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Stentor roeseli
Scientific classification
Domain: Eukarya
Kingdom: Chromalveolata
Superphylum: Alveolata
Phylum: Ciliophora
Class: Heterotrichea
Order: Heterotrichida
Family: Stentoridae
Genus: Stentor
Lorenz Oken, 1815

Stentor is a genus of filter-feeding ciliates. They are usually horn-shaped, and reaching lengths of 2 millimeters, they are among the biggest known unicellular organisms.

They are a type of protist ciliate in the heterotrich class. The old-fashoned name for them was trumpet animalcules.

The body is horn-shaped, with a ring of prominent cilia around the anterior "bell" that sweeps in food and helps swimming. Some reach several millimeters in length, making them among the largest single celled organisms. As in many freshwater protozoans, Stentor has a contractile vacuole. Because the concentration of salt inside the cell and in the surrounding freshwater is different, Stentor must store water that enters it by osmosis and then discharge it from the vacuole. They can regenerate, and small fragments can grow into full organisms. Each cell has one (often elongated) macronucleus and several micronuclei. Having two types of nuclei is charcteristic of ciliates.