30 MYCETOZOA AND BELATED OKGANISMS 



In this genus the plasmodium heaps itself up, at first, into a conical 

 structure in the basal part of which cellulose walls are produced, sepa- 

 rating it into the more or less hexagonal cells of the stalk. In the upper, 

 head-like part it breaks into separate, rounded cells, which also develop 

 cellulose walls and become the spores. These are embedded in a slimy 

 drop. Sexual reproduction and the eventual formation of a true Plas- 

 modium have not been reported but possibly occur in the other genera of 

 the order. The myxamoebae of Copromyxa and Guttulina lack con- 

 spicuous pseudopodia. The fruiting bodies of the former are sessile, of 

 the latter short-stalked. Dichjostelium, Acrasis, and Polysphondylium pro- 

 duce myxamoebae with well-developed pseudopodia. All produce stalked 

 fruiting bodies (branched in the last named) with ovoid or spherical 

 heads of spores. Olive (1902) gives a monograph of this order, describing 

 all known species. (Fig. 5.) 



The Acrasiales differ from the Myxogastrales in the absence of a 

 flagellate swarm-spore stage preceding the formation of the myxamoebae, 

 and in the fact that the naked cells remain distinct for a long time in the 

 pseudoplasmodium, forming a true plasmodium only a short time before 

 the fruiting body is produced, if at all. Furthermore no capillitium is 

 formed nor a peridium, although the slime that lies between and around 

 the spores may represent these structures. 



The Labyrinthulales include parasitic forms attacking algae and other 

 aquatic plants in both marine and fresh-water habitats, and possibly 

 one genus of dung-inhabiting saprophytes. There are several species in 

 the genus Lahyrinthula, of which L. macrocystis Cien. was shown by 

 Renn (1935) to be the cause of the destruction of most of the eel grass 

 (Zostera marina L.) of the shallow waters of the North Atlantic Ocean 

 in recent years, both on European and on North American shores. This 

 genus, according to Young (1943) consists of naked, spindle-shaped cells 

 with a single nucleus and a vacuole which may contract at intervals. 

 These cells divide transversely or obliquely and form a rope-hke mass. 

 Those near the tips of this mass send out, apically, thin colorless fila- 

 ments, one to each cell, eight to ten times its length. These filaments fuse 

 to form a net-like track along which the cells glide— externally to the 

 track, not in it as in a tube as Valkanov (1929) claimed. Eventually the 

 cells may assemble into a pseudoplasmodial mass embedded in a gelati- 

 nous matrix. Some of these cells may encyst or encystment of individual 

 cells may occur without the formation of a pseudoplasmodium. The 

 encysted cells apparently give rise to four naked cells which penetrate 

 through the cell walls of the host plant and in their turn become spindle- 

 shaped and start the development of new "net-plasmodia." Dangeard 

 (1932) suggested that possibly there is a sexual stage somewhere in the 

 life cycle but did not demonstrate where it occurred. The naked cells 



