Intermediate somites forming a mesosoma occur, but tend to fuse superficially with the metasomatic carapace or to become co-ordinated with the somites of the metasoma, whether fused or distinct to form one region, the opisthosoma (abdomen of authors).
- One of the Nymphonomorphous Pantopoda, Nymphon hispidum, showing the seven pairs of appendages I to 7; ab, the rudimentary opisthosoma; s, the mouth-bearing proboscis.
We shall, therefore, ignoring the ocular somite, speak of the first, second, third, fourth, fifth and sixth legbearing somites of the prosoma, and indicate the appendages by the Roman numerals, I, II, III, IV, V, VI, and whilst ignoring the praegenital somite we shall speak of the first, second, third, &c., somite of the mesosoma or opisthosoma (united mesosoma and metasoma) and indicate them by the Arabic numerals.
Not freeswimming, none of the prosomatic appendages modified to act as paddles; segments of the mesosoma and metasoma (= opisthosoma) not more than ten in number, distinct or coalesced.
- Free-swimming forms, with the appendages of the 6th or 5th and 6th pairs flattened or lengthened to act as oars; segments of mesosoma and metasoma (= opisthosoma), twelve in number.
- The primitive distinction between the mesosoma and the metasoma wholly or almost wholly obliterated, the two regions uniting to form an opisthosoma, which never consists of more than twelve somites and never bears appendages or breathing-organs behind the 4th somite.
The breathing-organs of the opisthosoma, when present, represented by two pairs of stigmata, opening either upon the 1st and 2nd (Pedipalpi) or the 2nd and 3rd somites (Solifugae, Pseudo-scorpiones), or by a single pair upon the 3rd (?
2nd) somite (Opiliones) of the opisthosoma, there being rarely an additional stigma on the 4th (some Solifugae).
The appendages of the 2nd somite of the opisthosoma absent, rarely minute and budlike (some Amblypygi), never pectiniform.
- Appendages of 1st pair bisegmented, without poison gland; of 2nd pair prehensile, their basal segments underlying the proboscis, and furnished with sterno 1 to i 1, Somites of the opisthosoma (mesosoma plus metasoma).
B, Dorsal view of the opisthosoma of the same.
A narrow prae-genital somite is present between opisthosoma and prosoma (figs.
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).
- 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 praegenital somite; opisth 1, first somite of the opisthosoma.
No distinct respiratory stigmata behind the sterna of the 1st and 2nd somites of the opisthosoma.
1 1, The eleventh somite of the opisthosoma.
opisth, First somite of the opisthosoma.
11 opisth, Eleventh somite of the opisthosoma.
Thelyphonusassamensis d .Ventral surface of theanteriorregionof the opisthosoma, the first somite being pushed upwards and forwards so as to expose the subjacent structures.
opistho I, First somite of the opisthosoma; opistho 2, second do.; g, genital aperture; 1, edges of the lamellae of the lung-books; m, stigmata of tergo-sternal muscles.
I to VI, the prosomatic appendages; 4, 5, 6, the fourth, fifth and sixth tergites of the opisthosoma.
Opisthosoma when segmented showing the same number of somites as in the Pedipalpi; usually unsegmented, the prae-genital somite constricted to form the waist; the appendages of its 3rd and 4th somites retained as spinning mammillae.
- Opisthosoma dis tinctly segmented, furnished with tergal plates, as in the Ambly pygi; the ventral surface of the 1st and 2nd somites with large sternal plates, covering the genital aperture and the two pairs of FIG.
In front of it the narrow waist is formed by the soft sternal area of the praegenital somite; 2, the sternite of the 2 second opisthosomatic somite covering the posterior pair of lung-sacs; and 4, the spinning appendages (limbs) of the opisthosoma; a, inner, b, outer ramus of the appendage; I I, sternite of the eleventh --
- Opisthosoma without trace of separate terga and sterna, the segmentation merely represented posteriorly by slight integumental folds and the sterna of the 1st and 2nd somites by the opercular plates of the pulmonary sacs.
The larger species of Bird's Nest Spiders (Avicularia), the opisthosoma of which is as large as a bantam's egg, undoubtedly attack young birds, and M'Cook gives an account of the capture in its web by an ordinary house spider of a small mouse.
Opisthosoma consisting of only ten somites, which have no tergal and sternal elements, the prae-genital somite contracted to form a " waist," as in the Pedipalpi; the last three narrowed to form a A B prae-1 2345 6789 io I I111I IV V VI gen Opisttaosoma Prosoma FIG.
C, Lateral view, I to VI, proprosternites; c, mesosternite; and d, metasternite of the somatic appendages; a,b,c, prosoma; f, ventral surface the three tergal plates of the of the prae-genital somite; prosoma; prae-gen, the prae g, sternite of the genital genital somite; 1 to 10, the somite (first opisthosomatic ten somites of the opisthosoma.
Opisthosoma composed of ten somites.
Respiratory organs tracheal, opening upon the ventral surface of the 2nd and 3rd, and sometimes also of the 4th somite of the opisthosoma.
opisth I, First or genital somite of the opisthosoma.
1 2, Anterior tracheal aperture of the opisthosoma in somite 2 of the opisthosoma.
I', Tracheal aperture in somite 3 of the opisthosoma.
The segmentation of the prosoma and the form of the appendages bear a homoplastic similarity to the head, pro-, meso-, and meta-thorax of a Hexapod with mandibles, maxillary palps and three pairs of walking legs; while the opistho io i e d c b o a S' S" 2 I VT V S IV III II I Opisthosoma Prosoma FIG.
2, 9, Io, First, second, ninth and tenth somites of the opisthosoma.
2 and 3, Sternites of the second and third somites of the opisthosoma, each showing a tracheal stigma.
to and it, Sternites of the tenth and eleventh somites of the opisthosoma.
10, Tergite of the tenth somite of the opisthosoma.
II, The evanescent eleventh somite of the opisthosoma.
Opisthosoma three minute and forming a slender generally-retracted tail like that of Thelyphonus.
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.
III to VI, The third, fourth, fifth followed by the opisthosoma of and sixth appendages of the four visible somites.
A movable membranous j oint between the prosoma and the opisthosoma, the generative aperture opening upon the ventral side of the membrane.
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.
,?, VI 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.
prosoma and opisthosoma; a, tracheal stigma; b, last somite.
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.
The prosoma and opisthosoma were broadly confluent and probably immovably welded together.
- Opisthosoma consisting of ten segments defined by integumental grooves, each of the anterior four of these furnished with a single pair of dorsally-placed spiracles or tracheal stigmata.
A, Lateral view with appendages III to VI removed; 1, plate covering the whole dorsal area, representing the fused tergal sclerites of the prosoma and opisthosoma; 2, similarly-formed ventral plate; 3, tracheal stigma.
a prae-gen 1 2 3 s Lam_ Opisthosoma FIG.
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 FIG.
- 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 united in the middle line behind the mouth, those of the 3rd, 4th, 5th and 6th pairs are widely separated and not provided with sterno-coxal (maxillary) lobes, and take no share in mastication; the respiratory stigmata, when present, belong to the prosoma, and the primitive segmentation of the opisthosoma has entirely or almost entirely disappeared.
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