262 ANIMAL PARASITES AND MESSMATES. 



among the most cm^ious worms that have been dis- 

 covered dm^ing late years. They are of small size, and 

 live in the gills of fishes, often in great numbers, and 

 move with considerable agility. They are armed wii^h 

 very variable hooks, which serve to anchor them ; and 

 sometimes a digestive canal and organs of sensation are 

 fomid in them. 



The Gyrodactylus elegans bears within it a young 

 one which already has hooks, and in this young one, 

 which is not yet born, we see another generation with 

 the same organs, so that three generations are thus 

 enclosed. The daughter is ready at the moment of 

 her birth to give birth to another daughter. According 

 to another mode of interpretation, the mother and 

 daughter are sisters ; the elder is found at the peri- 

 phery, the younger at the centre. These worms are 

 found abundantly in the gills of the cyprinidse, or white 

 fishes. We have only to scrape gently the surface of the 

 gills with a scalpel, and thus remove a small quantity of 

 a mucous substance, place it on a slide of a microscope, 

 cover it with thin glass, and examine it immediately with 

 the compound microscope. We cannot repeat this three 

 times without finding gyrodactyles. 



There are also many insects which live as parasites 

 on plants, and demand from them both a resting-place 

 and their food. Almost all the Hemiptera are among 

 these ; we have already mentioned them. The hemip- 

 tera, which live on the sap of vegetables, are parasites 

 in the same manner as those which live at the expense 

 of animals. We ought not to make a difference between 

 the manner of life of the bugs of plants and those of 

 animals. It may be said that Providence has placed 



