MORPHOLOGY AND TAXONOMY OF THE SARC0D1NA 445 



plasm. Encystment, with cellulose cyst walls, is common. Nuclei 

 are multiple as a rule; reproduction by plasmotomy or by division 

 into uninucleate amebae; flagellated swarmers unknown. Accepted 

 genera: Nuclearia Cienkowsky, Arachnula Cienkowsky and Vam- 

 pyrella Cienkowsky (Fig. 183). 



Sub-class II. MYCETOZOA de Bary. 



The Mycetozoa were formerly regarded as low types of fungi and 

 under the name of Myxomycetes or "slime molds" were included 

 among the lower plants. The investigations of de Bary, however, 

 revealed the rhizopod affinities, and the relationship with other 

 Sarcodina is now clearly recognized. There is little doubt, how- 

 ever, that Mycetozoa are borderline organisms and their semi-ter- 

 restrial habitat leads to modifications and adaptations not met with 

 elsewhere. Many of them are highly complex both as to organiza- 

 tion and as to life history and by no stretch of the imagination can 

 they be regarded as simple organisms. 



A general idea of the essential characteristics of the Mycetozoa 

 may be gained by following through a typical life history beginning 

 with a recently germinated "spore." This is a small uninucleate 

 ameboid organism known as a "myxameba;" it is active, throwing 

 out pseudopodia and moving energetically about the field. It has 

 a contractile vacuole, and takes in solid food which is digested in a 

 gastric vacuole, or it may live upon dissolved proteins from decom- 

 posing organic matter. It may also reproduce by division while 

 in this ameboid condition. 



The naked ameboid condition is usually temporary; sooner or 

 later the "myxameba" turns into a " myxoflagellate " by the devel- 

 opment of a flagellum. The contractile vacuole is retained and the 

 body, usually ellipsoidal, is highly metabolic and may even give 

 rise to pseudopodia, particularly at the posterior end where the 

 pseudopodia aid in the ingestion of solid food in the form of bacteria, 

 small Protozoa or bits of organic detritus; saprozoic nutrition, how- 

 ever, is also common. Like the "myxamebae" the "myxoflagel- 

 lates" may reproduce by longitudinal division, in which case the 

 centrioles of the mitotic figure become the basal bodies of the 

 flagella. Myxoflagellates are apparently rather sensitive and show 

 a ready tendency to encyst. Such "microcysts" are temporary 

 and the excysted organism again passes through myxameba and 

 myxoflagellate stages. 



According to later investigations of Jahn these myxoflagellates 

 ultimately become gametes; the last division, prior to gamete 

 formation is a chromosome-reducing division, and the haploid 

 gametes fuse to form diploid zygotes. In Physarum didymoides the 

 gametes have 8, the zygotes 16 chromosomes. 



