LAG EN IDl ALES 927 



gation of the receptive thallus with from one to several smaller contribut- 

 ing thalli, upon germination functioning as a sporangium. 



Primarily parasites of fresh-water fungi, but also occurring in fresh- 

 water and marine algae. 



The zoospore was said by Cornu to be posteriorly uniflagellate. 

 Fischer (1882) described it as biflagellate in the species studied by him. 

 His generic description (1892) allows for both types. Schroeter's generic 

 description (1893) mentions only the biflagellate type, as does that of 

 Minden (1915). 



Work by Shanor (1939a, 1939b) and McLarty (1939, 1941a) with 

 single-spore cultures of various species of Olpidiopsis and forms which 

 would ordinarily be placed in Fischer's genus Pseudolpidium, showed 

 that all the fungi parasitic on Phycomycetes studied by them were 

 species of Olpidiopsis. Fischer's genus is maintained tentatively (p. 955) 

 for the three algal parasites which lack resting spores. Their true 

 affinities cannot be determined at this time. 



As here understood, Olpidiopsis includes all forms with unlobed or 

 nontubular endobiotic holocarpic thalli, with biflagellate zoospores, 

 with sporangia bearing one to several discharge tubes, and with resting 

 spores produced either asexually or after conjugation of thalli. In the 

 past the marine forms have been referred both to Pleotrachelus (Petersen, 

 1905) and to Peter senia (Sparrow, 1936b), but they differ from the first 

 in having biflagellate zoospores and from the second in having unlobed 

 nontubular sporangia. Peronium aciculare Cohn (1853) may possibly 

 have been a species of this genus, but it is too imperfectly known to be 

 considered here. 



Butler (1907), Barrett (1912b), Scherffel (1925a), and Diehl (1935) all 

 describe what might be considered a poorly defined type of diplanetism 

 of the zoospores. After the zoospores escape from the sporangium there 

 is a short period of motility, which, in Olpidiopsis saprolegniae and O. 

 schenkiana, may include slight amoeboid motion; this is followed by a 

 period of quiescence accompanied by retraction of the flagella. During 

 the rest period contractile vacuoles may appear in the plasma. At its 

 conclusion the flagella initiate movement and elongate, and the spore, 

 after rocking for a while, once more assumes motility. No encystment 

 occurs, and the primary zoospore is interpreted as having been directly 



