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 vegetation on the walls of their ships.
Xyrillian males usually carry infants to term in a pouch on the upper chest. 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.