572 



CHAPTER XXXI. 



ARTHR OPOD A— continued. 



ARACHNIDA AND MYRIOPODA 



Class II. Arachnida. 



The Arachnida — including the Spiders, Scorpions, Mites, &c. — 

 possess almost all the essential characters of the Crustacea, to which 

 they are very closely allied. Thus, the body is divided into a vari- 

 able number of somites, some of which are always provided with 

 articulated appendages. A pair of ganglia is primitively developed 

 in each somite, and the neural system is placed ventrally. The 

 heart, when present, is always situated on the opposite side of the 

 alimentary canal to the chain of ganglia. The respiratory organs, 

 however, whenever these are differentiated, are never in the form of 

 branchiae as in the Crustacea, but are in the form either of pul- 

 monary vesicles or sacs, or of ramified tubes, formed by an involu- 

 tion of the integument, and fitted for breathing air directly. Fur- 

 ther, there are never more than four pairs of ambulatory limbs, and 

 no locomotive appendages are developed upon any of the abdominal 

 segments. Antennae are not present (as such), and the eyes are 

 simple and sessile. The head and thorax are united to form a 

 cephalothorax (the thoracic segments in rare cases remaining free), 

 and in some cases the cephalothorax is fused with the abdomen. 



Speaking generally, therefore, the Arachnida are distinguished 

 from the Crustacea by the possession of four pairs of walking-legs, 

 the want of locomotive appe?idages on the abdomen, and the absence of 

 a?itennce ; while the breathing-organs (when differentiated) are adapted 

 ^or breathing air directly, and are never in the form of branchice. 



The integument of the Arachnida is in general more or less ex- 

 tensively hardened by chitine, and commonly forms a resistant exo- 

 skeleton. Viewed under the microscope, the chitinous exoskeleton 

 of such an Arachnidan as a Scorpion (fig. 430, a and b) exhibits a 



