The amoebas Sarcodina

The amoebas are characterised by the possession of pseudopodia (='false feet'), temporary projections from the cell into which cytoplasm flows until the organism has moved forward (Figure 9.14). This means that amoebas are continually changing their body shape and the position of their internal organelles. Pseudopodia are also used to capture and engulf food, forming a vacuole around it. Once again, digestive enzymes are released from lysosomes and the food particle dissolved. Once absorption of soluble nutrients has taken place, undigested waste is ejected by the vacuole moving back to the cell surface.

Reproduction in the amoebas is by simple binary fission. Most amoebas are free-living, in aquatic environments; their mode of movement and feeding makes them well adapted to life on the bottom of ponds and lakes, where there is a good supply of prey organisms and suspended organic matter. Also included in the group, however, are some important parasites, including Entamoeba histolytica, which causes amoebic dysentery

Pseudopodium

Contractile vacuole

Pseudopodium

Contractile vacuole

Endoplasm

Ectoplasm

Food vacuole

Nucleus

Endoplasm

Ectoplasm

Food vacuole

Nucleus

Figure 9.14 Amoeboid structure. The internal features of an amoeba change position as the cell changes its shape by cytoplasmic streaming in humans. Ingested in faecally contaminated water, it is responsible for some 50 000 -100 000 deaths world-wide every year. Unlike its free-living relatives, Entamoeba is unable to reproduce outside of its host.

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