PHYLUM ARTHROPODA 



219 



mandibles. The classes of crustaceans have 

 become different from one another, prin- 

 cipally as a result of specialization of ap- 

 pendages, shortening of the body, and de- 

 velopment of a carapace. Internal changes 

 are associated with these external modifi- 

 cations. Orders of insects are arranged in 

 the phylogenetic tree (p. 229) to suggest 

 possible relationships. The centipedes are 

 more closely related to the insects than are 

 the millipedes. 



Trilobites 



The name trilobite (Fig. 122) means 

 "three-lobed" and refers to the fact that the 

 dorsal surface is divided by longitudinal fur- 

 rows into three lobes. They were covered by 

 a hard shell. On the head was a pair of an- 

 tennae, 4 pairs of two-branched appendages, 

 and often a pair of compound eyes. 



These primitive fossil arthropods date 

 from early geologic time. Although trilobites 



Figure 122. Fossil trilobites; these extinct arthropods lived in warm primeval seas about 

 500,000 million years ago. They dominated hfe on earth for a span many, many times as long 

 as man's whole existence. (Courtesy of American Museum of Natural History.) 



probably did not give rise directly to any 

 other group of arthropods, they appear to 

 be most closely related to crustaceans. 



RELATIONS OF 

 CRUSTACEA TO MAN 



Crustacea are of considerable value as 

 food for man, either directly or indirectly. 

 The smaller species may be present in 

 enormous numbers in both fresh water 



and salt water and constitute an important 

 part of the food chain of many fish and 

 other aquatic animals that eventually come 

 to our table. Commercially, the shrimp is 

 the most important of the crustaceans as 

 human food; crabs, lobsters, and crayfishes 

 follow in this order. Blue or edible crabs are 

 eaten extensively in certain regions; they are 

 called hard-shelled crabs except just after 

 molting, when they become soft-shelled. The 

 crayfish, especially the soft-shelled individ- 

 ual, is very popular among fishermen as 



