Xyrillians have an unusual physiology. They do not have a metabolism based on water or breathe an oxygen-based atmosphere. In fact, their high-pressure atmosphere is hallucinogenic to most species, and as a result, most other humanoids need to be weaned onto and off their atmosphere to avoid undesired effects. When traveling, they grow edible vegetation on the walls of their ships, and had eel-like creatures swimming in tanks, although it is unknown if they are food, decoration, or served any other purpose.
Xyrillian males usually carry infants to term in a pouch on the upper chest. They serve only as hosts, and the offspring only contain genetic material from the mother. Impregnation can occur with only minor physical contact. Humans and Xyrillians are genetically compatible, as Commander Tucker discovered when he was impregnated by a Xyrillian female. He also developed a nipple on his wrist, but it is unknown if this occurs with Xyrillian males as well or if it is a weird reaction of inter-species reproduction with Humans.