Xyrillians had an unusual physiology. They did not have a metabolism based on water or breathe an oxygen-based atmosphere. In fact, their high-pressure atmosphere was hallucinogenic to most species, and as a result, most other humanoids needed to be weaned onto and off their atmosphere to avoid undesired effects. When traveling, they grew edible vegetation on the walls of their ships, and had eel-like creatures swimming in tanks.
Xyrillians also had dermal plating which they found necessary for detecting each other's moods. Apart from this plating and the fact they were slightly shorter than the typical Human, there wasn't a lot of obvious physical characteristics that distinguished Xyrillians from Humans.
Xyrillian males usually carried infants to term in a pouch on the upper chest. They served only as hosts, and the offspring only contained genetic material from the mother. Impregnation could occur with only minor physical contact. Humans and Xyrillians were 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 occurred with Xyrillian males as well or if it was a weird reaction of inter-species reproduction with Humans.
The Xyrillians had their own language. Their translation of "We are complying with your request. We ask you not to harm us," was "Kaja lakala royj kaiti."