Portrait of Spirostomum teres, a large heterotrich ciliate. S. teres differs from the similar S. ambiguum and S. minus in that it is smaller (usually 150-600 microns long; this individual is 424 microns long) and has an ellipsoid macronucleus (seen here in midbody). Otherwise, the overall appearance is quite similar. The body is vermiform and cylindrical, rounded anteriorly and truncate posteriorly. The somatic kineties are longitudinal and uniform. There is an adoral zone of membranelles along the left margin of the peristome. Although often described as lacking an undulating membrane, an inconspicuous UM is demonstrated by protargol staining (not seen here). The peristome occupies about 50% of the body length. A posterior contractile vacuole has a long dorsal collecting canal. From organically enriched stagnant fresh water near Boise, Idaho. DIC optics.
