INTRODUCTION 15 



spiral path, turning on its axis, the whole movement being more awk- 

 ward and irregular than in normal spores. 



The secondary biflagellate zoospore of Saprolegnia, which emerges 

 from the cyst formed by the primary one, is in contrast to it an extremely 

 vigorous and more durable swimmer. The reason for this may be the fin- 

 like construction of the posterior flagellum mentioned earlier (Manton, 

 et a/., 1951). Both the anterior and the (longer) posterior flagellum pre- 

 sent alternating double and single images but, since they do not undu- 

 late in the same plane, the two do not show like images at the same time. 

 The spore swims in a spiral path, rotating on its own axis, the flagella 

 presenting at one time single, at other times double images. As Couch 

 points out, these observations, indicating that both flagella are active, 

 are at variance with the generally accepted idea that the posterior fla- 

 gellum is dragged passively along behind and functions mainly as a 

 rudder. This passive appearance is in reality due to the fact that at 

 such times the flagellum is undulating in a plane vertical to the observer. 

 Further studies on zoospore motility, using phase and other types of 

 equipment, will undoubtedly be productive and supplement the pio- 

 neering studies of Couch. 



Schussnig (1949) applied the terms "monokont"' and (because the 

 single flagellum is posterior) "opisthokont" to the chytridiaceous swarm- 

 er. That of the Hyphochytriales he called "akrokont." The kidney- 

 shaped bilaterally symmetrical secondary swarmer of the Lagenidiales 

 -Saprolegniales-Peronosporales series he designated "subakrokont," 

 if the flagella are subapical, and "pleurokont," if they are laterally 

 attached in the groove. Whether these and other of his terms will 

 convey a clearer picture of the zoospore than those in present usage 

 seems questionable. His lengthy discussion of zoospore morphology 

 and its bearing on origin and relationships contains, however, many 

 stimulating passages. 



A bilaterally symmetrical, dorsiventrally flattened zoospore with a 

 laterally attached, posteriorly directed flagellum was reported by Geit- 

 ler (1942) to be characteristic of Zygorhizidium verrucosum. Schussnig 

 points out that it is strikingly different from the radially symmetrical 

 "opisthokont" of other chytrid spores, and he even advocates erection 

 of a new genus to accommodate the fungus. Laterally attached, poste- 

 rior flagella have, however, been reported in other chytrids, namely in 



