g remains in
the adult scorpion. In all the embryonic or permanent opening is on
the coxa of the fifth pair of prosomatic limbs. Thus an organ newly
discovered in Scorpio was found to have its counterpart in Limulus.

The name "coxal gland" needs to be carefully distinguished from
"crural gland," with which it is apt to be confused. The crural
glands, which occur in many terrestrial Arthropods, are epidermal in
origin and totally distinct from the coxal glands. The coxal glands of
the Arachnida are structures of the same nature as the green glands of
the higher Crustacea and the so-called "shell glands" of the
Entomostraca. The latter open at the base of the fifth pair of limbs
of the Crustacean, just as the coxal glands open on the coxal joint of
the fifth pair of limbs of the Arachnid. Both belong to the category
of "coelomoducts," namely, tubular or funnel-like portions of the
coelom opening to the exterior in pairs in each somite (potentially,)
and usually persisting in only a few somites as either "urocoels"
(renal organs) or "gonocoets" (genital tubes). In Peripatus they occur
in every somite of the body. They have till recently been very
generally identified with the nephridia of Chaetopod worms, but there
is good reason for considering the true nephridia (typified by the
nephridia of the earthworm) as a distinct class of organs (see
Lankester in vol. ii. chap. in. of _A Treatise on Zoology_, 1900). The
genital ducts of Arthropoda are, like the green glands, shell glands
and coxal glands, to be regarded as coelomoducts (gonocoels). The
coxal glands do not establish any special connexion between Limulus
and Scorpio, since they also occur in the same somite in the lower
Crustacea, but it is to be noted that the coxal glands of Limulus are
in minute structure and probably in function more like those of
Arachnids than those of Crustacea.

[Illustration: FIG. 15.--The remaining three pairs of mesosomatic
appendages of Scorpio and Limulus. Letters as in fig. 14. l130
indicates that there are 130 lamellae in the scorpion's lung-book,
whilst l150 indicates that 150 similar lamellae are counted in the
gill of Limulus.

(After Lankester, _loc. cit._)]

4. _The Entosternites and their Minute Structure._--Strauss-Durckheim
(1) was the first to insist on the affinity between Limulus and the
Arachnids, indicated by the presence of a free suspended en