An internal skeletal plate, the so-called "entosternite " of fibrocartilaginous tissue, to which many muscles are attached, is placed between the nerve-cords and the alimentary tract in the prosoma of the larger forms (Limulus, Scorpio, Mygale). In the same and other leading forms a pair of much-coiled glandular tubes, the coxal glands (coelomocoels in origin), is found with a duct opening on the coxa of the fifth pair of appendages of the prosoma. The vascular system is highly developed (in the non-degenerate forms); large arterial branches closely accompany or envelop the chief nerves; capillaries are well developed. The blood-corpuscles are large amoebiform cells, and the blood-plasma is coloured blue by haemocyanin. The alimentary canal is uncoiled and cylindrical, and gives rise laterally to large gastric glands, which are more than a single pair in number (two to six pairs), and may assume the form of simple caeca. The mouth is minute and the pharynx is always suctorial, never gizzard-like. The gonadial tubes (gonocoels or gonadial coelom) are originally reticular and paired, though they may be reduced to a simpler condition. They open on the first somite of the mesosoma. In the numerous degenerate forms simplification occurs by obliteration of the demarcations of somites and the fusion of body-regions, together with a gradual suppression of the lamelliferous respiratory organs and the substitution for them of tracheae, which, in their turn, in the smaller and most reduced members of the group, may also disappear. The Eu-arachnida are divided into two grades with reference to the condition of the respiratory organs as adapted to aquatic or terrestrial life. Grade a (of the Eu-arachnida). DELOBRANCHIA (Hydropneustea). Mesosomatic segments furnished with large plate-like appendages, the 1st pair acting as the genital operculum, the remaining pairs being provided with branchial lamellae fitted for breathing oxygen dissolved in water. The prae-genital somite partially or wholly obliterated in the adult. The mouth lying far back, so that the basal segments of all the prosomatic appendages, excepting those of the 1st pair, are capable of acting as masticatory organs. Lateral eyes consisting of a densely packed group of eye-units (" compound eyes). Order 1. Xiphosura.-The prae-genital somite fuses in the embryo with the prosoma and disappears (see fig. 19). Not freeewimming, 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. Family Limulidae (Limulus). In none of them are the appendages known, but in the form of the intermediate between Limulus and the Trilobitae. The young form two carapaces and the presence of free somites they are distinctly of Limulus itself (fig. 40) is also similar to a Trilobite so far as its segmentation and trilobation are concerned. The lateral eyes of Limulus appear to be identical in structure and position with those of certain Trilobitae. Order 2. Gigantostraca (figs. 46, 47).-Free-swimming forms, with the appendages of the 6th or 5th and 6th pairs flattened or lengthened Appendages of anterior pair very large and chelate. Sub-order Pterygotomorpha, Pterygotidae (Pterygolus). Appendages of anterior pair minute and chelate. Stylonuridae (Stylonurus). Sub-order Eurypteromorpha Eurypteridae (Eurypterus, Slimonia). Remarks.-The Gigantostraca are frequently spoken of as "the Eurypterines." Not more than thirty species are known. They became extinct in Palaeozoic times, and are chiefly found in the Upper Silurian, though extending upwards as far as the Carboniferous. They may be regarded as " macrourous" Xiphosura; that is to say, Xiphosura in which the nomomeristic number of eighteen FIG. 47. Plerygotus osiliensis, Schmidt. Silurian of Rootzikil. Restoration of the ventral surface, about a third natural size, after Schmidt. a, Camerostome or epistoma. oc, The compound eyes. 1 to 8, Segments of the sixth prosomatic appendage. I'to V', First five opisthosomatic somites. 7', Sixth opisthosomatic somite. Observe the powerful gnathobases of the sixth pair of prosomatic limbs and the median plates behind m. The dotted line on somite I indicates the position of the genital operculum which was probably provided with branchial lamellae.] well-developed somites is present and the posterior ones form a long tail-like region of the body. There still appears to be some doubt whether in the sub-order Eurypteromorpha the first pair of prosomatic appendages (fig. 46) is atrophied, or whether, if present, it has the form of a pair of tactile palps or of minute chelae. Though there are indications of lamelliform respiratory appendages on mesosomatic somites following that bearing the genital operculum, we cannot be said to have any proper knowledge as to such appendages, and