SUMMARY 
67 
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 ir¬ 
regular 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. Lob¬ 
sters 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. 
