198 



COLLEGE ZOOLOGY 



Figure 109. Representatives of the major classes of arthropods, showing body divisions and 

 appendages. The lines suggest possible relationships. The figures are not drawn to scale. 



states, and Astacus in the western United 

 States. The lobster Homarus americanus, 

 differs in structure from the crayfish only in 

 minor details. In Europe the most common 

 crayfish is Astacus fluviatilis. 



External anatomy 



Exoskeleton 



The outside of the body is covered by a 

 hard cuticle containing chitin* and impreg- 



* The best-known component of the cuticle is 

 chitin, a nitrogenous polysaccharide; it is a very re- 



nated with lime salts. This exoskeleton (Fig. 

 112) is thinner and flexible at the joints, 

 allowing movement. 



sistant substance that is insoluble in water, alcohol, 

 dilute acids, alkalies, and the digestive juices of 

 many animals. Formerly it was thought that the 

 chitin was responsible for the hardness of the 

 cuticle; now, however, it is definitely known that 

 the hardness of the cuticle is due to nonchitinous 

 substances. The softer parts of the cuticle usually 

 contain more chitin than the harder parts. The 

 hard parts of the cuticle are said to be "sclerotized," 

 not chitinized. 



Chitin also occurs in some sponges, hydroids, 

 bryozoans, brachiopods, annelids, and mollusks. 



