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Morphology

When you look at an amoeba in a freshly made wet mount under the light microscope, it has usually been disturbed by its transfer from the sample jar onto the slide. For some minutes or even for some hours, depending upon the species, it doesn’t really move and has a rounded irregular or even a star shaped form. It bulges, turns or stays immobile in this stationary form, also called non directed locomotion.

Disturbed amoebae may also float.  Usually they have a more or less star shaped body, with several pseudopodia radiating from the central body. These floating forms  are characteristic for some taxa.

Amoeba stationary form
Amoeba floating form
Stationary form of Amoeba proteus and floating form of Polychaos dubium

After a while an amoeba in a stationary or floating form starts to move and during this continuous directional locomotion it maintains this shape until it stops or changes direction. During this movement the shape usually undergoes some minor changes, but this stable locomotive form of an actively, continuously moving amoeba is very characteristic and can be used as a taxonomical instrument.

Amoeba schematic
Basic morphological features of a naked lobose amoeba u uroid, d dorsal fold, w wing, n nucleus, v vacuole, g granuloplasm, h hyaloplasm.

The main mass of the cytoplasm, called granuloplasm, contains optically visible granules, crystals and other inclusions. During locomotion the leading edge of the cell shows an area of transparent hyaloplasm, which looks as clear as water. This anteriorly situated hyaloplasm forms a hyaline crescent and is usually called the hyaline cap.

While the hyaline cap is the anterior part, the uroid is the posterior part of a locomotive amoeba. In many amoebae species, the uroid has a distinct characteristic shape. These uroid can be hairy, morulate or rounded. Some species like Rhizamoeba  show trailing filaments, which originate from sticky contact with the substrate.

Uroid amoebae

Different shapes of the uroid: a with villi, b with papillae, c pseudovilli or small papillae, d remnants of a retracted pseudopodium, e trailing filaments or collopodium.

 

Pseudopodia

Amoebae without any pseudopodia are usually monopodial.  The cell is cylindrical or sub-cylindrical and behaves as one large pseudopodium. It may put out a lateral pseudopodium when changing direction. The term monotactic or limax form is applied to amoebae which are consistently monopodial.

Cylindrical or sub-cylindrical amoebae which form several distinct pseudopodia are called polypodial. These cells can be hand-shaped (palmate), with the pseudopodia as “fingers”. Polypodial amoebae can adopt a monopodial form when active, in fast movement. Distinction between monopodial and polypodial forms can only be applied to amoebae advancing in a constant direction.

monopodial amoeba
Amoeba polypodial
Monotactic =monopodial or limax form and polypodial form

Most small and medium sized amoebae don’t form distinct pseudopodia, but smaller hyaline projections which doesn’t have any locomotive function and are called subpseudopodia. They are completely hyaline and can have different shapes.  Mayorella species have typical conical subpseudopodia.

Some amoebae have folds on the dorsal surface of the locomotive form. Some large amoebae may have folds or  wings on their lateral surface.

Amoeba pseudopodia
Amoeba subpseudopodia
Pseudopodia (arrows) and subpseudopodia (arrows)
crystals
Crystal forms: a bi-pyramidal shape, b blunt pyramidal shape, c double crystal, d-e elongated forms as found in Polychaos fasciculatum, f strongly flattened crystal with two aberrant forms, g-h as observed with Polychaos dubiumi-j observed crystals with Saccamoeba spec.,  k-l paired crystals, characteristic for some larger Mayorella species, croissant-shaped crystal, characteristic for Saccamoeba wellneri.
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