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ARACHNIDA of Scorpio, but there is one additional posterior pair. The origin of the paired arteries from the heart differs in Limulus from the arrangement obtaining in Scorpio, in that a pair of lateral commissural arteries exist in Limulus (as described by Alphonse Milne - Edwards (6)) leading to a suppression of the more primitive direct connexion of the four pairs of posterior lateral arteries, and of the great median posterior arteries with the heart itself (Fig. 29). The arterial system is very completely developed in both Limulus and Scorpio, branching repeatedly until minute arterioles are formed, not to be distinguished from true cap■VPM* illaries ; these open into irregular swollen vessels VPM5 which are the veins or venous sinuses. A very art — VPM remarkable feature in Limulus, first described by Owen, is the close accom7 - VPM paniment of the pro-

the heart. The blood is brought to the respiratory organs in both cases by a great venous-collecting sinus having a ventral median position. In both animals the wall of the 'pericardial sinus is connected by vertical muscular bands to the wall of the ventral venous sinus (its lateral expansions around the lung-books in Scorpio) in each somite through which the pericardium passes. There are seven pairs of these veno-pericardiac vertical muscles in Scorpio, and eight in Limulus (see Figs. 30, 31, 32). It is obvious that the contraction of these muscles must cause a depression of the floor of the pericardium and a rising of the roof of the ventral blood sinus, and a consequent increase of volume and flow of blood to each. Whether the pericardium and the ventral sinus are made to expand simultaneously or all the movement is made by md, Py’

dv3 dv* sag' Fig. 31.—Diagram of a lateral view of a longitudinal section of a Scorpion. d, chelicera ; ch, chela; cam, camerostome ; m, mouth ; ent, entosternum ; p, pecten ; stig1, first pulmonary aperture ; stig4, fourth pulmonary aperture ; dam, muscle from carapace to a praeoral entosclerite ; ad, muscle from carapace to entosternum; md, muscle from tergite of genital somite to entosternum (same as dpm in Fig. 30) ; dv1 to 4 dtf, dorso-ventral muscles (same as the series labelled tsm in Fig. 30); pv to pvt, the seven veno-pericardiac muscles of the right side (labelled VPM in Fig. 30). (After Beck, Trans. Zool. Soc. vol. xi. 1883.) one only of the surfaces concerned, must depend on conditions of tension. In any case it is clear that we have in these muscles an apparatus for causing the blood to flow differentially in increased volume into either the pericardium, through the veins leading from the respiratory organs, or from the body generally into the great sinuses which bring the blood to the respiratory organs. These muscles act so as to pump the blood through the respiratory organs. It is not surprising that with so highly developed an arterial system Limulus and Scorpio should have a highly developed mechanism for determining the flow of blood to the respiratory organs. That this is, so to speak, a need of animals with localized respiratory organs is seen by the existence of provisions serving a similar purpose in other animals, e.g., the branchial hearts of the Cephalopoda. The veno-pericardiac muscles of Scorpio were seen and dv

Fig. 30.—View from below of a Scorpi°n somatic nerve centres and (B. occitanvs) opened and dissected so ^gryeg py arteries, SO close as to show the pericardium with its. > muscles, the lateral arteries, and the indeed that the great tergo-stemal muscles. PRO, prosoma; (r,,TUri;„T1 Tmi„„ + clpm, dorso-plastral muscle; art, lateral ganglion mass anti llS nll OUL artery; tsm1, tergo - sternal muscle running nerves are actually (labelled dv in Fig. 31) of the second, P . , , , (pectiniferous) mesosomatic somite: this SUUK m Or invested. Dy is the most anterior pair of the series arteries. The connexion of six, none 4are present m the genital . ... somite; tsm , tergo-sternal muscle of is not SO intimate in the fifth mesosomatic somite; tsm6, a„nrr.;n },nr novortfip tergo-sternal muscle of the enlarged first OCOrpiO, DUt IS nevertne metasomatic somite Per, pericardium; l yerv close One, closer XTDAT1 4-^ VPM7, X7r>Af 7 the +V, r. series of oatran VPMi to seven roiT»C! pairs ess . a ^r: j • ,, of veno-pericardiac muscles (labelled pv than W6 finu. in any Otlier F in, ig/I). There is some reason to Arthropods in which the admit the existence of another more . y . ^ anterior pair of these muscles in Scor- arterial system IS welln depio; this would make the number i i vr,.exactly correspond with the number veiopeQ, C.g., me IViyria in Limulus. (After Lankester, Trans, poda and Some of the 1 Zool. Soc. vol. xi. 1883.) ,,, , r, arthrostracous Crustacea. It seems that there is a primitive tendency in the Arthropoda for the arteries to accompany the nerve cords, and a “ supra-spinal ” artery—that is to say, an artery in close relation to the ventral nerve cords—has been described in several cases. On the other hand, in many Arthropods, especially those which possess trachese, the arteries do not have a long course, but soon open into wide blood sinuses. Scorpio certainly comes nearer to Limulus in the high development of its arterial system, and the intimate relation of the anterior aorta and its branches to the nerve centres and great nerves, than does any other Arthropod. An arrangement of great functional importance in regard to the venous system must now be described, which was shown in 1883 by Lankester to be common to Limulus and Scorpio. This arrangement has not hitherto been detected in any other class than the x Arachnida, and if it should ultimately^ prove 32.—Diagram of a lateral view ofPh,a longitudinal Site, suctorial - - to - be - Fig. pharynx; al, alimentary canal; pharynx ; M,section mouthof; Limulus Est, entostemum peculiar to that group, would have considerable ventral venous sinus ; chi, chilaria ; go, genital operculum ; hr1 to fcr6, branchial weight as a proof of the close genetic affinity of appendages left side VPM11 to VPM8, the eight pairs of veno-pericardiac muscles (labelled pv in Fig. 31). Limulus and Scorpio. VPM is probably represented in Scorpio, though not marked in Figs. 30 and 31. The great pericardial sinus is strongly developed in (After Benham, Trans. Zool. Soc. voL xi. 1883.) both animals. Its walls are fibrous and complete, and it holds a considerable volume of blood when the heart itself figured by Newport but not described by him. Those of is contracted. Opening in pairs in each somite, right and Limulus were described and figured by Alphonse Milneleft into the pericardial sinus are large veins, which bring Edwards, but he called them merely “transparent ligathe blood respectively from the gill-books and the lung- ments,” and did not discover their muscular structure. books to that chamber, whence it passes by the ostia into They are figured and their importance for the first time S. L —67
 * VS,
 * met, unsegmented metasoma; EntujA, fourth dorsal entapophysis of
 * tsm, tergo-sternal muscles, six pairs as in Scorpio (labelled dv in Fig 31);