THALLOPHYTA: FUNGI 



113 



wall (Fig. 88B). These are liberated by decay of the host. Upon 

 germination, each gives rise to a zoospore. 



4. Saprolegniales 



The Saprolegniales, or water molds, are an order of aquatic fungi, 

 usually occurring in ponds and streams. Most of them are saprophytic 

 on plant or animal remains lying in the water, while a few are parasitic. 

 Many also occur on damp soil. The order includes 20 genera and about 

 120 species, representative members being Saprolegnia and Achlya. 



/ 



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A B 



Fig. 89. Saprolegnia. A, a sporangium and three escaped zoospores, X350; B, an 

 oogonium with many eggs and with two antheridia in contact with it, X350. 



Saprolegnia. This common water mold usually lives on dead insects, 

 fishes, tadpoles, and other aquatic animals. Sometimes it attacks 

 living fishes and fish eggs. The vegetative body consists of a delicate, 

 branching, coenocytic mycelium that penetrates the food supply. Some 

 of the hyphae form terminal sporangia, each of which is a slender elon- 

 gated cell, cut off by a basal wall, and giving rise to many uninucleate 

 zoospores (Fig. 89A). These are developed by progressive cleavage of 

 the cytoplasm within the sporangium. The zoospores escape singly 

 into the water through a terminal pore in the sporangial wall. In Achlya 

 they escape as a mass. 



The zoospores of Saprolegnia are oval and have two equal cilia attached 

 apically. After swimming for a while, they become quiescent, form a 

 cell wall, and go into a dormant stage. After about 24 hours, the proto- 

 plasts escape and again become motile, but this time the spores are 

 kidney-shaped and laterally biciliate. Finally they settle down and, 

 on a suitable substratum, each produces a new mycelium. The occur- 



