16 AQUATIC PHYCOMYCETES 



Olpidiomorpha, where they are nearly apical (Fig. 12 J, p. 124), and in 

 Pleotrachelus wildemani (Ingold, 1952). Furthermore, all chytrid zoo- 

 spores are not radially symmetrical. 



Occurrence 



Aquatic Phycomycetes occur on a wide variety of substrata, princi- 

 pally in fresh but also, although to lesser extent, in marine waters. Prob- 

 ably the only limiting factors are inimical physical conditions and 

 lack of suitable substrata. Members of the Chytridiales are found as 

 saprophytes or parasites on many diverse hosts. These hosts include al- 

 gae, other aquatic fungi, aquatic and terrestrial herbaceous angio- 

 sperms, the spores of vascular plants, and vegetable debris; also, the 

 eggs, embryos, and adults of microscopic animals, the empty chitinous 

 integuments of aquatic insects, and keratinized substances, such as 

 snake-skin casts and (especially) hair. Since other chitinous materials 

 and kinds of keratin, when used as bait, retrieve many fungi from the 

 aquatic habitat, they most certainly must be utilized by them under 

 natural conditions. The Hyphochytriales are similar to the Chytridiales 

 in occurrence. The Blastocladiales, Saprolegniales, Leptomitales, and 

 Pythiaceae (Peronosporales) are predominantly saprophytes, although 

 a few species of the Saprolegniales and Pythiaceae are parasitic on aquat- 

 ic and terrestrial plants and on aquatic animals. Among the substrata 

 utilized by the fresh-water saprophytes belonging to these orders are 

 twigs, rosaceous fruits , and cadavers offish and insects. The Monobleph- 

 aridales are usually found on sunken twigs or fruits in cool water, 

 although several species of Monoblepharella, a member of this order, 

 have, on the contrary, been isolated on hempseed from tropical soils. 

 The Plasmodiophorales are all obligate parasites of other aquatic fungi 

 and of terrestrial and aquatic (both fresh-water and marine) angio- 

 sperms. The Lagenidiales are primarily parasitic and usually live on 

 fresh-water algae, fresh- water Phycomycetes, and microscopic animals. 

 However, one family, the Sirolpidiaceae of this order, includes facul- 

 tative parasites of certain marine algae and invertebrates. Members 

 of the one genus (Ancylistes) of the order Entomophthorales that is 

 treated here are parasites of desmids. 



The great majority of the aquatic Phycomycetes are found in fresh 

 waters, but the Chytridiales, Saprolegniales, Hyphochytriales, Plas- 



