ANATOMY AND METAMORPHOSIS OF CYCLOPS. 223 



it quiet, examine the basal joints of the antennules with 

 the highest magnifying power that can be used (from fifty 

 to one hundred diameters). Notice the thin-walled, trans- 

 parent vesicle, the otocyst, which nearly fills the enlarged 

 basal joint of the antennule. On the upper surface of the 

 antennule notice the transverse slit through which the oto- 

 cyst opens to the exterior. Notice the row of hooked 

 hairs which project over this slit, as a sort of thatch. In 

 the cavity of the otocyst a number of small grains of sand. 

 A row of long, slender, sensory hairs, which project from 

 the wall of the otocyst into its cavity, on the side nearest 

 the median line of the body. On the posterior or basal 

 side an irregular cluster of shorter hairs. 



An examination of the hearing organ of the adult crab, 

 and of the lobster or crayfish, will show that the megalops 

 larva differs from the adult and resembles the lobster, in 

 having the otocyst open to the exterior. The grains of 

 sand also are present in the otocysts of the lobster, but 

 absent in that of the adult crab. 



XXII. THE ANATOMY AND METAMORPHOSIS OF 

 CYCLOPS. 



SMALL Copepods are usually abundant in both fresh and 

 salt water, and there is never any difficulty in obtaining 

 them. As Cyclops is one of the most common and widely 

 diffused fresh- water genera it has been selected for de- 

 scription, but there should be no difficulty in studying other 

 forms, for although the generic differences are very con- 

 siderable, they are not of such a character as to confuse 

 the student. In order to obtain a supply of specimens of 

 Cyclops, carefully examine the sides of an aquarium in 

 which water plants have been growing, and search for 



