28 STRUCTURE AND CLASSIFICATION OF FUNGI 



the class can best be described by considering each subclass sepa- 

 rately. Indeed, as stated above, many mycologists consider them 

 so diverse that they divide the Phycomycetes into two or three 

 classes, each equal in rank to the other two classes of perfect fungi. 



The Archimycetes are primitive forms, mainly aquatic and para- 

 sitic on water plants and animals, though some are important causes 

 of disease of land plants. They are characterized by the absence of 

 mycelium, the thallus being a single cell or an irregular mass, though 

 a rudimentary mycelium is formed by some. They reproduce mainly 

 by motile, flagellated spores called zoospores. In some, sexuality 

 is very primitive. In others, well-defined gametes, oogonia, and 

 antheridia, are formed. There are no forms of importance to the 

 bacteriologist. 



The Oomycetes are also mostly aquatic forms; some are parasitic 

 on land plants, and some live in soil. They reproduce by sexual 

 spores called oospores and non-sexual zoospores which in many forms 

 are motile. 



Life Cycle of Saprolegnia. The common water molds of the genus 

 Saprolegnia may be considered to illustrate the life history of an 

 Oomycete. They are found especially on dead animal matter sub- 



Fig. 22. A minnow infected with Saprolegnia. 



merged in water, as insects and fish. Some are parasitic on fish. 

 The scum which develops frequently on goldfish in aquaria or min- 

 nows kept in bait-pails is due to the growth of this mold. They are 

 especially prone to develop on fish after the latter have been handled 

 or bruised (Fig. 22). 



The non-sexual spores are formed at the tips of filaments of my- 

 celium projecting into the water. The terminal portion of the fila- 

 ment becomes separated from the rest by a crosswalk The cell so 



