Page:EB1911 - Volume 02.djvu/324

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309
ARACHNIDA
composed, at least in many cases, of eleven somites, the 11th somite very small, often hidden within the 10th. Respiratory organs in the form of tracheal tubes opening by a pair of stigmata in the 2nd and 3rd somites of the opisthosoma. Intromittent organ of male beneath sternum of the 1st somite of the opisthosoma.
Sub-order a. Panctenodactyli.—Dorsal plate of prosoma (carapace) narrowed in front; the appendages of the 1st pair small, much narrower, taken together, than the posterior border of the carapace. Serrula on movable digit of appendages of 1st pair fixed throughout its length, and broader at its proximal than at its distal end; the immovable digit with an external process.
Family—Cheliferidae (Chelifer (figs. 70, 71, 72), Chiridium).
 ”   Garypidae (Garypus).


1911 Britannica-Arachnida-Garypus litoralis3.png

Fig. 72.—Garypus litoralis, one of the Pseudoscorpiones.
Lateral view.

I to VI, Basal segments of the six         2, 3, 10, The second,
prosomatic third and tenth
appendages. somites of the
o, Eyes. opisthosoma.
prae-gen, Tergite of the 11, The minute eleventh
prae-genital somite. somite;
1, Genital or first an, The anus.
opisthosomatic
somite.

(Original.)


Sub-order b. Hemictenodactyli.—Dorsal plate of prosoma scarcely narrowed in front; the appendages of the 1st pair large, not much narrower, taken together, than the posterior border of the carapace. The serrula or the movable digit free at its distal end, narrowed at the base; no external lamina on the immovable digit.
Family—Obisiidae (Obisium, Pseudobisium).
 ”   Chthoniidae (Chthonius, Tridenchthonius).
Remarks.—The book-scorpions—so called because they were, in old times, found not unfrequently in libraries—are found in rotten wood and under stones. The similarity of the form of their appendages to those of the scorpions suggests that they are a degenerate group derived from the latter, but the large size of the prae-genital somite in them would indicate a connexion with forms preceding the scorpions. Reference to literature (37).


1911 Britannica-Arachnida-Cryptostemma Karschii.png

Fig. 73.—Cryptostemma Karschii, one of the Podogona.
Dorsal view of male.

III to VI, The third, fourth, an, Orifice within which the
fifth and sixth caudal segments are
appendages of the withdrawn.
prosoma. E, Extremity of the fifth
a, Movable (hinged) sclerite           appendage of the male
(so-called hood) modified to subserve
overhanging the first copulation.
pair of appendages.
b, Fused terga of the
prosoma followed by
the opisthosoma of four
visible somites.

(Original drawing by Pocock and Pickard-Cambridge.)


Order 7. Podogona = Ricinulei (see figs. 73 to 76).—Dorsal area of prosoma furnished with two shields, a larger behind representing, probably, the tergal elements of the somites, and a smaller in front, which is freely articulated to the former and folds over the appendages of the 1st pair. Ventral area without distinct sternal plates. Appendages of 1st pair, bi-segmented, completely chelate. Appendages of 2nd pair, with their basal segments uniting in the middle line below the mouth, weakly chelate at apex. Appendages of 3rd, 4th, 5th and 6th pairs similar in form; their basal segments in contact in the middle line and immovably welded, except those of the 3rd pair, which have been pushed aside so that the bases of the 2nd and 4th pairs are in contact with each other. A movable membranous joint between the prosoma and the opisthosoma, the generative aperture opening upon the ventral side of the membrane. Prae-genital somite suppressed; the opisthosma consisting of nine segments, whereof the first and second are almost suppressed and concealed within the joint between the prosoma and the opisthosoma; the following four large and manifest, and the remaining three minute and forming a slender generally-retracted tail like that of Thelyphonus. Respiratory organs tracheal, opening by a pair of spiracles in the prosoma above the base of the fifth appendage on each side. Intromittent organ of male placed at the distal end of the appendage of the 5th pair.
Family—Cryptostemmidae (Cryptostemma, Poliochera), Carboniferous.
1911 Britannica-Arachnida-Cryptostemma Karschii2.png
Fig. 74.—Cryptostemma Karschii, anterior aspect of the prosoma with the “hood” removed. I to IV, first to fourth
appendages of the prosoma; a, basal segment of the second pair of appendages meeting its fellow in the middle
line (see fig. 75).


