232 THE CRAYFISH 



animals are barely visible to the naked eye. They are found in almost 

 every part of the world, from the arctic seas to those of the tropics, and 

 in fresh as well as salt water. They are so numerous that the sea in places 

 is colored by their bodies. So prolific are they that it is estimated that 

 one copepod may produce in a single year four billion five hundred million 

 offspring. These animals form a large part of the food supply of many 

 of our most important food fishes as well as the food of many other aquatic 

 animals. The whale, for example, subsists largely on this kind of food. 

 They are, then, in an indirect way, of immense economic value. 



Degenerate Crustaceans. One of the most interesting forms to a 

 zoologist is the goose barnacle. This crustacean, like all others of the 

 group, is free-swimming during its early life. Later, however, after passing 

 through several changes in form during its development, the barnacle 

 settles down on a rock or some floating object, fastens itself along the 

 dorsal surface, and remains fastened during the rest of its life. Food 

 conies to it in a current of water, which is set in motion by the rhyth- 

 mical beating of the appendages. Thus food particles are carried along 

 the ventral side of the body to the mouth. Such animals, having lost 

 the power of locomotion, are said to be degenerate. 



Parasitic Crustaceans. Other crustaceans have become even more 

 helpless and have come to take their living from other animals. In some 

 cases they become simply a bag for absorbing nourishment from the host 

 on which they are fastened. Such is the sacculina, a degenerate crusta- 

 cean that lives attached to the body of the crab. Others attach them- 

 selves to fishes and are known to fishermen as fish lice. 



REFERENCE BOOKS 

 ELEMENTARY 



Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American 



Book Company. 



Burnet, School Zoology, pages 67-73. American Book Company. 

 Davison, Practical Zoology, pages 133-141. American Book Company. 

 Herrick, Textbook in General Zoology, Chap. XIII. American Book Company. 

 Jordan and Kellogg, Animal Life, Chap. VIII. D. Appleton and Company. 

 Jordan, Kellogg, and Heath, Animal Studies, Chap. IX. D. Appleton and Com- 

 pany. 



ADVANCED 



Herrick, The American Lobster, Report of U.S. Fish Commission, 1895. 



Huxley, The Crayfish. D. Appleton and Company. 



Mead, Reports of the R.I. Inland Fisheries Commission. 



Parker, Elementary Biology. The Macmillan Company. 



Parker and Haswell, Textbook of Zoology. The Macmillan Company. 



