Water Molds 



143 



Beggiatoa (fig. 56 h) is the commonest of the so-called 

 sulfur bacteria. Its cylindric unbranched and unat- 

 tached filaments are motile, and rotate on the long axis 

 with swinging of the free ends. The boundaries be- 

 tween the short cylindric cells are often obscure, 

 especially when (as is often the case) the cells are filled 

 with highly refractive granules of sulfur. Considerable 

 deposits of sulfur, especially about springs, are due to 

 the activities of this and allied forms. 



Water molds — True fungi of a larger growth abound 

 in all fresh w^aters, feeding on almost every sort of 

 organic substance contained 

 therein. The commonest of 

 the w^ater molds are the Sap- 

 rolegnias, that so quickly 

 overgrow any bit of dead 

 animal tissue which may 

 chance to fall upon the sur- 

 face of the water and float 

 there. If it be a fly, in a 

 day or two its body is sur- 

 rounded by a white fringe of 

 radiating fungus filaments, 

 outgrowing from the body. 

 The tips of many of these 

 filaments terminate in cylin- 

 dric sporangia, which when 

 mature, liberate from their ruptured tips innumerable 

 biciliated free-swimming swarm spores. These wander 

 in search of new floating carcases, or other suitable food. 

 Certain of these water molds attack living fishes, 

 entering their skin wherever there is a a slight abrasion 

 of the surface, and rapidly producing diseased condi- 

 tions. These are among the worst pests with which the 

 fish culturist has to contend. They attack also the 



Fig. 57. A common water mold, 



Saprolegnia. (After Engler and 



Prantl.) 



a, a colony growing on a dead fly; h, a bit 

 of the mycelium that penetrates the 

 fly's body; c. a fruiting tip, with escap- 

 ing swarm spores. 