further evidence with regard to them is much to be desired. (For literature see Zittel, 22*.) Grade b (of the Eu-arachnida). EMBOLOBRANCHIA (Aeropneustea). In primitive forms the respiratory lamellae of the appendages of the 3rd, 4th, 5th and 6th, or of the 1st and 2nd mesosomatic somites are sunk beneath the surface of the body, and become adapted to breathe atmospheric oxygen, forming the leaves of the so-called lung-books. In specialized forms these pulmonary sacs are wholly or partly replaced by tracheal tubes. The appendages of the mesosoma generally suppressed; in the more primitive forms one or two pairs may be retained as organs subservient to reproduction or silkspinning. Mouth situated more forwards than in Delobranchia, no share in mastication being taken by the basal segments of the 5th and 6th pairs of prosomatic appendages. Lateral eyes, when present, presented by separate ocelli. The prae-genital somite, after appearing in the embryo, either is obliterated (Scorpio, Galeodes, Opilio and others) or is retained as a reduced narrow region of the body, the "waist," between prosoma and mesosoma. It is represented by a full-sized tergal plate in the Pseudo-scorpiones. Section a. Pectinifera.-The primitive distinction between the mesosoma and the metasoma retained, the latter consisting of six somites and the former of six somites in the adult, each of which is furnished during growth with a pair of appendages. Including the prae-genital somite (fig. 16), which is suppressed in the adult, there are thirteen somites behind the prosoma. The appendages of the 1st and 2nd mesosomatic somites persisting as the genital operculum and pectones respectively, those of the 3rd, 4th, 5th and 6th somites (? in Palaeophonus) sinking below the surface during growth in connexion with the forma tion of the four pairs of pulmonary sacs (see fig. 17).. Lateral eyes monostichous. Order 1. Scorpiones.-Prosoma covered by a single dorsal shield, bearing typically median and lateral eyes; its sternal elements reduced to a single plate lodged between or behind the basal segments of the 5th and 6th pairs of appendages. Appendages of 1st pair tri-segmented, chelate; of 2nd pair chelate, with their basal segments subserving mastication; of 3rd, 4th, 5th and 6th pairs similar in form and function, except that in recent and Carboniferous forms the basal segments of the 3rd and 4th are provided with sterno-coxal (maxillary) lobes, those of the 4th pair meeting in the middle line and underlying the mouth. The five posterior somites of the metasoma constricted to form a "tail," the post-anal sclerite persisting as a weapon of offence and provided with a pair of poison glands (see figs. 8, 10, 12, 13, 14, 15, 21 and 22). Sub-order Apoxypoda.-The 3rd, 4th, 5th and 6th pairs of appendages short, stout, tapering, the segments about as wide as long, except the apical, which is distally slender, pointed, slightly curved, and without distinct movable claws. Family-Palaeophonidae, Palaeophonus (figs. 48 and 49). Restored after Thorell's indications by R. I. Pocock. FIG. 48.-Dorsal view of a restoration of Palaeophonus nuncius, Thorell. The Silurian scorpion from Gothland. Sub-order Dionychopoda.-The 3rd, 4th, 5th and 6th pairs of appendages slender, not evenly tapering, the segments longer than wide; the apical segment short, distally truncate, and provided with Ia pair of movable claws. Basal segments of the 5th and 6th pairs of appendages abutting against the sternum of the prosoma (see fig. 10 and figs. 51, 52 and 53).. Family-Pandinidae (Pandinus, Opisthophthalmus, Urodacus). Vejovidae (Vaejovis, Jurus. Euscorpius, Broteas). Bothriuridae (Bothriurus, Cercophonius). Buthidae (Buthus, Centrurus). *Cyclophthalmidae (Cyclophthalmus) " " " Carbon *Eoscorpiidae (Eoscorpius, Centromachus) iferous. Remarks on the Order Scorpiones.-The Scorpion is one of the great animals of ancient lore and tradition. It and the crab are FIG. 49.-Ventral view of a restoration of Palaeophonus Hunteri, Pocock, the Silurian scorpion from Lesmahagow, Scotland. Restored by R. I. Pocock. The meeting of the coxae of all the prosomatic limbs in front of the pentagonal sternum; the space for a genital operculum; the pair of pectens, and the absence of any evidence of pulmonary stigmata are noticeable in this specimen. (See Pocock, Quart. Jour. Micr. the only two invertebrates which had impressed the minds of carly men sufficiently to be raised to the dignity of astronomical representation. It is all the more remarkable that the scorpion proves to be the oldest animal form of high elaboration which has persisted to the present day. In the Upper Silurian two specimens of a scorpion have been found (figs. 48, 49), one in Gothland and one in Scotland, which