obation of the head and body--due to the expansion and
flattening of the sides or "pleura" of the tegumentary skeleton--is so
closely repeated in the young of Limulus that the latter has been called
"the trilobite stage" of Limulus (fig. 42 compared with fig. 41). No
Crustacean exhibits this trilobite form. But most important of the
evidences presented by the trilobites of affinity with Limulus, and
therefore with the Arachnida, is the tendency less marked in some,
strongly carried out in others, to form a pygidial or telsonic shield--a
fusion of the posterior somites of the body, which is precisely
identical in character with the metasomatic carapace of Limulus. When to
this is added the fact that a post-anal spine is developed to a large
size in some trilobites (fig. 38), like that of Limulus and Scorpio, and
that lateral spines on the pleura of the somites are frequent as in
Limulus, and that neither metasomatic fusion of somites nor post-anal
spine, nor lateral pleural spines are found in any Crustacean, nor all
three together in any Arthropod besides the trilobites and Limulus--the
claim of the trilobites to be considered as representing one order of a
lower grade of Arachnida, comparable to the grade Entomostraca of the
Crustacea, seems to be established.

The fact that the single pair of prae-oral appendages of trilobites,
known only as yet in one genus, is in that particular case a pair of
uni-ramose antennae--does not render the association of trilobites and
Arachnids improbable. Although the prae-oral pair of appendages in the
higher Arachnida is usually chelate, it is not always so; in spiders it
is not so; nor in many Acari. The bi-ramose structure of the post-oral
limbs, demonstrated by Beecher in the trilobite Triarthrus, is no more
inconsistent with its claim to be a primitive Arachnid than is the
foliaceous modification of the limbs in Phyllopods inconsistent with
their relationship to the Arthrostracous Crustaceans such as Gammarus
and Oniscus.

Thus, then, it seems that we have in the trilobites the representatives
of the lower phases of the Arachnidan pedigree. The simple anomomeristic
trilobite, with its equi-formal somites and equi-formal appendages, is
one term of the series which ends in the even more simple but degenerate
Acari. Between the two and at the highest point of the arc, so far as
morphological differentiation is concerned, stands the scorpion; near to
it in the trilobite's direction (