(Original drawing by Pocock and Pickard-Cambridge.)


Remarks on the Podogona.—The name given to this small but remarkable group has reference to the position of the male intromittent organ (fig. 73, e). They are small degenerate animals with a relatively firm integument. Not more than four species and twice that number of specimens are known. They have been found in West Africa and South America. A fact of special interest in regard to them is that the genus Poliochera, from the Coal Measures, appears to be a member of the same group. The name Cryptostemma, given to the first-known genus of the order, described by Guérin-Méneville, refers to the supposed concealment of the eyes by the movable cephalic sclerite. Reference to literature (38).


1911 Britannica-Arachnida-Cryptostemma Karschii3.png

Fig. 75.—Cryptostemma Karschii, one of the Podogona. Ventral view.

I to VI, The six pairs of appendages of the prosoma, the last three cut short.
1, 2, 3, 4, The four somites of the opisthosoma.
a. Visible hood overhanging the first pair of appendages.
b, Position of the genital orifice.
c, Part of 3rd appendage.

d, Fourth segment of 2nd appendage. Observe that the basal segment of appendage III does not meet its fellow in the
middle line.

(Original drawing by Pocock and Pickard-Cambridge.)

1911 Britannica-Arachnida-Cryptostemma Karschii4.png
Fig. 76.
Cryptostemma
Karschii
. Extremity of
the fifth pair of
appendages of the
female for comparison
with that of the male E
in fig. 73.
Order 8. Opiliones (see fig. 77).—Dorsal area of prosoma covered by a single shield usually bearing a pair of eyes. Sternal elements much reduced. Appendages of 1st pair large, three segmented and completely chelate; of 2nd pair either simple and pediform, or prehensile and subchelate; of remaining four pairs, similar in form, ambulatory in function; the basal segment of the 2nd, 3rd and sometimes of the 4th pairs of appendages furnished with sterno-coxal (maxillary) lobe. Opisthosoma confluent throughout its breadth with the prosoma, with the dorsal plate of which its anterior tergal plates are more or less fused; at most ten opisthosomatic somites traceable; the generative aperture thrust far forwards between the basal segments of the 6th appendages. Prae-genital somite suppressed. Respiratory organs tracheal, opening by a pair of stigmata situated immediately behind the basal segments of the 6th pair of appendages on what is probably the sternum of the 2nd opisthosomatic somite and also in some cases upon the 5th segment of the legs.
Intromittent organ of male lying within the genital orifice.
Sub-order a. Laniatores.—Orifice of foetid glands opening above the coxa of the 4th appendage, not raised upon a tubercle. Orifice of coxal gland situated just behind that of the foetid gland. Sternal plate of prosoma long and narrow, with a distinct prosternal element underlying the mouth. Coxae of 4th, 5th and 6th appendages immovable. Appendages of 2nd pair, strong, usually prehensile and spiny. Genital orifice covered by an operculum.
Families—Gonoleptidae (Gonoleptes, Goniasoma).
      Biantidae (Biantes).
      Oncopodidae (Oncopus, Pelitnus).
      Trioenonychidae (Trioenonyx, Acumontia).
Sub-order b. Palpatores.—Orifice of foetid glands opening above the coxa of the 3rd appendage, not raised upon a tubercle. Orifice of coxal gland situated between the coxae of the 5th and 6th appendages. Sternal plate of prosoma usually short and wide, rarely longer than broad; with a larger or smaller prosternal element underlying the mouth. Coxae of 3rd, 4th, 5th and 6th appendages movable