14 AQUATIC PHYCOMYCETES 



occurs in the Hyphochytriales, Couch raises the question: how is it 

 possible for the undulatory movement of an anteriorly directed flagel- 

 lum to pull the spore forward, when in the Chytridiales, in which it is 

 posteriorly directed, it pushes the spore ahead? The answer is that, in 

 general, in the anterior type (represented by Rhizidiomyces of the 

 Hyphochytriales) undulations are propagated just back of the tip, 

 travel toward the body of the spore, and pull it forward. In the posterior 

 type the tip does not undulate but, rather, waves start near the base 

 of the flagellum and travel toward the tip and push it forward. Accord- 

 ing to Couch, the anterior type of spore is able to reverse its path 

 and swim for a short distance with the flagellum directed backwards. 

 In such instances he surmises wave propagation is from spore body 

 to flagellar tip. 1 Karling (1943) in describing zoospore movement in the 

 marine Anisolpidium ectocarpii says that in it the single flagellum ex- 

 tends straight forward and appears to be almost stationary save for 

 the anterior portion which whips rapidly back and forth. This distal 

 lashing propels the spore forward rapidly and evenly, the movement 

 being interspersed by sudden stops and changes of direction. He records 

 no undulations of the whole flagellum as noted by Couch in Rhizidio- 

 myces, another member of the same order. 



The primary biflagellate zoospore of Saprolegnia (Couch, 1941) has 

 two subapically attached flagella which, when the spore is moving, are 

 oppositely directed. These spores, which Couch found to be poor swim- 

 mers, exhibit two main methods of locomotion: (1) They may swim 

 slowly and awkwardly in broad spirals while rotating on their axes, 

 in which case, although the anterior is the more active, both flagella 

 produce an alternating single and double image under dark-field illu- 

 mination, indicating that each undulates in only one plane with respect 

 to the zoospore; (2) the spore may move in a narrow circle making about 

 a one-half turn on its axis as it completes a circle, with both flagella 

 active but the anterior one always the more vigorous. Occasionally, one 

 or the other flagellum is missing. If the posteriorly directed one is lost 

 the zoospore swims smoothly, without rotating on its axis or describing 

 a spiral path, but if the anterior one, the spore travels a very irregular 



1 Such a reversal of spore movement has also been noted, under certain conditions, 

 by Gaertner (1954a) in a member of the chytrid genus Phlyctochytrium. 



