w; I to VI, the six prosomatic appendages.
B, Ventral view of the prosoma and of the first somite of the
opisthosoma, with the appendages I to VI cut off at the base; a,
tracheal stigma; mx, maxillary processes of the coxae of the 3rd
pair of appendages; g, genital aperture.
C, Ventral surface of the prosoma and opisthosoma; a, tracheal
stigma; b, last somite.
D, Lateral view of the 1st and 2nd pair of appendages.
E, Lateral view of the whole body and two 1st appendages, showing
the fusion of the dorsal elements of the prosoma into a single
plate, and of those of the opisthosoma into an imperfectly
segmented plate continuous with that of the prosoma.]

Apparently related to the Opiliones are two extinct groups, the
Anthracomarti and Phalangiotarbi, which are not known to have survived
the Carboniferous period. In the Anthracomarti the opisthosoma was
movably articulated to the prosoma, and consisted of from eight to ten
segments furnished with movable lateral plates, the anal segment being
overlapped dorsally by a laminate expansion of the preceding segment.
The carapace of the prosoma was unsegmented and often bore a pair of
eyes. The appendages of the 2nd pair were slender and pediform; those
of the 3rd, 4th, 5th and 6th pairs were similar in form and ambulatory
in function with their basal segments arranged round a sternal area as
in the order Araneae. The best-known genera were _Anthracomartus_ and
_Eophognus_.

In the Phalangiotarbi the appendages resembled those of the
Anthracomarti, except that the basal segments of the last four pairs
were usually approximated in the middle line leaving a long and narrow
sternal area between; and the carapace of the prosoma was unsegmented.
The prosoma and opisthosoma were broadly confluent and probably
immovably welded together. The opisthosoma consisted of eight or nine
segments, whereof the anterior five or six were very short in the
dorsal region, and the posterior three exceptionally large with the
anal orifice terminal.

Several genera have been established, the best-characterized being
_Geraphognus_ and _Architarbus_.

Order 9. Rhynchostomi = Acari (see fig. 78).--Degenerate Arachnids
resembling the Opiliones in many structural points, but chiefly
distinguishable from them by the following features:--The basal
segments of the appendages of the 2nd pair are