oa f A fnrrn a, 



ARACHNIDS. 557 



to form a single piece, to which LATREILLE 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 SAVIGNY, 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 



i which succeed these, and which in the scorpions sustain large shear- 

 shaped feelers, are, according to SAVIGNY, 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, 



v 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 chelce or shears of cray-fish and crabs, would thus, like the 



i proper jaws (mandibles), be wanting in arachnids 1 . 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 antennae. 

 [EKICHSON rejects this opinion of LATREILLE ; EntomograpTiien, 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, p. 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. DE 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 dicren in VAN HALL, VROLIK and MULDER, 

 Bijdragen tot de natuurk. Wetensch. n. 1827, bl. 134, afterwards by DUGES Ann. dcs 



