ABACHXTOA. 449 



The dense chitinous covei-ing of the cephalothorax in the great 

 tropical spiders {Mygale), and of a larger proportion of the skeleton 

 in the African scorpion, consists of a series of layers of a brown 

 colour; the superficial are darker than the rest: the layers are 

 traversed, in the scorpions, by minute tubes.* 



The legs, answering to the six in hexapod insects, consist each of 

 a coxa, a short trochanter, a longer stiff Jemur, a tibia divided by an 

 articulation into two unequal parts, and a tarsus composed of a long 

 and short joint ; the latter is commonly armed by two claws. In 

 some spiders these have a pectinated appendage on their convex side. 

 There is a spine on the end of the tarsus of eacli hind leg opposed to 

 the terminal hooks. In the aquatic HydrachncB the legs are thickly 

 set with hairs on one side. 



The muscular system is principally aggregated in the cephalo- 

 thorax for working the organs of mastication and locomotion : some 

 small muscular bands are attached to a median ventral raphe of 

 the abdomen in certain spiders. There are better developed muscles 

 in the slender jointed abdomen of the scorpions for its inflection and 

 extension, and more especially for the purpose of wielding the 

 poisonous weapon with which it is terminated. The elementary 

 muscular fibres are transversely striated. 



A well-marked gradation of structure may be traced in the nervous 

 pystem of the Arachnids. The brainless condition of Macrobiotus 

 has already been alluded to. In the Pycnogonidte the first of the 

 four ventral ganglions is connected by a slender oesophageal collar 

 with an ovoid cerebral ganglion ; this is divided in the Phalangidce. 



The principal masses or ganglions of the nervous system are con- 

 centrated around the oesophagus in the cephalothorax of the scorpion. 

 From the small superoesophageal or cephalic bilobed mass are sent 

 upwards the optic filaments, forwards the nerves of the forcipated 

 mandibles or " chelicera," and, backwards, the stomato-gastric nerves ; 

 the sub-oesophageal ganglionic columns distribute nerves to the 

 great maxillary cheliform palpi, and to the four pairs of thoracic 

 legs ; two slender continuations of the median columns are continued 

 along the jointed abdomen or tail, and seven small ganglions are 

 developed upon them, from which and from the interganglionic 

 chords nervous filaments are distributed to the surrounding parts. 



The ventral continuation of the anterior aorta, which lies loosely 

 upon the dorsal aspect of the ganglionic chords, must be injected in 

 order that its branches, which accompany the nervous filaments, may 

 be distinguished from them. The vessel itself has been mistaken for 



* CCXXXL p. 405. 



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