Chap. 29 



ARTHROPODS CRUSTACEANS 



579 



Fig. 29.8. Section of earth showing types of crayfish burrows, c, chimney and 

 opening of burrow; cc, closed chimney; s, stream; wl, ground water level; x, place 

 from which crayfish was taken. (After Ortman. Courtesy, Robert W. Pennak, 

 Fresh-Water Invertebrates of the United States. Copyright 1953, The Ronald 

 Press Company.) 



jointed appendages of arthropods are among the most versatile of nature's in- 

 ventions. The abdominal ones are built on the basic plan nearest the original 

 pattern (Fig. 29.10). 



Homology and Evolution of Appendages. The appendages of crayfishes and 

 lobsters are homologous structures with like parts in similar relation to one 

 another. They are striking examples of serial homology, all of them variables 

 of a common pattern. In the developing young, the basic pattern is clear, espe- 

 cially in lobsters. 



Internal Organs and Metabolism. Digestion. Food is cut, shredded and 



Fig. 29.9. Female crayfish in a resting position. Eggs are carried glued to the 

 swimmerets. After they are hatched the young ones hold on for a time with their 

 pincers in the exact fashion of young lobsters. 



