to the surface at frequent intervals to take in a quantity of air, and if they be kept beneath the surface by means of a gauze net, they perish from suffocation. The special contrivances consist of additional respiratory organs, lodged in cavities either adjoining the gill-cavity or communicating with the ventral side of the œsophagus, or of the air-bladder which enters upon respiratory functions (Dipnoi, Lepidosteus, Amia). The water used by fishes for respiration is received by the mouth, and by an action similar to that of swallowing driven to the gills, and expelled by the gill-openings, of which there may be one or several on each side behind the head; rarely one only in the median line of the ventral surface. [Illustration: Fig. 58.--Fore-part of the body of an embryon of Carcharias, showing the branchial filaments (natural size).] The _gills_ or _branchiæ_ consist essentially of folds of the mucous membrane of the gill-cavity (_laminæ branchiales_), in which the capillary vessels are distributed. In all fishes the gills are lodged in a cavity, but during the embryonic stage the Chondropterygians have the gill-laminæ prolonged into long filaments projecting beyond the gill-cavity (Fig. 58), and in a few young Ganoids external gills are superadded to the internal. In _Branchiostoma_ the dilated pharynx is perforated by numerous clefts, supported by cartilaginous rods (Fig. 29, _h_). The water passes between these clefts into the peritoneal cavity, and makes its exit by the porus abdominalis situated considerably in advance of the vent. The water is propelled by cilia. In the _Cyclostomes_ the gills of each side are lodged in a series of six or more antero-posteriorly compressed sacs, separated from each other by intervening septa. Each sac communicates by an inner duct with the œsophagus, the water being expelled by an outer duct. In _Bdellostoma_ each outer duct has a separate opening, but in _Myxine_ all the outer ducts pass outwards by one common gill-opening on each side. In the Lampreys the ducts are short, the outer ones having separate openings (Fig. 2, p. 39). The inner ducts lead into a single diverticulum or bronchus, blind behind, situated below the œsophagus, and communicating in front with the pharynx, where it is provided with two valves by which the regurgitation of the water into the buccal cavity is prevented.