130 ARACHNIDA. 



to illustrate the general structure of the order. To 

 distinguish the man'dibles (mandib'ula, a jaw) or 

 proper jaws, the foot-jaws, and the branchial legs, 

 the animals must be dissected in water with the 

 mounted needles. The very delicate feathery fila- 

 ments of the branchiae may be best observed when 

 these organs are dried on a slide. 



The Entromostaca may be kept alive in a jar 

 of water with water-plants for a long period. They 

 may be removed from the water for examination 

 by the dipping-tube, and are best observed in a live- 

 box. 



ARACH'NIDA (apa^vrj, spider) is the Class of spiders, 

 scorpions, and mites. 



Araneida. This Order contains the more highly 

 developed forms of the Class, among which are the 

 common spiders of houses and gardens ; and some of 

 their structures are very curious and interesting. 



The head of spiders is united or fused with the 

 thorax, forming one piece, which is called the ceph'alo- 

 ihorax (tce<f>a\rj, head, copal;, chest). 



The claw-jaws, or mandibles, are terminated by a 

 curved and pointed claw, with which the spiders hold 

 their prey. It is traversed by a slender canal, con- 

 taining a slender tube or duct leading from a poison- 

 gland, and opening near its point; and when the 

 insect prey is transfixed by the mandible, the poison 

 is pressed out and enters the wound. 



Near the root or base of the mandibles on each 

 side is a jointed feeler, or pal'pus ; but spiders have 

 no anten'nse. The eyes are simple, forming separate 

 round shining dots, and are called ocel'li (ocel'lus, 

 a little eye) ; they are usually placed on the top of 

 the head, and are often arranged in a geometrical 

 form, as a triangle, &c. 



The legs are four pairs ; they are hairy, and ter- 

 minated by two or three claws, which are fringed 

 with minute teeth, or pec'tinate. These claws serve 



