INTRODUCTION 7 



constitution of the families and subfamilies and are chiefly based on 

 methods of development and thallus structure. 



The Hyphochytriales at present are considered an isolated group of 

 problematical affinities, whose members have, perhaps, evolved from 

 anteriorly rather than posteriorly uniflagellate monads. In spite of 

 their close resemblance in body plan to the chytrids, they are not be- 

 lieved closely allied to them. Of the orders Blastocladiales, Mono- 

 blepharidales, Saprolegniales, Leptomitales, and Peronosporales, each 

 appears to be composed of clearly related fungi. Whether or not the 

 Lagenidiales as here defined will prove to be a natural group must 

 await the test of time. The same may be said of our present concept of 

 the Entomophthorales (Ancylistes). Until adequate studies have been 

 made of the zoospores and the life cycles of a number of the species, 

 little can be said concerning the interrelationships within the Plasmo- 

 diophorales. Indeed, this also holds true for their alliance to the Phy- 

 comycetes as a whole. x 



The Zoospore 



The now universal acceptance of the significance of the structure of 

 the zoospore in determining the relationships of aquatic Phycomycetes 

 makes it essential at this point to give consideration to the different 

 types that are thus far known. More details concerning these spores 

 will be found under the respective orders. 



Five principal types of swarmers are distinguished in the zoosporic 

 series, viz.: (1) posteriorly uniflagellate, (2) anteriorly uniflagellate, (3) 

 unequally ("heterokont") biflagellate, (4) anteriorly biflagellate ("pri- 

 mary"), and (5) laterally biflagellate ("secondary"). In general they 

 may be characterized as follows: 



1. The posteriorly uniflagellate zoospore is typical of the orders 

 Chytridiales, Blastocladiales, and Monoblepharidales. In the Chytri- 



1 It is becoming increasingly clear that the "Phycomycetes" as now constituted is 

 an artificial grouping of coenocytic fungi, which embraces among its aquatic 

 members (exclusive of Ancylistes) at least four distinct lines of descent: (1) the 

 Chytridiales-Blastocladiales-Monoblepharidales, (2) the Hyphochytriales, (3) the 

 Plasmodiophorales, (4) the Saprolegniales-Leptomitales-Lagenidiales-Peronosporal- 

 es. The last-named galaxy probably has some forms derived from coenocytic algae. 

 See Sparrow, in Mycologia, 50 : 797. 1958 (1959.) 



