ORDERS ACRASIALES AND LABYRINTHULALES 



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Orders Acrasiales and Labyrinthulales. Sometimes associated with 

 the Subclass Mycetozoa are the two orders Acrasiales and Labyrinthulales. 

 Whether they should be included in this subclass or in distinct subclasses 

 the author will not seek to decide. In the first of these two orders the 

 swarm spores do not possess flagella although they are amoeboid upon 

 emerging from the spore wall. They consist of saprophytic or parasitic 

 organisms occurring on dung, decaying wood, leaf mold or other organic 

 matter. The spores upon germination give rise to naked amoeboid cells 

 (myxamoebae) with or without conspicuous pseudopodia and containing 

 a single nucleus and one or more food vacuoles. Within these vacuoles are 

 digested the bacteria and other bits of organic matter that serve as food 

 for the organism. Raper (1937) has shown that the myxamoebae of 

 Dictyostelium discoideum Raper are strictly parasitic upon bacteria, many 

 kinds of which may serve as their food. There is no indication of a 

 symbiotic relation between the bacteria and the myxamoebae. The latter 

 enlarge and divide several times and in D. mucoroides Bref., according to 

 Skupienski (1920), then unite by twos. The resulting zygotes seem now 

 to be mutually attracted to one another and draw together into heaps of 

 naked cells that maintain their individuality. These heaps of separate 

 cells are called pseudoplasmodia in contrast to the true plasmodia that 

 are found in the Myxogastrales. At this stage pressure or, in some cases, 

 exposure to bright light will cause the pseudoplasmodium to separate into 

 its individual cells which reassemble again elsewhere. Skupienski claims 

 that in Dictyostelium the cells eventually unite into a true Plasmodium 

 within which the nuclei undergo two more divisions (probably meiotic). 



Fig. 5. Acrasiales. (A) Pseudoplasmodium of Polysphondtjlium violaceum Bref. 

 (B, C) Dictyostelium mucoroides Bref. (B) Stalk and terminal ball of spores. (C) 

 Details of cells of stalk. (After Olive: Proc. Boston Soc. Natural History, 30:451-513.) 



