606 AQUATIC PHYCOMYCETES 



Further, they resulted in the recognition of a series of thallus types 

 (Matthews, 1937; Stuben, 1939), which, together with the reproductive 

 structures, unquestionably link the Blastocladiales to the Chytridiales 

 on the one hand and possibly to the Monoblepharidales on the other. 



So far as known, species of the order are either parasites of aquatic 

 larvae of insects, eggs of liver flukes, adult nematodes, and aquatic 

 fungi or they are saprophytes in fresh water or in soil. The saprophytes 

 in water occur most commonly on rosaceous fruits, twigs of ash, birch, 

 oak, and horse chestnut, or on bits of animal debris. The precise char- 

 acter of the substratum of only a few of the terricolous species is known, 

 since these forms are ordinarily obtained through methods of indirect 

 culture. 



The astute observations of Couch (1945b) led him to the discovery 

 that the curious group of parasites of mosquito larvae first described 

 by Keilin (1921) as Coelomomyces and long of problematical relation- 

 ship belonged in the Blastocladiales. 



Members of the Blastocladiales are in general characterized mor- 

 phologically by the possession of posteriorly uniflagellate zoospores 

 with a prominent nuclear cap (Fig. 46 F, p. 680) and bipolar mode of 

 germination, dark-colored resting spores with minutely punctate thick 

 walls, and a more or less well-defined basal cell anchored to the sub- 

 stratum by a series of strongly tapering, branching, chytrid-like rhizoids. 

 These rhizoids not only act as holdfasts, but by their extensive growth 

 and profuse branching undoubtedly provide the developing thallus 

 with a well-organized and effective nutrient-gathering system. 



Development and Morphology 



the thallus 



The character of the thallus differs widely among members of the 

 Blastocladiales. It varies in complexity from a simple unwalled and 

 almost plasmodial type (in the Coelomomycetaceae) to one resembling 

 that of a monocentric or polycentric chytrid (Fig. 44 A, G, I, p. 651) 

 or one produced by a true mycelial fungus. Indeed, the order is remark- 

 able because it provides examples of parallelism of body form with the 

 Chytridiales, the Leptomitales, and the higher fungi. If the thallus is 



