In the _Teleostei_ the gills with their supporting branchial arches lie in one undivided cavity; more or less wide clefts between the arches lead from the pharynx to the gills, and a more or less wide opening gives exit to the water after it has washed the gills. The interbranchial clefts have sometimes nearly the same extent as the branchial arches; sometimes they are reduced to small openings, the integuments stretching from one arch to the other. Sometimes there is no cleft behind the fourth arch, in which case this arch has only an uniserial gill developed. The gill-opening likewise varies much in its extent, and when reduced to a foramen may be situated at any place of the posterior boundary of the head. In the _Symbranchidæ_ the gill-openings coalesce into a single narrow slit in the median line of the isthmus. In the majority of Teleosteans the integument of the concave side of the branchial arches develops a series of horny protuberances of various form, the so-called _gill-rakers_. They are destined to catch any solid corpuscles or substances which would be carried into the gill-cavity with the water. In some fishes they are setiform, and form a complete sieve, whilst in others they are merely rough tubercles, the action of which must be very incomplete if they have any function at all. Most Teleosteans possess four complete gills, but frequently the fourth arch is provided with an uniserial gill only, as mentioned above, or even entirely gill-less. The most imperfect gills are found in _Malthe_, which has two and a half gills only, and in _Amphipnous cuchia_, in which one small gill is fixed to the second arch.