562 CLASS ix. 



tions 1 . In many of the inferior arachnids neither heart nor vesseL 

 have been met with. 



In the spiders and scorpions, on the contrary, there are vessels 

 present for the circulation of the blood. The heart has, indeed, stil 

 the form of a longitudinal vessel, but other vessels arise from it 

 whilst from the respiratory organs the returning arterialised blooc 

 penetrates the heart through lateral, transverse fissures on the 

 upper or dorsal surface, which are provided with valves. The 

 course of these~returning vessels, corresponding to the pulmonar} 

 veins of the human body, is not yet perfectly understood. Pro- 

 bably the whole heart is surrounded by a sinus, into which the 

 arterial blood is poured, before it penetrates the above-men tionec 

 fissures. Vessels also from the heart proceed to the respirator} 

 organs, but these probably serve for their nutrition, and not foi 

 respiration. The heart of arachnids is then, beyond doubt, ar 

 arterial heart, like that of Crustacea and of mottusca 2 . 



We have already said that the respiratory organs are in soni( 

 arachnids air-tubes, in others pulmonary sacs. In both cases th( 

 air has access to the respiratory organs by air-slits (stigmata], as ir 

 insects ; but these stigmata are always in less number than ir 

 most insects. In some arachnids no respiratory organs at all have 

 been discovered (Pycnogonum, the so-named tardigrada, man} 

 Acari). In these also no stigmata are present. Sometimes, indeed 

 parts have been taken for stigmata, which have an entirely different 

 signification ; I allude to two rows of spots on the back, which arc 

 for the attachment of muscles, which, connecting the dorsal and 

 ventral surfaces, contract the abdomen, and which we also meel 

 with again in Limulus, amongst the Crustacea 5 . In the scorpions 



1 TREVIRANUS Verm. ScJir. i. s. 31, Tab. in. fig. 18, TULK 1. c. p. 249, PL iv. 

 fig. 1 7 a", H t p. 



3 The most complete description of the heart and vessels of the scorpions has been 

 given by NEWPOKT in Phil. Transact, for 1843, Part I. pp. 286 298, with beautiful 

 figures. From the dorsal vessel, that as an aorta springs from the heart at the fore part, 

 there arises, before the origin of the arteries for the last pair of feet, an artery on each 

 side, which passes beneath the oesophagus ; the two form a single vessel lying on the 

 nervous cord. It is this artery which was described by TREVIRANUS as a third nervous 

 cord, and by MUELLER as a ligament. Under the nervous cord in the abdomen a 

 venous trunk is situated. 



8 This mistake was made even by the celebrated TREVIRANUS in Chelifer (Verm. 

 Schr. I. s. 18, 19, Tab. n. figs. 6, 7, A), who could not on that account find any air- 

 tubes there. 



