262 MANUAL OF ZOOLOGY SECT. 



abdomen (nine in number) are united together, being cov- 

 ered dorsally by a continuous abdominal carapace. At the 

 posterior end is attached a very long, narrow, caudal spine 

 which is a modification of the ninth abdominal segment of 

 the larva. The anterior appendages (Fig. 157) somewhat 

 resemble those of the scorpion. In front of the mouth is 

 a pair of short, three-jointed, chelate appendages, the cheli- 

 cercE, at the sides of a labrum or upper lip. Behind these 

 follow a series of five pairs of legs, the bases of all of which, 

 with the exception of the last, are covered with spines, and 

 have the action of jaws, while the extremities are for the 

 most part chelate. The first pair of appendages of the 

 abdomen are flat plates, which are united together in 

 the middle line and together form the broad operculum 

 (operc) , overlapping all the posterior appendages ; on its 

 posterior face are the two genital apertures. The posterior 

 appendages, of which there are five pairs, are thin, flat 

 plates to which the gills are attached ; each of them is 

 divided by sutures into a small inner ramus or endopodite, 

 and a larger external ramus or exopodite. A labrum (ros- 

 trum) lies in front of the mouth, and between the sixth pair 

 of appendages is a pair of processes, the chilaria. 



In their mode of life the Arachnida present almost as great 

 a diversity as the Insecta. Some Acarida are parasites 

 throughout life. Most of the other groups of Arachnida 

 are predaceous, preying for the most part on insects or 

 other arachnids. To capture the insects which constitute 

 their food, the majority of spiders construct a web formed 

 of the threads secreted by the arachnidium. The primary 

 function of the threads formed from the secretion of the 

 spinning organ is to constitute the material for the manufac- 

 ture of a cocoon for enclosing the eggs, and in some arach- 

 nids this is the sole purpose to which they are devoted. In 



