INTRODUCTION 23 



a population of microscopic animals, usually rotifers and nematodes. 

 The adults and their eggs should be watched for evidences of parasitic 

 organisms, and all dead animals should be examined carefully for 

 traces of fungi. The layer of scum formed on the sides of the culture 

 jar at the water line is a particularly favorable place to find them. One 

 of the most curious and productive habitats for chytrids is the empty 

 cases (integuments or exuviae) of aquatic insects such as caddis flies, 

 mayflies, midges, and the like. 1 After the insect emerges from the water 

 and sheds its skin this integument is frequently washed back into the 

 water. Within it no doubt persist the remnants of the molting fluid, 

 which are attractive to the zoospores of certain chytrids. These fungi 

 soon establish themselves within the integument and may flourish there 

 in great abundance for a few days, after which they either disappear 

 or form resting spores and become quiescent. It is possible. that the 

 material of the integument itself is utilized by the chytrids, but this 

 does not seem probable in view of the rarity of their occurrence in in- 

 teguments which have obviously been in the water for a long period. 

 The collection of the exuviae offers no difficulty if they are searched 

 for in early spring among stands of emergent phanerogams such as 

 Seirpus or Juncus in shallow water. 



Of late, many new chytrids have been discovered through the use 

 as bait of such substances of animal origin as purified shrimp chitin 

 and various forms of keratin (snake-skin casts, defatted hair and skin, 

 horn shavmgs, and the like). Materials of vegetable origin, as unwater- 

 proofed cellophane, have been similarly utilized. These baits tend to 

 attract chitinophilic, keratinophilic, and cellulolytic fungi which are 

 not ordinarily found. 



Members of the Blastocladiales occur for the most part in two very 

 different habitats. The most widespread genus of this order, Blastocladia, 

 is primarily an inhabitant of submerged twigs and rosaceous fruits. 

 On these it forms white, generally rather crisp and granulated hemis- 

 pherical pustules of densely compacted plants. The collection of fruit- 

 and twig-inhabiting fungi is relatively simple. Such substrata can be 

 searched for at a likely aquatic site, but a surer method is to place 



1 Although dragonfly exuviae are excellent habitats for chytrids, they are too 

 large and too opaque for good observation of the fungi. 



