A Testy Blob - Arcella sp.

Searching the detritus sediment at the bottom of the jar of pond water you find amber colored structures shaped like the cap of a mushroom, with a round opening where the mushroom's stalk would be. The shell is transparent and inside can be seen an amoeboid cell attached to the inner surface by fine threads of cytoplasm. Suddenly a pseudopod emerges from the central hole. You have found Arcella, a common type of shelled amoeba.

Under the coverglass oxygen is decreasing and Arcella is feeling stress, but not to worry, natural selection has equipped it with a means of escape. The little amoeba can secrete a bubble of gas into its shell, and if it were not restrained by the coverglass, it would rise into the water column and drift away--presumably to a less hostile environment. This escape strategy is just one of the many survival adaptations seen in amoeboid protists.