ARTHROPODS IN GENERAL 305 



Section I. Branchiata (bran ki a' ta; G., hranchia, gill). Mostly- 

 aquatic, possessing gills. 

 Class 1. Crustacea. 

 Section II. Tracheata (tra ke a' ta; G., tracheia, rough). Air-breath- 

 ing, with tracheae or book lungs, or water-breathing, with book gills. 

 Division A. Protracheata (pro tra ke a' ta; G., pro, before, -^trache- 

 ata). Primitive, annelid-like. 

 Class 2. Onychophora. 

 Division B. Antennata. With antennae. 

 Class 3. Myriapoda. 

 Class 4. Insecta. 

 Division C. Arachnoidea (a rak noi' de a; G., arachnoeides, like a 

 spider). Without antennae. 

 Class 5. Arachnida. 

 333. 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 intelligence. 

 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 intelligence is attributed to the two groups men- 

 tioned, it should also be recognized in them. 



