een separately evolved as
distinct lines of descent from the ancient aquatic Arachnida. The
Holosomata and Rhynchostomi are probably offshoots from the stem of
the Araneae, and it is not unlikely (in view of the structure of the
prosomatic somites of the Tartarides) that the Solifugae are connected
in origin with the Pedipalpi. The appearance of tracheae in place of
lung-sacs cannot be regarded as a starting-point for a new line of
descent comprising all the tracheate forms; tracheae seem to have
developed independently in different lines of descent. On the whole,
the Epectinata are highly specialized and degenerate forms, though
there are few, if any, animals which surpass the spiders in rapidity
of movement, deadliness of attack and constructive instincts.

[Illustration: From Lankester, _Q. J. Mic. Sci._ N.S. _vol_. xxi.,
1881.

FIG. 54.--_Thelyphonus_, one of the Pedipalpi.

A, Ventral view.
I, Chelicera (detached).
II, Chelae.
III, Palpiform limb.
IV to VI, The walking legs.
stc, Sterno-coxal process (gnathobase) of the chelae.
st^1, Anterior sternal plate of the prosoma.
st^2, Posterior sternal plate of the prosoma.
pregen, Position of the prae-genital somite (not seen).
l, l, Position of the two pulmonary sacs of the right side.
1 to 11, Somites of the opisthosoma (mesosoma plus metasoma).
msg, Stigmata of the tergo-sternal muscles.
an, Anus.
B, Dorsal view of the opisthosoma of the same.
pregen, The prae-genital somite.
p, The tergal stigmata of the tergo-sternal muscles.
paf, Post-anal segmented filament corresponding to the post-anal
spine of Limulus.]

Order 2. Pedipalpi (figs. 54 to 59).--Appendages of 1st pair
bisegmented, without poison gland; of 2nd pair prehensile, their basal
segments underlying the proboscis, and furnished with sterno-coxal
(maxillary) process, the apical segment tipped with a single movable
or immovable claw; appendages of 3rd pair different from the
remainder, tactile in function, with at least the apical segment
many-jointed and clawless. The ventral surface of the prosoma bears
prosternal, metasternal and usually mesosternal chitine-plates (fig.
55). A narrow prae-genital somite is present between opisthosoma and
prosoma (figs. 55, 57). Opisthosoma consisting of eleven somites,
almost wholly without visible appendages. Intromittent organ of male
b