Aspects of functional anatomy with respect to water and blood flow through the gills of Squalus acanthias and one specimen of Mustelus mustelus are described. A reconstruction of an entire filament has been made. The cavernous body in the proximal segment of the filament is relatively small, contains a small amount of elastic tissue, and has no connections with the underlying afferent filament artery, except at its junction to the middle segment. In the middle segment of the filament, the wall and columns of the cavernous body are well supplied with elastic tissue. The size of the cavernous body increases gradually distally. The underlying afferent filament artery opens into it with numerous pores and eventually becomes incorporated in the cavernous body. The distal segment of the filament is characterized by the rapid increase in size of the cavernous body. Toward the free edge of the filament it almost reaches the efferent filament artery; toward the diaphragm it forms an anastomosing bridge with the neighboring filaments. Wall and columns in the distal segment of the cavernous body are richly supplied with elastic tissue. The afferent filament artery is no longer discernible. In living specimens the tips of opposite filaments are in close apposition during the whole respiratory cycle. Application of artificial ventral aortic pressure to freshly killed specimens evoked the same reaction in the filament tips. The cavernous bodies provide the hydrostatic skeleton, necessary to keep the gill sieve closed. Lamellae are equipped with finger-like appendages to facilitate adherence to the opposite filament tips. Lamellar appendages also reduce the dead space between lamellae of neighboring filaments. The whole of cavernous bodies may also serve as an elastic blood reservoir with a function similar to that of the bulbus arteriosus in teleost fish. Of all suitable places, only the lamellar streambed was clear and convincing in its response to perfusion with adrenalin or acetylcholine. Structural details of the nonrespiratory circulation within the gills are described.