ANNULOSA ARACHNIDA. 67 



the Insecta. There is one pair of antennae attached to 

 the head. The eyes are simple, and vary in number. 



81. Development. The young animal, on emerging from 

 the egg, differs little from the adult, except that it has a 

 smaller number of segments and limbs. At each change 

 of skin it increases in size. Some of them, at first, are 

 quite destitute of feet; others are furnished with three 

 pairs, like the insects. 



CLASS 3. Arachnida. 



82. The class Arachnida (Gr. amchne, a spider) comprises 

 the spiders, scorpions, and mites. The head and thorax 

 are united, forming a mass termed the cephalothorax. 

 The abdomen is distinct. The antenna are converted 

 into a pair of mandibles or prehensile jaws. There are 

 four pairs of limbs, borne by the cephalothorax. The 

 abdomen is destitute of legs. Respiration is effected 

 either by tracheae or pulmonary sacs. 



83. Skeleton. The integument is usually hardened by 

 chitine, but is sometimes soft and elastic. In the higher 

 groups, the body is composed of twenty somites, six of 

 which form the head. The segments of the thorax are 

 amalgamated with those of the head, forming what is 

 termed the cephalothorax. There are four pairs of limbs 

 attached to the cephalothorax. The joints of the limbs 

 are similar to those of the insects. Each limb is furnished 

 with two claws at its extremity. The abdomen in the 

 spiders is unsegmented; but in the scorpions it consists of 

 a long, flexible, jointed tail. It is always destitute of 

 legs. 



The abdomen in the scorpions is terminated by a 

 hooked claw, which is its principal weapon. This claw 

 is perforated, and communicates with a poison gland 

 situated at its base. 



There are glands, situated in the abdomen of the spiders, 

 which secrete a glutinous substance that has the property 

 of hardening when exposed to the air. These glands 



