INTRODUCTION 3 



The Phycomycetes comprise the following orders, which are here 

 grouped in three series. 1 



Chytridiales 



Blastocladiales 



Monoblepharidales 



Hyphochytriales 



Plasmodiophorales 



Saprolegniales 



Leptomitales 



Lagenidiales 



> Zoosporic; aquatic 



Peronosporales 



) Zoosporic or "conidial"; terrestrial except 

 \ for the Pythiaceae, which are amphibious 



Mucorales } Conidial; terrestrial except for Ancylistes of 



Entomophthorales \ the Entomophthorales 



The zoosporic aquatic series, with which we are primarily concerned, 

 is composed of fungi which live as saprophytes or parasites on various 

 plants and animals or their parts, in water, or in damp soil. In this field, 

 however, as elsewhere in biology, no hard and fast distinction can be 

 drawn between aquatic, amphibious, and terrestrial organisms. The 

 many and diverse soil-inhabiting zoosporic types isolated in recent 

 years are all clearly related to and usually congeneric with purely aquat- 

 ic forms and have no doubt only secondarily invaded land. Another 

 even more striking instance of what may be termed "adaptive radiation," 

 is to be seen in Ancylistes, a member of a conidial, terrestrial order (the 

 Entomophthorales), which occurs as a parasite of desmids, a group of 

 strictly aquatic algae. Also, there is found in the elegant series of aquatic, 

 amphibious, and terrestrial fungi belonging to the Peronosporales, at 

 least one genus, Phytophthora, that is well known both to plant pathol- 

 ogists and to students of aquatic fungi. The present volume does not, 



1 This arrangement was for the most part suggested by Professor W. H. Weston 

 of Harvard University. It does not include the Zoopagales. a group (tentatively 

 brought together by Bessey, 1950) of predaceous fungi of uncertain affinities which 

 prey upon soil- and water-inhabiting animals. 



