Physolisti - fish in which the swim bladder is closed (perch, stickleback, pikeperch, etc.). Filling the swim bladder with gas is accomplished by a well-developed red body (gas gland), located in the anterior part of the swim bladder. Absorption of gases is accomplished by the oval, which is a branching of a huge number of capillaries that are located in the posterior part of the swim bladder. When moving the fish to depth, the pressure increases, the saturation of blood gases accordingly changes, gas from the swim bladder enters the blood, and when the fish rises to the surface layers of water, the pressure decreases and excess gas from the blood is released into the swim bladder.

Physolisti - all bony fishes having closed swim bladder without ductus pneumaticus and contrasted with Physostomi, y which swim bladder by means of d. pneumaticus opens to intestine. All Physolisti in embryonic state are Physostomi.

Closed-bellied fish include: perch, cod, cod, mullet, stickleback, mackerel and others.