264 A YEAR IN SCIENCE 



organ. When this is filled with air, the body of the 

 fish has nearly the same weight as the water which it 

 displaces, and the fish is thus buoyed up. 



Fig-. "120. White perch. 



Arthropods. This group of animals, together with 

 all those which follow, belong to the invertebrates. 



The arthropods include many animals which you 

 already know. It is an exceedingly 'large group, and 

 we will consider it in three divisions: (1) Crustacea, 

 (2) Insects, (3) Spiders. All arthropods resemble each 

 other in a few characters: their bodies are made up 

 of divisions or segments; attached to some of these 

 divisions are jointed appendages; all have a hard outer 

 covering on the body. 



To the Crustacea belong the crayfish, lobster, crab, 

 barnacle, etc. These animals have a hard outer shell. 

 The body is made up of a number of segments to which 

 are attached many appendages, some of which are used 

 for claws, others for walking legs, and still others for 

 swimming. Crustacea, for the most part, live in the 