would be recognized at once as true scorpions by a child or a savage. The Silurian scorpion Palacophonus, differs, so far as obvious points are concerned, from a modern scorpion only in the thickness of its legs and in their terminating in strong spike-like joints, instead of being slight and provided with a pair of terminal claws. The legs of the modern scorpion (fig. 10: fig. 51) are those of a terrestrial Arthropod, such as a beetle; whilst those of the Silurian scorpion are the legs of an aquatic Arthropod, such as a crab or lobster. It is probable that the Silurian scorpion was an aquatic animal, and that its respiratory lamellae were still projecting from the surface of the body to serve as branchiae. No trace of "stigmata," the FIG. 50.-Comparison of the sixth prosomatic limb of a recent scorpion (B), of Palaeophonus (C), and of Limulus (A), showing their agreement in the number of segments; in the existence of a movable spine, Sp, at the distal border of the fifth segment; in the correspondence of the two claws at the free end of the limb of Scorpio with two spines similarly placed in Limulus; and, lastly, in the correspondence of the three talon-like spines carried on the distal margin of segment six of recent scorpions with the four larger but similarly situated spines on the leg of Limulus; s, groove dividing the ankylosed segments 4 and 5 of the Limulus leg into two. (After Pocock, Q. J. Mic. Sci., 1901.) orifices of the-lung-chambers of modern scorpions, can be found in the Scottish specimen of Palaeophonus, which presents the ventral C 2... 3. surface of the animal to view. On the other hand, no trace of respiratory appendages excepting the pectens can be detected in the specimen (see fig. 49). Fossil scorpions of the modern type are found in the Coal Measures. At the present day scorpions of various genera are found in all the warm regions of the world. In Europe they occur as far north as Bavaria and the south of France. The largest species measure 9 in. from the front of the head to the end of the sting, and occur in tropical India and Africa. Between 200 and 300 species are known. From Lankester, Journ. Linn. Soc. Zool. vol. xvi, 1881. (Calmette), and rapidly paralyses animals which are not immune From Lankester, Journ. Linn. Soc. From Lankester, Journ. FIG. 53-The same FIG. 52.-Drawing from life of the Italian scorpion Euscorpius italicus, Herbst, holding a blue-bottle fly with its left chela, and carefully piercing it be Two insertions of the sting are effected tween head and thorax with its sting, and the fly is instantly paralysed by the poison so introduced into its body. All Arachnida, including Limulus, feed by suctorial action in essentially the same way as Scorpio. 014 Scorpions of various species have been observed to make a hissing noise when disturbed, or even when not disturbed. The sound is produced by stridulating organs developed on the basal joints of the limbs, which differ in position and character in different genera (see Pocock, 27). Scorpions copulate with the ventral surfaces in contact. The eggs are fertilized, practically in the ovary, and develop in silu. The young are born fully formed and are carried by the mother on her back. As many as thirty have been counted in a brood. For information as to the embryology of scorpions, the reader is referred to the works named in the bibliography below. Scorpions do not possess spinning organs nor form either snares or nests, so far as is known. But some species inhabiting sandy deserts form extensive burrows. The fifth pair of prosomatic appendages is used by these scorpions when burrowing, to kick back the sand as the burrow is excavated by the great chelae. References to works dealing with the taxonomy and geographical distribution of scorpions are given at the end of this article (28). Section B. Epectinata.-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. A prae-genital somite FIG. 51.-Drawing from life of the desert scorpion, Buthus australis, is often present either in a reduced condition forming a waist (PediLin., from Biskra, N. Africa. palpi, Araneae, Palpigradi) or as a full-sized tergal plate (Pscudoscorpiones); in some it is entirely atrophied (Solifugae, Holosomata, The scorpions use their large chelae for seizing prey and for fighting and Rhynchostomi). Lateral eyes when present diplostichous. with one another.. They never use the sting when (as frequently Remarks.