562 ‘ CLASS IX. 
tions! In many of the inferior arachnids neither heart nor vessels 
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, still 
the form of a longitudinal vessel, but other vessels arise from it, 
whilst from the respiratory organs the returning arterialised blood 
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 pulmonary 
veins of the human body, is not yet perfectly understood. Pro- 
bably the whole heart is surrounded by a snus, into which the 
arterial blood is poured, before it penetrates the above-mentioned 
fissures. Vessels also from the heart proceed to the respiratory 
organs, but these probably serve for their nutrition, and not for 
respiration. The heart of arachnids is then, beyond doubt, an 
arterial heart, like that of crustacea and of mollusca?. 
We have already said that the respiratory organs are in some 
arachnids air-tubes, in others pulmonary sacs. In both cases the 
air has access to the respiratory organs by air-slits (stégmata), as in 
insects; but these stigmata are always in less number than in 
most insects. In some arachnids no respiratory organs at all have 
been discovered (Pycnogonum, the so-named tardigrada, many 
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 are 
for the attachment of muscles, which, connecting the dorsal and 
ventral surfaces, contract the abdomen, and which we also mect 
with again in Limulus, amongst the Crustacea*. In the scorpions 
1 TREVIRANUS Verm. Schr. I. 8. 31, Tab. 111. fig. 18, TULK l. c. p. 249, Pl. Iv. 
fig. 17a, H, p. 
2 The most complete description of the heart and vessels of the scorpions has been 
given by Newport in Phil. Transact. for 1843, Part 1. 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 cesophagus; 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. 
3 This mistake was made even by the celebrated TREVIRANUS in Chelifer (Verm. 
Schr. 1. 3. 18, 19, Tab. it. figs. 6, 7, A), who could not on that account find any air- 
tubes there. 
