ARACHNIDS. 557 
as to form a single piece, to which LaTretutn has given the name 
of Cephalothorax: to this piece alone are the feet attached. Be- 
hind the cephalothorax is connected with the abdomen, the second 
principal piece of the body. A longitudinal arterial heart or dorsal 
vessel is present, and in many a more or less developed vascular 
system for the circulation of blood. Respiration is effected either 
by means of air-tubes, as in insects, or of lungs in determinate 
parts of the body. In all, however, there are lateral openings or 
air-slits (see above, p. 260, 261), which conduct to the respiratory 
organs. The sexes are distinct. 
According to the theory of Savrany, no oral parts are present 
in this class, which correspond to the upper and lower jaws of 
insects. The parts, which in spiders and scorpions are usually 
called upper jaws (mandibles), are, according to SAVIGNy, to be 
compared with the second pair of auxiliary jaws, or feet changed 
into jaws in the cray-fish and other ten-footed crustaceans. In 
some arachnids these parts undergo such a change, that they 
assume a flattened form and compose a sucker. The under-jaws 
which succeed these, and which in the scorpions sustain large shear- 
shaped feelers, are, according to SAvIGNny, to be compared with the 
third pair of auxiliary jaws, or feet changed into jaws of decapod 
crustaceans. To these, in the arachnids, four pairs of feet succeed, 
of which the first pair, according to the same writer, corresponds to 
the second pair of untransformed feet of decapod crustaceans. The 
untransformed first pair of feet of the decapod crustaceans, the so- 
called chele or shears of cray-fish and crabs, would thus, liké the 
proper jaws (mandibles), be wanting in arachnids!. The abdomen 
is never provided with feet. 
1 This view, however, is not altogether free from objection. Thus LATREILLE con- 
siders the first pair of jaws (the upper-jaws) of arachnids to be modified antenne. 
[ERIcHSon rejects this opinion of LATREILLE; Entomographien, erstes Heft. Berlin, 
1840, s. 9; OWEN, on the contrary, on the ground of the origin of the nerves distributed 
to these parts, defends it. Lectures on Comp. Anat. I. 1843, Pp. 253, 2nd edit. p. 448.] 
Still more may it be doubted whether the first pair of feet of arachnids really corresponds 
to the second pair of unaltered feet in decapod crustaceans; this comparison may be 
looked on as merely an arbitrary conception. Rather does the opinion deserve the 
preference, that these parts correspond to the lateral parts of the under-lip. [This 
opinion, I think, was first offered by W. Dr Haan in an essay, of which the other 
propositions appear to me to be less happy, entitled: Vergelijking tusschen de tast- 
kaauw-en bewegings-werktuigen der gelede dieren in Van Hat, Vrorrk and Munper, 
Bydragen tot de natuurk. Wetensch. 1. 1827, bl. 134, afterwards by DugEs Ann. des 