-The Epectinate Arachnids do not stand so close to the happens) they attack another scorpion, because, as was ascertained aquatic ancestors of the Embolobranchia as do the Pectiniferous by A. G. Bourne (24), the poison exuded by the sting has no in- scorpions. At the same time we are not justified in supposing that jurious effect on another scorpion nor on the scorpion itself. The the scorpions stand in any way as an intermediate grade between stories of a scorpion stinging itself to death when placed in a circle any of the existing Epectinata and the Delobranchia. It is probable of burning coals are due to erroneous observation. When placed in that the Pedipalpi, Araneae, and Podogona have been separately such a position the scorpion faints and becomes inert. It is found evolved as distinct lines of descent from the ancient aquatic Arach(Bourne, 24) that some species of scorpion faint at a temperature nida. The Holosomata and Rhynchostomi are probably offshoots of 40° Cent. They recover on being removed to cooler conditions. from the stem of the Araneae, and it is not unlikely (in view of the A scorpion having seized its prey (usually a large insect, or small structure of the prosomatic somites of the Tartarides) that the reptile or mammal) with the large chelae brings its tail over its head, Solifugae are connected in origin with the Pedipalpi. The appearand deliberately punctures the struggling victim twice with its sting ance of tracheae in place of lung-sacs cannot be regarded as a start(fig. 52). The poison of the sting is similar to snake-poisoning-point for a new line of descent comprising all the tracheate forms: two anterior mesosomatic somites, besides the prae-genital somite, would then have to be supposed to have occurred also. 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; appendged ages of 3rd pair with only the apical seg algodobno-ds? please lo for od to 15 From Lankester, Q. J. Mic. Sci. N.S. vol. xxi., 1881.90g bayne bas bab Ter igo od FIG. 54.-Thelyphonus, one of the Pedipalpi. innige odT A, Ventral view.1auns die 1 to 11, Somites of the opistho1, Chelicera (detached).79512610 soma (mesosoma plus meta11, Chelae, onde obrvihn in soma).pulsdonians yd III. Palpiform limb. msg, Stigmata of the tergoIV to VI, The walking legs. a sternal muscles. stc, Sterno-coxal process (gnatho- an, Anus. on base) of the chelae. B, Dorsal view of the opisthost, Anterior sternal plate of the. soma of the same. prosoma. SP, Posterior sternal plate of the prosoma. pregen, Position of the praegenital somite (not seen). 11. Position of the two pulmonary sacs of the right side. the post analol lens sclerite, which has gibson lenno no poison glands. Tribe 1. Uro-ion baɛ ada lomme ora no zogehnsage tricha.-Dorsal area FIG.56.-Thelyphonus assamensis Ventral surfaceoftheanteriorregion of the opisthosoma, of prosoma covered with a single shield the first somite being pushed upwards and for(? two in Geralin- wards so as to expose the subjacent structures. ura), bearing median opistho 1, First somite of the opisthosoma; and lateral eyes. opisthon2, second do.; g, genital aperture; Post- anal sclerite, edges of the lamellae of the lung-books; m, modified as a long, stigmata of tergo-sternal muscles.linode3 many-jointed feeler. ogrom (Original drawing by Pocock.) obvod 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. TDTO 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 evan escent 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). of the IV he pre to to, the the originally ribest by Dorsal 2a freely movable; claw free or fused; basal segments of 4th and 5th | in the Amblypygi, or with the posterior pair, rarely the anterior pair pairs widely separated by the sternal area; appendages of 3rd pair with all the segments except the proximal three, forming a manyjointed flagellum. Opisthosoma without post-anal sclerite and posterior caudal elongation: with frequently a pair of small lobate III IV V VI as well, replaced by tracheal tubes. Intromittent organ of male in the apical segment of the 2nd prosomatic appendage. Sub-order a. Mesothelae (see figs. 60 to 62).-Opisthosoma distinctly segmented furnished with 11 tergal plates, as in the Amblypygi; the ventral surface of the 1st and 2nd somites with large sternal plates, covering the genital aperture and the two pairs of smoor Family-Phrynichidae (Phrynichus, Damon). 10 bar ebru,qub Admetidae (Admetus, Heterophrynus). Ambur pad Charontidae (Charon, Sarax). Samopod (Family ?)-Graeophonus. odgo Remarks. The Pedipalpi are confined to the tropics and warmer temperate regions of both hemispheres. Fossil forms occur in the Carboniferous. The small forms known as Schizomus and Hubbardia are of special interest from a morphological point of view. The Pedipalpi have no poison glands. (Reference to literature (29).) Order 3. Araneae (figs. 60 to 64).-Prosoma covered with a single shield and typically furnished with median and lateral eyes of diplostichous structure, as in the Amblypygi. The sternal surface wide, continuously chitinized, but with prosternal and metasternal nebisida ori di bomu? mot it to su and estimespot Tis atiel sdtime FIG. 60.