The tribble has a thick, muscular skin, which is used to move not unlike a small, living tank. A tribble can also start itself rolling, and can even climb reasonably well by pulling itself up with folds of its skin.
Directly underneath the skin is a sensory layer, with which the tribble feels, hears, smells, and detects waves much like sonar.
The mouth opening, located at the underside of the tribble, can expand greatly to take in large food particles. It ends in a 'multifunctional tube', which extends through the whole tribble and serves as an oesophagus, trachea, and birth canal.
The stomach is a large digestive sack, in which almost anything can be completely digested and absorbed. In cast a tribble swallows something that can not be digested, it can be excreted through the multifunctional tube.
The liver, gall bladder, and the two digestive glands which are unique to the tribble and play a large role in its enormous digestive capabilities, empty right into the stomach.
Circulation takes place by the pumping of a blood-like substance through ducts to the walls of the various organs. Within the organ walls it flows freely, and the oxygen is released in an exchange involving a substance found only in tribbles, called Trey's fluid, after the Earth biologist who discovered it. Only in the lungs is there a similarity with the alveoli found in most higher oxygen breathing life forms. Its hart, though muscular, is small, as are all organs apart from stomach and uterus, taking a little of the available space as possible.
Half the tribble is uterus. Because of its relatively large size, the uterus is divided in smaller cavities. This facilitates transport of oxygen and nutrients to the embryos.
Ripened eggs and sperm are released immediately into the uterus. In case of heterosexual fertilization, eggs and sperm of both tribbles are mixed in the process. Thus, some eggs will be fertilized by the other tribble's sperm, and some by its own. Still, a larger percentage of offspring will usually be the result of the crossing.
A tribble has no skeleton as we know it, but the skin is so thick and tough that it acts as an exoskeleton.