Publisher Summary Fish live in an environment that is considerably different from that encountered by terrestrial vertebrates. Compared with air, water is 800 times more dense, 60 times more viscous, has 1/30 the capacity to hold oxygen and oxygen diffuses at 1/8000 the rate. Thus this respiratory medium is more abrasive and oxygen depleted than air. An aquatic environment also fosters the untoward transfer of water, salts, acid or base, as well as toxic substances. These necessitate corrective measures, many of which also occur at the gill. The fish gill has become a multifunctional organ designed to deal with the vagaries of an aquatic medium. The anatomy of gill tissue and its vasculature is as diverse and specialized as the functions it performs and the gill may well be the most highly differentiated of any vertebrate organ. Because all of these activities are predicated upon cellular contact with environmental water or blood, the hallmark of gill tissue is an extensive elaboration of both epithelial and vascular surfaces.