CliUSTACEA 141 



sow bugs, or wood lice abound everywhere under logs and 

 stones during most of the year. They are about a half 

 inch long and have a convex grayish dorsal surface. They 

 feed on decaying vegetable matter. Respiration is effected 

 by gills instead of by lungs or trachea characteristic of 

 most land forms. The inner terminal divisions of the 

 abdominal legs, which are covered and kept moist by the 

 outer division, are the gills. 



Crustaceans are remarkable for their perfect adaptation 

 to widely different 

 conditions of life. 

 They dwell in fresh 

 water, in the sea, 

 on land, and as 

 parasites. In the 

 Mammoth Cave of 

 Kentucky are blind 



FIG. 165. Photograph of the shrimp. Natural size. 



crayfish which 



show by certain features that they are undoubtedly de- 

 scended from the forms outside of the cave enjoying perfect 

 sight. Some species live in the ocean at a depth of nearly 

 four miles. While most crustaceans are solitary, some 

 like the shrimp much used for food occur in vast shoals in 

 the ocean. 



