36 MYCETOZOA AND BELATED ORGANISMS 



In some regards this seems to lie in a position intermediate between the 

 Myxogastrales and the Proteomyxa, while in some structural features it 

 shows similarity to the Labyrinthulales. No definite formation of spo- 

 rangial structures has been observed. 



In the author's opinion the primitive ancestors of these four groups 

 were more or less colonial amoeboid organisms consisting of encysted 

 cells at one stage of their life history. From these emerged biflagellate 

 swarm cells, both fiagella being of the whiplash type and nearly or quite 

 equal. This biflagellate condition has persisted in Plasmodiophora and is 

 found in a considerable number of the swarm cells of the Myxogastrales, 

 the presence of two blepharoplasts remaining even when one flagellum is 

 missing. In the Labyrinthulales in one genus no flagella are known and 

 in one genus the swarm spores are described as anteriorly uniflagellate. 

 The questions as to blepharoplast number and type of flagellum are not 

 solved. In the Acrasiales the flagella are entirely lacking. The Myxo- 

 gastrales are undoubtedly the furthest developed from the evolutionary 

 standpoint, in the development of sporangia and capillitium and adapta- 

 tion to aerial dispersal of the encysted spores. The Acrasiales are probably 

 closely related. The Labyrinthulales and Plasmodiophora are water or soil 

 organisms and lack the complicated structure of the Myxogastrales. 



Key to the More Important Orders of Mycetozoa 



Saprophytes or surrounding and ingesting fungi, bacteria, etc. Sporangia aerial. 

 Sporangia with thin or thick peridium, and mostly with a capillitium. 



Spores upon germination producing an anteriorly uni- or biflagellate swarm 

 cell (rarely nonflagellate myxamoeba). Order Myxogastrales 



Sporangia without peridium and capillitium, spores embedded in a mass of 

 slime. 

 Spores producing nonflagellate myxamoebae. Order Acrasiales 



Parasites in the cells of algae and of submerged aquatic. plants, forming net- 

 plasmodia. 

 No aerial sporangia. Spores upon germination producing myxamoebae or 

 anteriorly uniflagellate zoospores. Order Labyrinthulales 



Parasites producing plasmodia within the cells of roots and stems of higher plants, 

 a few in algae and aquatic fungi. Swarm spores anteriorly l>iflagellate. 



Order Plasmodiophorales 



Key to the More Important Families and Genera of Myxogastrales 



(Based in Part Upon Macbride and Martin) 



Spores produced externally. Family Ceratiomyxaceae 



Only genus. Ceratiomyxa 



Spores produced internally. 



Spores violet, brown, or purplisli gray (rarely ferruginous or colorless). 

 Capillitium always present. 

 Sporangia with lime granules (calcium carbonate) in capillitium and often 

 in peridium also. Family Physaraceae 



