ARTHROPOD A. 



291 



There are two orders : 



1. Chilognatha, having a cylindrical body, each segment 

 furnished with two pairs of legs. They are of slow loco- 

 motion, harmless, and vegetarian. The Thousand-legged 

 Worm (Julus) is a common representative. 



2. Chilopoda, characterized by having a flattened body 

 composed of about twenty segments, each carrying one 

 pair of legs, of which the hindermost is converted into 

 spines. They have longer antennae than the preceding, 

 and the mouth is armed with two formidable fangs con- 

 nected with poisonous glands. They are carnivorous and 

 active. Such is the Centipede (Seolopendra, Fig. 259). 



Class IV. — Insecta. 

 Insects are distinguished by having head, thorax, and 

 abdomen distinct, three pairs of jointed legs, one pair of 

 antennae, and generally two pairs of wings. The number 

 of segments in the body never exceeds twenty. The head, 

 apparently one, is formed by the union of four segments. 

 The thorax consists of three — the prothorax, mesothorax, 

 and metathorax — each bearing a pair of legs ; the wings, 

 if present, are carried by the last two segments. The ab- 

 domen is normally composed of ten segments, more or less 

 movable upon one another. The skin is hardened with 

 chitine, and to it, as in all Arthropods, the muscles are at- 

 tached. The organs of sense are confined to the cephalic 

 division of the body, the motor organs to the thoracic, and 

 the vegetative to the abdominal. All the appendages are 

 hollow. 



The antennae are inserted between or in front of the 

 eyes. There is a great variety of forms, but all are tubu- 

 lar and jointed. They are supposed to be organs of touch, 

 and also seem to be sensitive to sound. The eyes are 

 usually compound, composed of a large number of hexago- 

 nal corneae, or facets (from fifty in the Ant to many thou- 



