There are over 300 species of Macrouridae of which 90 per cent or more live close to the continental slopes between depths of some 200 and 2000 m. In general organization, particularly that involving parts of the olfactory, gustatory and lateralis systems, the macrourids are closely similar to gadid and related groups of the order Anacanthini. There are correlations between the unpaired fin pattern, jaw position and gill raker structure in the two main subfamilies, Macrourinae and Bathygadinae. These correlations are evidently linked to ways of swimming and feeding (pp. 304–305). In size, macrourid eyes are inversely related to the depths of their living spaces. Slope-dwelling species have the largest and most elaborate eyes, while abyssal species have the smallest eyes. Judged by the pattern of nerves belonging to the ramus lateralis accessorius system, macrourids have an extensive gustatory surface over the body and fins. There is a highly developed lateralis system on the head, where very wide canals contain large neuromasts capped with prominent cupulae. The long axes of the latter are set across the canals. The functional significance of such orientation — and of other features of the lateralis system — is discussed. Whether slope or abyssal species, nearly all macrourids have a well developed swimbladder. The deeper their living space, the longer are the capillaries in the retia mirabilia. This is understandable considering the main functions of the retia : to keep gases within the swimbladder and to generate requisite gas tensions for secretion by the gas gland. Both functions become harder with increasing depth, but the efficiency of both directly depends— inter alia—on the length of the retial capillaries. The males of most macrourine species have a pair of large drumming muscles attached to the forward part of the swimbladder. These muscles are doubtless used to produce sounds. Abyssal macrourines and bathygadines do not have such muscles. These facts are considered in the light of present ideas concerning the very limited range in which fishes can home to sources of sound and water displacements. Unlike most benthic fishes of the deep-sea floor, numerous macrourids have a light organ which contains luminous bacteria and is housed along the mid-line of the underparts. The possible biological roles of luminescence are discussed. Limited evidence suggests that macrourids lay bouyant eggs close to the bottom. The youngest larvae are found in depths around 200 meters : older ones occur at deeper levels. The life-history pattern is discussed in relation to the problem facing slope species : the maintenance of populations that live over a relatively narrow strip of the deep-sea floor. The few bathypelagic macrourids seem to have wide distributions. The slope dwellers, and even most abyssal species, range much less widely. Most often, the former are confined to one side of an ocean. Though most species are found in tropical regions, the diversity of the group does not follow the rather regular pole-to-equator increase shown by bathypelagic fishes. Certain oceanic regions, such as the Sulu Sea, the Gulf of Mexico and the Caribbean, and the Hawaiian Islands, have been centers for extensive speciation.