The siphonophores are free-swimming or floating colonies. Unlike species of the two other major orders of Hydrozoans, the medusae are never released from the stem. The colony is often held together by a long thread where different kinds of specialized animals are attached. Some digest food, while others use stinging tentacles to catch their prey. They hand the food over to the "stomachs" that swallow and digest the food, and thus provide nutrition for the entire colony. Some even emit light.