NO. 2063. NORTH AMERICAN PARASITIC COPEPODS— WILSON. 567 



locomotion. Such dimorphism as has to do with structure will be 

 treated under morphology (see p. 571), while that which concerns 

 locomotion and prehension follows under those headings respectively. 



Locomotion. — The larvae alone in this family possess the abilitj^ 

 to swim about freely and that only for a remarkably short period, 

 during which they must seek out a host and attach themselves to it. 

 During this free stage the larva swims about actively with a jerky 

 spiral motion by means of its two pairs of swimming legs. Fasten in 

 his experiments with the brook trout parasite, Salmincola edwardsii, 

 found that the average distance covered by a single contraction of 

 the swimming legs was about an inch. When the motion ceased the 

 larva at first took an upright position, then turned upside down and 

 sank to the bottom where it lay as it struck, with either the dorsal 

 or ventral side uppermost. It was also able by means of its first 

 antennae to cling to the side of the aquarium, or to suspend itself 

 from the surface film of the water. By means of its peculiar spiral 

 locomotion it is enabled to cover the maximum of territory with the 

 minimum of muscular energy. 



Female. — After the female larva has once become attached it can 

 not loosen this connection and form another, but must remain in the 

 same place all the rest of its life. If the arms (second maxillae) are 

 short, as in many species of Clavella, scarcely any freedom of motion 

 can be enjoyed. The parasite is not only fixed in position, but it 

 can not turn or flex its body, and becomes fully as helpless as some of 

 the Lernaeidae or Chondracanthidae, whose head and neck are buried 

 in the flesh of their host. On the other hand, if the attachment 

 organs are long and slender as in many species of BracTiiella, Lernae- 

 opodina, etc., while the tips of these arms remain at a fixed point the 

 body of the parasite can describe quite an arc. The arms are sup- 

 plied with powerful muscles which swing the body to one side or 

 restore it again when once displaced. In this way the animal often 

 escapes being brushed or scraped off from the fish, especially those 

 species which attach theselves to the fins, where, of course, there is the 

 greatest danger. After the female is once fastened to her host she 

 loses during subsequent moults the two pairs of swimming legs which 

 were all the locomotor organs she ever possessed. She finds excellent 

 aeration for her eggs and a good position from whence to discharge 

 the copepodid larvae when once they have matured. And there is 

 also an abundance of food, so that she is well provided for during life. 



Male. — The free-swimming period of the male larva is no longer 

 than that of the female, and the two often fasten themselves side by 

 side upon the same fish. But while this attachment is permanent 

 in the case of the female, it is only temporary for the male. 



When the two have become sexually mature the male, on swinging 

 around upon its attachment filament and coming in contact with the 



