1 68 BULLETIN OF THE BUREAU OF FISHERIES. 



erable sexual dimorphism, a loss of the powers of locomotion, and modified means of 

 prehension. But the peculiar life history greatly modifies these characters and even 

 does away with some of them. 



Sexual dimorphism. — Since the females attain a length in some species of more than 

 20 mm. we would naturally look for a much smaller male, with marked differences between 

 the sexes. But a study of the development convinces us that there is no sexual dimor- 

 phism for the following reasons: There is no disparity in size up to the close of the 

 copepodid stages, at which time both sexes become mature and their union takes place. 

 After this union the male either dies at once or becomes a free swimmer for a short time 

 and then dies. And while the female subsequently seeks out a final host and undergoes 

 upon it marvelous changes in size and form, we can not in fairness compare her altered 

 proportions with those of the male before his death, and hence there is no real dimorphism. 



Locomotion. — The adults are incapable of locomotion, but the nauplius, metanau- 

 plius, and copepodid larvae are exceedingly active and move about constantly and 

 vigorously. When placed in a watch crystal of water the copepodid larvae swim around 

 in search of something to which they can cling. 



If there is a fragment of a gill filament present they all congregate upon it and 

 remain there. Every now and then they leave in search of a better location, but finding 

 none return to the filament. 



If there is nothing present to which they can fasten they swim about until tired, 

 then come to rest upon their backs on the bottom of the watch glass, and may 

 remain there an hour or more. They can be kept alive in this way for several days. 

 When swimming or when moving about over the gills the motion of the larva is jerky 

 and spasmodic. They swim with the back downward, darting for a short distance in 

 almost any direction, sometimes in a straight line, sometimes in a wide curve, stopping 

 wherever it happens, usually in a horizontal position, and then sinking slowly to the 

 bottom. 



But they also have the ability, possessed by many of the free swimmers, of holding 

 themselves suspended from the surface film of the water by means of their antennae, 

 their bodies hanging in a vertical position, and they often come to rest in this manner 

 and remain for some minutes. 



On the gills of the fish they always move about with their ventral surface next to 

 the filament, using the swimming legs for locomotion and the second antennae and 

 maxillipeds for prehension. They catch hold of the filament and kick themselves free 

 with a jerk that helps to send them forward. When resting the first antennae are ex- 

 tended horizontally, the two being in the same straight line, while the second pair are 

 turned ventrally and parallel to each other. The maxillipeds also stand out ventrally 

 at right angles to the surface of the head, ready for use. During locomotion the posterior 

 half of the body is often inclined ventrally, the flexure occurring between the fourth and 

 fifth-sixth (fused) thorax segments. After the short rest at the surface or the long rest 

 on the bottom the larva moves about as actively as ever, and this alternate motion and 

 resting is kept up until a host is found. 



Prehension. — The organs of prehension are the second antennae and the maxillipeds; 

 these are similar in both sexes and are the only means of prehension possessed by the 

 male. The larvae never develop a frontal filament like that of the Lemaeopodidae and 

 the other L,emaeidae, but continue to move about freely during their entire larval life. 



But the female, after she has sought out her final host and fastened herself to it 

 by means of the second antennae and maxillipeds, develops an entirely different method 



