Ramicristates

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The Ramicristates are a diverse group of amoeboid protists, comprising most forms with lobose and filose pseudopods, which appear to have a monophyletic origin. Common characteristics are the presence of branching, tubular cristae within the mitochondria - hence the name - and open mitosis without centrioles. The following groups are known to be ramicristates:

The best-known member is Amoeba proteus, found on decaying vegetation in freshwater streams and ponds. A number of lobse pseudopods extend from the front of the creature, fusing around food to trap it in internal vacuoles and spreading to pull the creature along the substrate. The outer layer of cytoplasm, the ectoplasm, streams back from the pseudopods so acts like a tank-tread. A single contractile vacuole expels water to maintain te osmotic balance.

This group includes all the lobose and filose amoebae that secrete shells. These include relatively simple shells like in Arcella, shells covered in sand grains and other collected detritus, as in Difflugia, and shells covered in internally formed scales, as in Euglypha. Some of the shells are fairly structured, though not nearly as ornate as those of other amoebae.

The Myxomycetes, called the true slime molds, are of special note. These grow to form a multinucleate plasmodium which may reach several square feet in size. Eventually this produces one or more fruting bodies, releasing spores that develop into biflagellate gametes, which then fuse to form new plasmodia. The life-cycles of the closely related Protostelids are similar, except that the amoebae remain mononucleate and form fruiting bodies directly.

The Dictyosteliids also form fruiting bodies, but these are formed by aggregates of cells rather than by individual amoebae or plasmodia, and supported by a stalk composed of dead amoebae. The aggregates (called slugs), which are formed by amoebae coming together under unfavorable conditions, have a distinct anterior and posterior end, with different parts ending up as different parts of the fruiting body, and thus are of interest to those studying cellular communication and differentiation. Similar life-cycles are found in other unrelated groups, such as the Acrasids and Myxobacteria, collectively referred to as cellular slime molds.