650 PROCEEDINGS OF THE NATIOXAL ^rmFA]M. vol.xxv. 



In .spitc of the fact that the Argulidae move about ,so freely and 

 show alnio.st no trace of degeneration, their food is exactly the same 

 as that of the other parasitic copepods, namely, the blood of their host. 

 They oljtain this by means of a long evertible proboscis or dart, which 

 is formed ])y a modification of certain of the mouth parts, and which 

 they thrust through the skin, afterwards sucking up the blood as it 

 flows from the wound. The base of the dart is continuous with the 

 base of the regular mouth parts, from whence it fxtends forward 

 between the bases of the two pairs of antenna?, lying in a shallow 

 groove which occupies the center of this ventral surface. 



When swimming about freely the proboscis is withdrasvn as far as 

 possible and carried in- the groove in a manner similar to that of the 

 hemipterous insects. But upon the iish's body the anterior end is 

 dropped down against the skin, into which it is pushed diagonally by 

 a slight forward movement of the body assisted by its own eversion. 



Naturally the parasites seek those portions of the body where the 

 skin is thin, but more especiall}' where the blood vessels are near the 

 surface, such as the inner surface of the operculum and the fins. When 

 fastened to the outside of the body of a scaly fish the scales naturally 

 obstruct the passage of the dart. To obviate this the Argulus burrows 

 beneath a scale with the anterior edge of the carapace, lifting the scale 

 up and pushing the body forward until the dart is brought to the naked 

 skin underneath. They remain in one position for an hour or more, 

 as though the blood were obtained slowh\ 



As to the relation between parasite and host, it is not probable that 

 the former ever become a serious menace to the life of the latter 

 except under favorable conditions. 



No matter how badly a fish may be infested with these pests it has a 

 chance to get rid of them pretty thoroughl}' at least three times a year 

 at the breeding seasons. Of course, if a fish for any reason has become 

 debilitated and then happens to get an extra dose of parasites the results 

 are likely to be fatal. But it is worthy of note that the Argulids prefer 

 strong, healthy lish and are not often found upon sickly ones. 



Their bodies are comparatively so small, are flattened so strongly, 

 and are held so firmly to the flsh's skin that they must occasion almost 

 no inconvenience in the natural movements of their host. 



A\'hen we consider the artificial propagation of flsh, however, the 

 conditions are entirely changed. Professor Smith long ago called 

 attention to the subject of copepod parasitism in its special practical 

 importance to all those engaged in raising flsh confined in ponds or 

 other restricted areas, especially in aquaria. The artificial surround- 

 ings always make more or less of a drain on the fish's vitality, and, 

 since the numbers are restricted, there is a resultant concentration of 

 the evil eft'ects produced by the parasites. The very fact that the latter 

 choose the strongest and most healthy fish is one of the worst things 



