744 AQUATIC PHYCOMYCETES 



single polycentric genus, ' comparable to the Megachytriaceae. Sepa- 

 ration at the family level of so few forms is open to criticism but does 

 seem justified. The small number of accredited species is probably due 

 either to insufficient or inaccurate observations on zoospore structure 

 or to their actually being of limited diversity, or possibly to the fact 

 that many as yet unknown species occupy habitats thus far unexplored 

 for fungi. 



The establishment and development of the thallus differs little from 

 that found in the true chytrids. Both chitin and cellulose are present in 

 the walls of some forms (Nabel, 1939), in others there is no evidence of 

 cellulose. During thallus development in the holocarpic Anisolpidium 

 ectocarpii, the nuclei divide mitotically and simultaneously, forming in- 

 tranuclear spindles (Karling, 1943). 



Zoospore production apparently differs somewhat among members 

 of the group. In Anisolpidium spores are cleaved out within the sporan- 

 gium (Karling, op. cit.). Furrows extend from the border of a large 

 irregular central vacuole outward, cutting out segments of contents of 

 various sizes. Further cleavage in a centrifugal direction gives rise to 

 the rudiments of the zoospores. Soon after delimitation these initiate 

 movement. This increases in intensity until the tips of the earlier-formed 

 discharge tubes open, whereupon the spores escape and aggregate in 

 an oscillating mass at the mouth of the tube. No vesicle surrounds them 

 and after several minutes they swim away. 



In Canteriomyces the protoplasmic contents of the sporangium emerges 

 as one or several naked masses and undergoes complete maturation 

 into zoospores outside in the water. In Rhizidiomyces the undifferen- 

 tiated contents usually issues to form a spherical or hemispherical mass 

 at the orifice of the discharge tube. It quickly divides and the zoospores 

 are delimited. In this genus Zopf (1884) and others have also noted some 

 differentiation of the zoospores prior to discharge. The rapidity with 

 which these mature after liberation certainly suggests some previous 

 cleavage. Evidence concerning the presence of a vesicle around the 

 spore mass in Rhizidiomyces is conflicting. Nabel (1939) states definitely 

 that in R. hive Hums there is such a membrane, but Karling (1944d) 

 failed to find one in his material of this species. In Rhizidiomycopsis 

 the spores mature completely within the sporangium and emerge 

 individually without formation of a vesicle. 



