portrait

Description:

Portrait of Spirostomum ambiguum, a large heterotrich ciliate. S. ambiguum, is among the largest protists (this individual is approximately 975 microns long). The body shape is a vermiform cylinder, rounded anteriorly and truncate posteriorly. Colorless or brownish. Highly contractile. Uniform longitudinal somatic kineties. S. ambiguum is distinguished from the similar but smaller species, S. minus by the length of the peristome. The peristome occupies about 75% of the body length in S. ambiguum and about 50% of body length in S. minus. An adoral zone of membranelles runs along the left border of the peristome. Although many descriptions state that an undulating membrane is absent, protargol staining shows a poorly developed UM along the right margin of the peristome. The macronucleus is long and moniliform. The contractile vacuole is posterior with a long dorsal collecting canal. Often found in sapropelic habitats. From organically enriched standing freshwater near Boise, Idaho. Phase contrast illumination.