, to use Baron von Liebig's expression,
amongst the "promenaders on the outskirts of Natural History," this
affected hesitation of the schoolmen cannot dissuade me from seeking an
answer, which indeed presents itself most naturally from Darwin's point
of view.

As not only the Tanaides (which reasons elsewhere stated (vide supra)
justify us in regarding as particularly nearly related to the primitive
Isopod) and the Amphipoda, but also the Decapod Crustacea, possess a
heart with three pairs of fissures essentially in the same position; and
as the same position of the heart recurs (vide infra) even in the
embryos of the Mantis-Shrimps (Squilla), in which the heart of the adult
animal, and even, as I have elsewhere shown, that of the larvae when
still far from maturity, extends in the form of a long tube with