Placopiscians, or "armoured fish", are a jawed fish-like animals with their head and thorax covered by articulated armoured plates. They are characterized by their bony skeleton instead of cartilage, and by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by fissure.
Placopiscians extract oxygen from seawater as it passes over their gills, pumping water over their gills to ensure a constant supply of oxygenated water. They have an operculum, which helps them breathe without having to swim.
The respiration and circulation process begins when deoxygenated blood travels to the two-chambered heart. Here the blood is pumped to the shark's gills via the ventral aorta artery where it branches off into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the ventricle of the heart and the cycle repeats. Placopiscians have a primitive gas-filled swim bladder
The blood and other tissue of placopiscians is isotonic to their marine environments, allowing them to be in osmotic balance with the seawater. This adaptation prevents most species from surviving in fresh water, and they are therefore confined to a marine environment.
Placopiscians do not have teeth, but rather multiple rows of sharp silica reinforced bony plates along the edges of their upper and lower jaws.
Placopiscian fins vary considerably between species and are adapted to the lifestyle. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes have evolved in adaptation to different environments.
Placopiscians have a complex dermal corset made of flexible collagenous fibres and large silica scales and plates.
Most species are mildly homeothermic - able to maintain their body temperature above the surrounding water temperature. This is possible because of the presence of the suprahepatic rete, a counter current exchange mechanism that reduces the loss of body heat. Muscular contraction also generates a mild amount of body heat. However, this differs significantly from true homeothermy, as found in mammals and birds, in which heat is generated, maintained, and regulated by metabolic activity.
Sex is easily determined. Males have modified pelvic fins which have become a pair of claspers. The name is somewhat misleading as they are not used to hold on to the female, but fulfill the role of the mammalian penis. Most chondrpiscians lay eggs. In most of these species, the developing embryo is protected by an egg case with the consistency of leather. Sometimes these cases are corkscrewed into crevices for protection.