osomatic and telsonic carapaces; D, adult condition, still with only
two free mesosomatic somites.

(From Korschelt and Heider.)]

[Illustration: FIG. 41.--Five Stages in the development of the
trilobite _Sao hirsuta_.

From Korschelt and Heider, after Barrande.

A, Youngest stage.
B, Older stage with distinct pygidial carapace.
C, Stage with two free mesosomatic somites between the prosomatic
and telsonic carapaces.
D, Stace with seven free intermediate somites.
E, Stave with twelve free somites; the telsonic carapace has not
increased in size.
a, Lateral eye.
g, So-called facial "suture" (not really a suture).
p, Telsonic carapace.]

The most important general change which has affected the structure of
the nomomeristic Arachnida in the course of their historic development
is the transition from an aquatic to a terrestrial life. This has been
accompanied by the conversion of the lamelliform gill-plates into
lamelliform lung-plates, and later the development from the
lung-chambers, and at independent sites, of tracheae or air-tubes (by
adaptation of the vasifactive tissue of the blood-vessels) similar to
those independently developed in _Peripatus_, Diplopoda, Hexapoda and
Chilopoda. Probably tracheae have developed independently by the same
process in several groups of tracheate Arachnids. The nomomeristic
Arachnids comprise two sub-classes--one a very small degenerate
offshoot from early ancestors; the other, the great bulk of the class.

[Illustration: FIG. 42.--So-called "trilobite stage" of _Limulus
polyphemus_. A, Dorsal; B, ventral view.

(from Korschelt and Heider, after Leuckart.)]

Sub-Class I. (of the Nomomeristica). PANTOPODA.--Nomomeristic
Arachnids, in which the somites corresponding to mesosoma and metasoma
have entirely aborted. The seventh, and sometimes the eighth,
leg-bearing somite is present and has its leg-like appendages fully
developed. Monomeniscous eyes with a double (really triple) cell-layer
formed by invagination, as in the Eu-arachnida, are present The
Pantopoda stand in the same relation to _Limulus_ and _Scorpio_ that
_Cyamus_ holds to the thoracostracous Crustacea. The reduction of the
organism to seven leg-bearing somites, of which the first pair, as in
so many Eu-arachnida, are chelate, is a form of degeneration connected
with a peculiar quasi-parasitic habit resembling