These combine to form parabranchial gill chambers, with the exception of the first visceral arch, which becomes the spiracle. The closing plates rupture in the embryo to establish the communication between the gill chamber and the surrounding medium. The general structure of a mature gill is shown in Figure in your text. Each gill bar is composed of several parts. Gill rakers are bony parts on the pharyngeal margin of the gill, and function in preventing food particles from entering the gill chambers.
Gill filaments are the feather-like projections of the gills across which diffusion of gases occurs.
Gill rays are found within the gill filaments, and provide support for the gill. Lamellae are oriented parallel to the flow of water through the gills, to maximize efficiency of diffusion. In addition, the blood flow through the gills opposes the flow of water through the lamellae. Such flow is called countercurrent flow, and maximizes the efficiency of diffusion. The Agnathans have external and internal pores rather than gill slits, and water is drawn into the gill chambers through the mouth and then passed over the gills.