-Liphistius desultor, Schiödte, one of the Araneae Mesothelae. Dorsal view. I to VI, the prosomatic appendages: 4, 5, 6, the fourth, fifth and sixth tergites of the opisthosoma. Between the bases of the sixth pair of limbs and behind the prosomatic carapace is seen the tergite of the small prae-genital somite. (Original by Pickard-Cambridge and Pocock.) elements generally distinguishable at the anterior and posterior ends respectively of the large mesosternum. Prosternum underlying the proboscis. Appendages of 1st pair have two segments, as in Pedipalpi, but are furnished with poison gland, and are retroverts. Appendages of 2nd pair not underlying the mouth, but freely movable and, except in primitive forms, furnished with a maxillary lobe; the rest of the limb like the legs, tipped with a single claw and quite unmodified (except in ). Remaining pairs of appendages similar in form and function, each tipped with two or three claws. 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: Respiratory organs (see fig. 63,stg), as V VI an FIG. 61.-Liphistius desultor. Ventral view with the prosomatic appendages cut short excepting the chelicerae (1) whose sharp retroverts are seen. Between the bases of the prosomatic limbs an anterior and a posterior sternal plate (black) are seen. 1. The sternum of the first opisthosomatic or genital somite covering the genital aperture and the first pair of lungsacs. In front of it the narrow waist is formed by the soft sternal area of the praegenital somite, 2, the sternite of the second opisthosomatic somite covering 3 the posterior pair of lung-sacs; 3 and 4. the spinning appendages (limbs) of the -4 opisthosoma; a, inner, b, outer ramus of the appendage; II, sternite of the eleventh "I somite of the opisthosoma: in front of it other rudimentary sternites; an, anus. (Original as above.) 2 pulmonary sacs, the sternal plates from the 6th to the 11th somites represented by integumental ridges, weakly chitinized in the middle. The two pairs of spinning appendages retain their primitive position in the middle of the lower surface of the opisthosoma far in advance of the anus on the 3rd and 4th somites, each appendage consisting of a stout, many-jointed outer branch and a slender, unsegmented inner branch. Prosoma as in the Mygalomorphae, except that the mesosternal area is long and narrow. Family-Liphistiidae (Liphistius, "Arthrolycosa). Sub-order b. Opisthothelae (see fig. 63).-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 spinning appendages migrate to the posterior end of the opisthosoma and take up a position close to the anus; the inner branches of the anterior pair either atrophy or are represented homogenetically by a plate, the cribellum, or by an undivided membranous lobe, the colulus. od1 in etsas Tribe 1. Mygalomorphae.-The plane of the articulation of the appendages of the 1st pair to the prosoma (the retrovert) vertical, the basal segment projecting straight forwards at its proximal end, the distal segment or fang closing backwards in a direction subparallel to the long axis of the body. Two pairs of pulmonary Plagiobothrus). Dipluri- I to VI, Appendages of the prosoma cut dae (Diplura, Macro- off at the base. thele). Ctenizidae o, Ocular tubercle. (Cleniza, Nemesia). praegen, The prae-genital somite. Atypidae (Atypus. 1 and 2, Sternites of the first and second Calommata) Tribe 2. Arachno morphae. The plane of the articulation of the 3 opisthosomatic somites. and 4, Appendages of the third and fourth opisthosomatic somites, which are the spinning organs, and in this genus occupy their primitive position instead of migrating to the anal region as in other spiders. somite. appendages of the 1st pair to the prosoma horizontal, the basal segment projecting vertically downwards, at 5 least eat its proximal end, the distal segment or fang closing inwards nearly or quite at right angles to the long axis of the body. The posterior pulmonary sacs (except in Hypochilus) replaced by tracheal tubes; the anterior and posterior pairs replaced by tracheal tubes in the Caponiidae. Tergite of the fifth opisthosomatic 11, Eleventh opisthosomatic somite; an, Anus. (Original.) Principal families-Hypochilidae (Hypochilus). Dysderidae (Dys dera, Segestria). Caponiidae (Caponia, Nops). Filistatidae (Filistata). Uloboridae (Uloborus, Dinopis). Argiapidae (Nephila, Gasteracantha). Pholcidae (Pholcus, Artema). Agelenidae (Tegenaria). Lycosidae (Lycosa). Clubionidae (Clubiona, Olios, Sparassus) Gnaphosidae (Gnaphosa, Hemiclaea). Thomisidae (Thomisus). Attidae (Salticus). Urocteidae (Uroctea). Eresidae (Eresus). Remarks on the Araneae.-The Spiders are the most numerous |