Volantipiscia are bipedal, warm-blooded, vertebrate animals that lay eggs. There are around 10,000 species. They inhabit a wide variety of ecosystems and range in size from 5 cm to 15 m. They are characterised by feather-like scales, elongated fully-toothed jaws, long tails, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, a lightweight but strong skeleton, a respiratory system that is highly adapted for flight, and forelimb-fins modified as wings. Most can fly.
The skeletal system is characterized by a stable pattern of fused cranial bones, a sclerotic ring of several small bones supporting the eye. The ocular orbits are large and separated by a bony septum. The inner ear contains large otoliths. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis. The ribs are flattened and the sternum is keeled for the attachment of flight muscles. In some species, the backbone over the shoulders is fused into a structure which serves to stiffen the torso during flight, and provide a stable support for the scapula.
The forelimbs are modified into wings. The wings are formed by membranes of skin and other tissues, strengthened by various types of closely spaced fibers called actinofibrillae. The membranes attach to the extremely long fourth finger of each limb and extend along the sides of the body. A bone connected to the wrist helps to support a membrane between the wrist and shoulder. In some species, the wing membrane is attached to the hindlimbs, at least in some species. However, there is considerable variation in the extent of the wing membranes, and different species have different wing designs. Many species also have webbed feet, for swimming. These also have an aerodynamic function.
The pulmonary system uses "flow-through ventilation," relying on a set of flexible air sacs that act like bellows to move air through the rigid boney lungs. These lungs do not have alveoli, but instead contain millions of tiny passages known as para-bronchi, connected at both ends by the dorsobronchi and that the airflow through the lung always travels in the same direction - posterior to anterior. By utilizing a unidirectional flow of air, they are able to extract a greater concentration of oxygen from inhaled air. The pulmonary cycle works as follows: air is inhaled into posterior air sacs, spent air is expelled into the anterior sacs and is expelled via anterior slits,. and the inhaled air moves from the posterior sacs into the lungs. This system allows flight at high altitudes and a higher metabolic rate. A portion of the air sac actually integrates with the skeleton, forming air pockets in otherwise dense bone. These skeletal air pockets lighten the bone structure, assisting in flight.