These large protozoans live in the intestines of invertebrates, fish, amphibians and mammals. Trophozoites feed on particulate material in the intestine and reproduce by fission. Occasionally, they may secrete proteolytic enzymes that cause an ulcer in the host's intestine and the trophozoites can invade and reproduce within the ulcer. Secondary infection may follow. As fecal material forms in the host's intestine and begins to dehydrate, trophozoites encyst. No multiplication occurs within the cyst. If kept moist, cysts can remain infective for 1-2 weeks, and infect a new host when ingested.