Gas exchange may occur through a single open spiracle, or the coordination of several spiracles. The anterior spiracle and anterior angle of antennal bases is prominent (most). This fish inhales through its spiracle (blowhole), an anatomical feature present in early tetrapods. Usually no spiracle or true operculum is present, though many species have operculum-like structures. In shallow water, "Polypterus" inhales primarily through its spiracle (blowhole). The inner part of the spiracle, still connected to the pharynx, forms the eustachian tube. The larva is amphipneustic (having only the anterior and posterior pairs of spiracle). It possessed large otic (spiracular) notches, probably accommodating a spiracle rather than a tympanic membrane. There is a tentacle-like structure on the inner posterior margin of each spiracle. The spiracle was situated in an otic notch behind each eye. A smaller opening, the spiracle, lies in the back of the first gill slit. The spiracle is thought to be homologous to the ear opening in higher vertebrates. They can differ in thoracic coloring, basicosta coloring, and spiracle coloring. A spiracle occurs beneath each eye which is used in respiration. They have elongated, oval-shaped eyes with a spiracle located directly behind on each side.