Crustacea. 83 



they are withdrawn and shut in if the animal is disturbed. In its devel- 

 opment the goose barnacle has about the same history as the acorn 

 barnacle. 



Other Degenerate Crustaceans. Degenerate as are the barnacles, 

 there are still lower crustaceans. Various crustaceans have become 

 parasites, living attached to whales, fishes, etc., and have become so 

 degenerate as to have lost all likeness to the typical crustacean struc- 

 ture, so that no one would, without pro- 

 longed investigation, even suspect that they 

 belonged to this group. 



All of the above cases, blind crayfish, 

 hermit crab, barnacles, and parasitic crus- 

 tacea, illustrate one general principle, that 

 disuse leads to atrophy, and that parasitic 

 habits involve degeneration in structure as 

 well as in function. 



Classification of Crustacea. The crus- 

 tacea are divided into two subclasses, the 

 Entomostraca and the Malacostraca. 



The Entomostraca are of comparatively FlG " 55 ' GooSE BARNACLES - 

 simple structure, usually small, sometimes ^^Lgy^"^ 6 



microscopic. The number of segments is 



variable, and the appendages are very similar throughout. We may 

 briefly consider some of the leading orders. 



Some of the Phyllopoda are covered by a flat, shield-shaped carapace ; 

 others have a bivalve shell which does not inclose the head. Some 

 Phyllopods have no carapace. In some forms the body is unsegmented, 

 and there are leaflike, lobed, swimming feet. 



The Ostracoda are small and the head as well as the rest of the 

 body is inclosed in a bivalve shell, somewhat resembling a little clam. 



The Copepoda may be represented by the cyclops, or water flea. 

 Fig. 56. This form is common in sluggish streams and ditches. It 

 is white, large enough to be seen by the naked eye, and swims by a 

 jerky motion of the antennae, which are its largest and strongest 

 appendages. The female bears two large egg masses. 



The Cirripedia comprise the barnacles above mentioned. 



The Malacostraca comprise the higher Crustacea. They are usually 

 of considerable size, and the number of segments is rather constant, 

 instead of variable as in the Entomostraca. There may or may not be 



