tosternum
or plastron or entosternite in both. We have figured here (figs. 1 to
6) the entosternites of Limulus, Scorpio and Mygale. Lankester some
years ago made a special study of the histology (3) of these
entosternites for the purpose of comparison, and also ascertained the
relations of the very numerous muscles which are inserted into them
(4). The entosternites are cartilaginous in texture, but they have
neither the chemical character nor the microscopic structure of the
hyaline cartilage of Vertebrates. They yield chitin in place of
chondrin or gelatin--as does also the cartilage of the Cephalopod's
endoskeleton. In microscopic structure they all present the closest
agreement with one another. We find a firm, homogeneous or sparsely
fibrillated matrix in which are embedded nucleated cells (corpuscles
of protoplasm) arranged in rows of three, six or eight, parallel with
the adjacent lines of fibrillation.

[Illustration: FIG. 16.--Diagram to show the way in which an
outgrowing gill-process bearing blood-holding lamellae, may give rise,
if the sternal body wall sinks inwards, to a lung-chamber with
air-holding lamellae.

I is the embryonic condition.
bs, Blood sinus.
L is the condition of outgrowth with gl, gill lamellae.
A is the condition of in-sinking of the sternal surface and
consequent enclosure of the lamelligerous surface of the appendage
in a chamber with narrow orifice--the pulmonary air-holding
chamber.
pl, Pulmonary lamellae.
bs, Blood sinus.

(After Kingsley.)]

A minute entosternite having the above-described structure is found in
the Crustacean Apus between the bases of the mandibles, and also in
the Decapoda in a similar position, but in no Crustacean does it
attain to any size or importance. On the other hand, the entosternite
of the Arachnida is a very large and important feature in the
structure of the prosoma, and must play an important part in the
economy of these organisms. In Limulus (figs. 1 and 2) it has as many
as twenty-five pairs of muscles attached to it, coming to it from the
bases of the surrounding limbs and from the dorsal carapace and from
the pharynx. It consists of an oblong plate 2 in. in length and 1 in
breadth, with a pair of tendinous outgrowths standing out from it at
right angles on each side. It "floats" between the prosomatic nerve
centres and the alimentary c