^ r2 Diseases of Fishes 



counteracting the bad influences of worms of this order, since 

 their larval state is passed, in some cases certainly, and m most 

 cases probably, in small crustacea, which constitute a constant 

 and necessary source of food for the fish. The same remark 

 which was made in .another connection with regard to the dis- 

 posal of the viscera of fish applies here. In no case should the 

 viscera of fish be throAvn back into the water. In this order 

 the sexes are distinct, and the females become at last veritable 

 sacs for the shelter and nourishment of enormous numbers of 

 embryos. The importance, therefore, of arresting the devel- 

 opment of as many embryos as possible is at once apparent. 



Nematodes. — The round worms or nematodes are very 

 especially abundant in marine fishes, and particularly in the 

 young. The study of these forms has a large importance to 

 man. Dr. Linton pertinently observes : 



"Where there is exhaustive knowledge of the thing itself 

 the application of that knowledge toward getting good out of 

 it or averting evil that may come from it first becomes possible. 

 For example, a knowledge of the Hfe-history of Trichina spiralis 

 and its pathological effects on its host has taught people a sim- 

 ple Avay of securing immunity from its often deadly effects. A 

 knowledge of the life-histories of the various species of tasniffi 

 which infest man and the domestic animals, frequently to their 

 serious hurt, has made it possible to diminish their numbers, 

 and may, in time, lead to their practical extinction. 



" So with the parasites of fishes. AVhenever for any reason 

 or reasons parasitism of any sort becomes so prevalent with any 

 species as to amount to a disease, the remedy will be suggested, 

 and in some cases may be practically applied. If, for example, 

 it were thought desirable to counteract the influences which are 

 at work to cause the parasitism of the trout of Yellowstone 

 Lake, it could be very largely accomplished by breaking up 

 the breeding-places of the pelican on the islands of the lake. 

 With regard to parasitism among the marine food-fishes, the 

 remedy while plainly suggested by the circumstances, might be 

 difficult of application. Yet something could be done even 

 there, if it were thought necessary to lessen the amount of 

 parasitism. If such precautions as the destruction of the para- 

 sites which abound in the viscera of fish before throwing them 



