Vary in size from 25 to 35 µ in diameter. Have a distinct, dense, hyaline covering. The pseudo-pods are large, finger-shaped refractile, and few in number. The granular cytoplasm contains bacteria, starch granules, and red and white blood-cells in various stages of disintegration. Vacuoles may be present. This protozoan is now conceded to be the cause of the acute and chronic dysenteries that occur in tropical and semi-tropical regions.

It is able to penetrate the mucous coat of the intestine and undergo multiplication in the submucosa. The blood-supply is shut off and the tissue breaks down, giving rise to the characteristic undermined ulcers of tropical dysentery.

The intestine is its usual habitat, but it may be found in solitary abscess of the liver that so commonly follows dysentery.

The E. tetragena is probably identical with the E. histolytica. The E. coli is very frequently found in the intestine under normal conditions but is probably non-pathogenic.