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305
ARACHNIDA
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.


1911 Britannica-Arachnida-Thelyphonus.png

From Lankester, Q. J. Mic. Sci. N.S. vol. xxi., 1881.
Fig. 54.—Thelyphonus, one of the Pedipalpi.

A, Ventral view. 1 to 11, Somites of the
I, Chelicera (detached). opisthosoma (mesosoma
II, Chelae. plus metasoma).
III, Palpiform limb. msg, Stigmata of the tergo-
IV to VI, The walking legs. sternal muscles.
stc, Sterno-coxal process an, Anus.
(gnathobase) of the chelae. B, Dorsal view of the
st1, Anterior sternal plate of the           opisthosoma of the same.
prosoma. pregen, The prae-genital
st2, Posterior sternal plate of
the prosoma. somite.
pregen, Position of the p, The tergal stigmata of the
prae-genital somite (not seen). tergo-sternal muscles.
l, l, Position of the two paf, Post-anal segmented
pulmonary sacs of the right filament corresponding to
side. 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 beneath the genital operculum (

=

sternum of the 1st somite of opisthosoma).
1911 Britannica-Arachnida-Thelyphonus sp.png
Fig. 55.—Thelyphonus sp. Ventral view of the anterior portion of the body to show the three prosomatic sternal plates a, b, c,
and the rudimentary sternal element of the prae-genital somite; opisth 1, first somite of the opisthosoma.




(From a drawing made by Pickard—Cambridge, under the direction of R. I. Pocock.)

Note.—The possibility of another interpretation of the anterior somites of the mesosoma and the prae-genital somite must be borne in mind. Possibly, though not probably, the somites carrying the two lung-sacs correspond to the first two lung-bearing somites of Scorpio, and it is the genital opening which has shifted. The same caution applies in the case of the Araneae. Excalation of one or of two anterior mesosomatic somites, besides the prae-genital somite, would then have to be supposed to have occurred also.
1911 Britannica-Arachnida-Thelyphonus assamensis.png
Fig. 56.—Thelyphonus assamensis ♂. Ventral surface of the anterior
region of the opisthosoma, the first somite being pushed upwards and
forwards so as to expose the subjacent structures. opistho 1, First
somite of the opisthosoma; opistho 2, second do.; g, genital aperture;
l, edges of the lamellae of the lung-books; m, stigmata of tergo-sternal
muscles.

(Original drawing by Pocock.)

Sub-order a. Uropygi.—Prosoma longer than wide, its sternal area very narrow, furnished with a large prosternal and metasternal plate, and often with a small mesosternal sclerite. Appendages of 2nd pair with their basal segments united in the middle line and incapable of lateral movement; appendages of 3rd pair with only the apical segment many-jointed. Opisthosoma without trace of appendages; its posterior somites narrowed to form a movable tail for the support of the post-anal sclerite, which has no poison glands.
Tribe 1. Urotricha.—Dorsal area of prosoma covered with a single shield (? two in Geralinura), bearing median and lateral eyes. Post-anal sclerite modified as a long, many-jointed feeler. Appendages of 2nd pair folding in a horizontal plane, completely chelate, the claw immovably united to the sixth segment. Respiratory organs present in the form of pulmonary sacs.
Family—Thelyphonidae (Thelyphonus (fig. 54), Hypoctonus, *Geralinura).
Tribe 2. Tartarides.—Small degenerate forms with the dorsal area of the prosoma furnished with two shields, a larger in front covering the anterior four somites, and a smaller behind covering the 5th and 6th somites; the latter generally subdivided into a right and left portion. There is also a pair of narrow tergal sclerites interposed between the anterior and posterior shields. Eyes evanescent or absent. Appendages of 2nd pair folding in a vertical plane, not chelate, the claw long and movable. Post-anal sclerite short and undivided. No distinct respiratory stigmata behind the sterna of the 1st and 2nd somites of the opisthosoma.
Family—Hubbardiidae (Schizomus, Hubbardia) (figs. 57-59).
1911 Britannica-Arachnida-Schizomus crassicaudatus.png

Fig. 57.—Schizomus crassicaudatus, one of the Tartarid Pedipalpi. Ventral view of a female with the appendages cut short near the base.

a, Prosternum of prosoma.
b, Metasternum of prosoma.
prae-gen, The prae-genital somite.
1 opisth, First somite of the opisthosoma.
11 opisth, Eleventh somite of the opisthosoma.
pa, Post-anal lobe of the female (compare the jointed filament in Thelyphonus, fig. 54).

(Original drawing by Pickard-Cambridge, directed by Pocock.)

1911 Britannica-Arachnida-Schizomus crassicaudatus2.png

Fig. 58.—Schizomus crassicaudatus, a Tartarid Pedipalp. Dorsal view of a male with the appendages cut short.

I to VI. The prosomatic appendages.
a, Anterior plate.
b, Posterior plate of the prosomatic carapace.
prae-gen, Tergum of the prae-genital somite.
11, The eleventh somite of the opisthosoma.
pa, Post-anal lobe of the male—a conical body with narrow basal stalk.

(Original as preceding.)


Sub-order b. Amblypygi.—Prosoma wider than long, covered above by a single shield bearing median and lateral eyes, which have diplostichous ommatea. Sternal area broad, with prosternal, two mesosternal, and metasternal plates, the prosternum projecting forwards beneath the coxae of the 2nd pair of appendages. Appendages of 2nd pair folding in a horizontal plane; their basal segments