Aquacutia are four-legged animals that do not have amniotic eggs, are ectothermic, have a two-lobed brain in a flattened skull, a wide mouth, a short snout, upward-facing eyes, and fleshy bone based "fins", and generally spend part of their time on land. They are able to spend very brief periods out of water, but primarily use their "legs" to paw their way through the mud. Most do not have the adaptations to an entirely terrestrial existence. There are around 20,000 species.
The boney limbs are weight bearing, with basic wrist bones and simple fingers. A robust ribcage supports the body. Generally the front "fins" feature arm-like skeletal structures including a shoulder, elbow, and wrist. The neck is able to move independently of its body. The skull is flat, resembling a crocodile's; eyes on top of its headwith well developed jaws suitable for catching prey. The front limbs are marked by being able to bend backward at the elbow and the hind limbs by being able to bend forward at the knee. They have fully developed backbones with vertebral columns, limbs with digits, and skulls. The number of digits on hands and feet is standardized at six.
The most notable characteristics of their skull is the relative frontal and rear portion lengths. The front of the skull is lengthened, positioning the ocular orbits farther back on the skull. The lacrimal bone is not in contact with the frontal, but is separated from it by the prefrontal bone. Also, the skull was now free to rotate from side to side, independent of the spine, on the newly forming neck. The tabular bones (which formed the posterior corners of the skull-table) were separated from the respective left and right parietals by a sutural junction between the postparietals and supratemporals. Also at the rear of the skull, all bones dorsal to the cleithrum were lost.
Aquacutians are able to breathe by using a primitive air-sac lung, suplimented by gills in some species. Spiracles on the top of the head allow the operation of primitive lungs in some species, as well as the lung-gills. They feature internal nostrils, to separate the breathing and feeding passages. Their palatal and jaw structures feature labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. breathe by inhaling air into lungs, where oxygen is absorbed. They also breathe through the moist lining of the mouth and skin, opening the mouth and nostrils, depressing the hyoid apparatus to expand the oral cavity, closing its mouth and nostrils finally, and elevating the floor of the mouth to force air back into the lungs - in other words, gulp then swallow. Other special respiratory methods also exist.
Aquacutians have a three-chambered heart, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel-aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.
They have have a wide gaping jaws lined with small, sharp, conical teeth set in the jaws with a second row of teeth on the roof of the mouth. Teeth are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the animal's life. All chondrpiscians have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. The tongue is quite fleshy and attached to the front of the lower jaw.
The skin is covered with small "dentine" silica scales, arranged as a helical network surrounding the body. They have special sense organs in the skin, that formed a system for perception of water fluctuations.
They lay eggs in the water, where their larvae developed into mature animals. The larvae breathe with exterior gills. After hatching, they start to transform gradually into the adult's appearance. Typically, the animals then leave the water and become terrestrial adults, but there are exceptions to this general way of reproduction.