920 AQUATIC PHYCOMYCETES 



or evanescent, contents in the vegetative stage often having a pale 

 gleaming luster, bearing irregular refractive granules and scattered oil 

 droplets, strongly vacuolate during maturation ; zoosporangia forming 

 one or more discharge tubes ; the zoospores of the laterally biflagellate 

 type, mono- or diplanetic, produced by successive division within the 

 sporangium or at the orifice of the discharge tube, where they are 

 generally surrounded by a vesicle, capable of repeated emergence; rest- 

 ing spore apparently asexually formed, or sexually after conjugation of 

 the receptive thallus with one or more contributing gametangia, ferti- 

 lization tube and periplasm present or absent, plants monoecious or 

 dioecious. 



Members of the order are most frequently parasites of fresh-water 

 algae, other Phycomycetes, and microscopic aquatic animals. A few are 

 parasitic or saprophytic in seaweeds. Several have been cultivated on 

 artificial media. 



The inclusion of the Olpidiopsidaceae in this order seems increasingly 

 justified as more simplified species of Lagenidium and Myzocytium with 

 endogenously formed zoospores are discovered (see, for example, L. 

 pythii, p. 991 ; M. microsporum, p. 980). As Whiffen (1946) pointed out, 

 if the oospore of L. pythii filled its container, it would be indistinguish- 

 able from a species of Olpidiopsis. Members of the family certainly 

 appear more closely allied to the Lagenidiaceae than to the chytrids. 



KEY TO THE FAMILIES OF THE LAGENIDIALES 



Thallus always one-celled, infection tube evanescent; zoospores formed 

 within the zoosporangium; resting spore lying free in the host, not 



formed in a gametangium Olpidiopsidaceae, p. 9 2 1 



Thallus predominantly multicellular, occasionally one-celled, infection 

 tube frequently persistent; zoospores formed inside the zoospo- 

 rangium or in a vesicle at the tip of the discharge tube; resting 

 spore formed in a gametangium 

 Zoospores formed within the zoosporangium, small, numerous; 



marine fungi Sirolpidiaceae, p. 965 



Zoospores formed or completing their maturation in a vesicle at the 



orifice of the discharge tube Lagenidiaceae, p. 972 



