SUMMARY 93 



is fatal to some of the smaller animals and painful to man. 

 The thousand-legged worms are harmless. 



Note. Insects have been studied also in Chapters I and 

 II, but it should be remembered that they are arthropods. 



SUMMARY 



An animal belongs to the arthropods if it has more than 

 two pairs of appendages which have several joints in them. 

 They also have an external skeleton which is shed at 

 irregular intervals in order to allow the animal to increase 

 in size. The body of the crayfish shows that part of the 

 segments have fused to form the head-thorax region. The 

 members of this group vary much in size and habits. 

 Lobsters and crabs are valuable for food and for this 

 reason should not be caught when they are small. 



QUESTIONS 



What kind of animals belong to the crustaceans ? How can you dis- 

 tinguish one from a worm ? From a hydroid ? Explain why insects are 

 arthropods. Which groups of arthropods are beneficial ? Which are 

 harmful ? What do you mean when you say that an insect is beneficial 

 or harmful ? 



REFERENCES 

 See Chapter II. 



