538 AQUATIC PHYCOMYCETES 



biotic part forming the rudiment of the sporangium 01 resting spore, 

 the endobiotic part producing the vegetative system; sporangium 

 epibiotic, operculate, formed from all or part of the enlarged body of 

 the encysted zoospore; zoospores posteriorly uniflagellate, usually with 

 a single globule, often undergoing a period of swarming in a vesicle 

 outside the sporangium before dispersing ; rhizoidal system endobiotic, 

 arising from the tip of the germ tube or from an apophysis; resting 

 spore epibiotic or extramatrical, thick-walled, apparently asexually 

 formed, upon germination functioning as a prosporangium. 



Saprophytes on plant and animal substrata and parasitic on diatoms. 



As originally diagnosed by Karling (1945c), the genus embraced only 

 epibiotic forms which had apophysate operculate sporangia, zoospores 

 that underwent a swarming period in a vesicle outside the sporangium, 

 and extramatrical resting spores. Although no comparison was then 

 made with the closely related Chytridium, the type species differed 

 significantly from a member of this genus only in having an extrama- 

 trical rather than an endobiotic resting spore. As more monocentric epi- 

 biotic operculate fungi were discovered which had extramatrical resting 

 spores but no apophysis 01 swarming period of zoospores, the concept 

 of Chytriomyces evidently was expanded to include them (Karling, 

 1948b: 332). Further, since close comparison was made by Karling 

 with Amphicypellus (p. 546), a fungus with a type of development like 

 Rhizidium, it is possible but by no means clear he also intended that 

 any operculate rhizidiaceous forms discovered in the future be included 

 under Chytriomyces. If that is so, the operculate Amphicypellus would 

 have nomenclatorial precedence. Karling's (1949d: 352) redefinition of 

 Chytriomyces is incorporated in the description. 



Whether or not a definite vesicle comparable, for instance, to that in 

 Pythium, always surrounds the zoospores upon discharge is uncertain 

 (see Chytriomyces appendicular). Doubt arises because other micro- 

 scopic organisms are able to move freely in and out of the swarming 

 group of zoospores. This swarming is found in other genera of chytrids 

 and is not unique to Chytriomyces. 



The commonest species is Chytriomyces hyalinus, which is readily 

 obtainable on chitin bait in bogs. 



