Portrait of Gonyostomum semen, a raphidomonad flagellate. Two flagella, the anterior beating rapidly, the posterior trailing (not seen in this image). Many small bright green discoid chloroplasts. Stigma absent. Contractile vacuole located anteriorly between large central nucleus and flagellar insertion. Prominent extrusomes are said to eject mucilaginous threads up to 100 microns long. From freshwater pond near Boise, Idaho. Phase contrast.