CHAPTER XXIV 

 THE OSTRACODA 



By R. W. SHARPE 

 Instructor in Biology, Vewitt Clinton High School, New York City 



An early author says of the Ostracoda, "these little creatures are 

 enclosed in a bivalve shell of lime and seem to be very lively in 

 their native element, being almost constantly in motion by the 

 action of their antennae, or walking upon plants and other soUd 

 bodies floating in the water." Also "by opening and closing their 

 valves, they enjoy light and move at their will, sometimes burying 

 themselves in the mud, sometimes darting through the water, the 

 humid air of their sphere. If they meet with any unforeseen object, 

 they conceal themselves all at once in their shells and shut the 

 valves, so that force and address seek in vain to open them." 



The Ostracoda are found abundantly in all kinds of fresh and 

 salt waters. They owe their name to the possession of a two- 

 valved limy shell, which is hinged dorsally, and encloses the entire 

 body. They are commonly more or less bean-shaped (Fig. 1244), 

 and seen from above (Fig. 1255 J) are usually oval or egg-shaped. 

 In many cases the shells overlap each other, or there may be a 

 ventral flange present. They average about i millimeter in length. 



The body of these little creatures is not segmented, and is com- 

 pletely enclosed in its bivalved shell, which is hinged along the 

 dorsal margin by means of a hinge ligament, somewhat as with 

 the molluscan bivalves. These valves are kept closed by adductor 

 muscles, their points of attachment being indicated by a nimiber 

 of lucid spots about the naiddle of each valve (Fig. 1255 a). 

 These are called "muscle impressions" and may often be of sys- 

 tematic value. At the anterodorsal end of the body is a single 

 eye, although it may occasionally be double. Most commonly the 

 shells of the sexes are of the same size and shape, although second- 

 ary sexual characters may appear here. For instance, the males 

 of the genus Candona are larger and of a different shape (Fig. 

 1300), while in Cypris and Notodromas the females are the larger. 



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