CHAPTER XXI 



ARTHROPODS 



Vocabulary 



Segmented, made up of joints or sections. 



Dorsal, the region of the back, usually, but not always, uppermost 



in animals. 

 Ventral, the side opposite the dorsal, the region of the belly, usually 



underneath. 

 Genus, next to the smallest general division in classification; 



plural is genera. 

 Species, the smallest general division in classification; plural 



is also species (specie means money). 



The group of animals next to be studied is called the arthropods 

 (jointed foot) because all their leg-like appendages are divided in 

 joints or segments. 



Characteristics. They are the largest group of living things in 

 the world, outnumbering all the other species of the animal king- 

 dom. These numerous forms all agree in the following points: 



1. They have jointed appendages. 



2. They have an external skeleton. 



3. The body is segmented and consists of three regions, 



(a) head specialized for food-getting and sensation. 



(b) thorax for locomotion. 



(c) abdomen not highly specialized. 



4. Heart is in the back (dorsal) region. 



5. Nervous system is beneath (ventral). 



Classes. The arthropods are divided into three or sometimes 

 four classes, the fourth being rather indefinite, and including the 

 worm-like forms such as the centipedes and " thousand legs." 



1. Crustacea, which include crayfish, lobster, crab, shrimp, and 

 many others. 



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