ARTHROPODS IN GENERAL 311 



1. Crustacea. Mostly aquatic, possessing gills, green glands, and two 

 pairs of antennae. 



2. Onychophora. Primitive, annelid-like, with nephridia, tracheae, 

 and one pair of antennae. 



3. Myriapoda. Body wormlike, either flattened dorsoventrally 

 (centipedes) or subcyUndrical (millipedes) ; possess tracheae, Malpighian 

 tubules, and one pair of antennae. 



4. Insecia. Body divided into head, thorax, and abdomen; have 

 tracheae, Malpighian tubules, three pairs of legs, and one pair of antennae. 



5. Arachnida. Body usually divided into cephalo thorax and abdo- 

 men ; have no antennae and no mandibles ; possess four pairs of walking 

 legs; respire by book gills, book lungs, or tracheae; have coxal glands and 

 Malpighian tubules. 



335. Behavior. — The most striking thing in the behavior of arthro- 

 pods is the great development of instincts. These frequently become so 

 complex and adjust the animal so perfectly to the conditions of its exist- 

 ence that to most people they seem to imply the exercise of inteUigence. 

 In addition to instinct most zoologists recognize a primitive form of 

 intelligence in the higher insects and in the spiders. However, the readi- 

 ness with which some higher crustaceans modify their behavior would 

 seem to indicate that if intelhgence is attributed to the two groups men- 

 tioned, it should also be recognized in them. 



