Genus Arachnula Cienkowski, 1876
Diagnosis: Cell colorless, forming several strands, long flat pseudopodia, which in turn branch into numerous granulopodia, usually at right angles. One of the most common forms is one long strand, which is spreading on both ends into a thin veil. Such veils may appear at any point of the cell body. Smaller veils can appear along the granulopodia where they travel up and down. Cytoplasm is very liquid with strongly refractive granules and a few dispersed contractile vacuoles. It shows a very characteristic energetic movement, giving the impression as if the whole cell is boiling. All granules show a fast bidirectional movement in every part of the cell. The amoeba moves trembling and staggering restlessly around (hence the name impatiens!) scanning its environment with its granulopodia, with movements resembling those of a spider (hence the name Arachnula.). At any part of the body granulopodia may shoot up, often at right angles and sometimes travelling along the pseudopodium where they originated. Granulopodia not only branch but also anastomose. No other known amoeboid shows such a fast energetic movement. Nuclei extremely difficult to observe. Digestion cysts observed.
Type species: Arachnula impatiens Cienkowski, 1876
Ecology: Freshwater and brackish water.
Remarks: Arachnula differs from Leptophrys and Vampyrella merely by the presence of granulopodia. Vampyrellids also have granules, but these are restricted to the base of the filopodia or to the so called pin needles and never run along the whole filopodium.
There are several species within this genus which have never been described and probably there are several genera.
yellow arrow: anastomosing granulopodia;
red arrows: small veils;
blue arrow: traveling pseudopodium;
white arrows: direction of ‘shooting’ granulopodia.