The centra of the vertebrae have no epiphyses. The skull may be (_a_) incomplete and membranous, or (_b_) more or less cartilaginous, or (_c_) bony. Membrane bones are not included in the cranial walls, and there are large unossified tracts in the skull. When membrane bones are developed in connection with the skull, a large parasphenoid occurs. The basisphenoid is always small or absent. The skull may be immovably fixed to the vertebral column, or may articulate with it by a single or double occipital condyle. When the occipital condyle is double, it is formed by the exoccipitals, and the basi-occipital is small or unossified. The mandible may be (_a_) cartilaginous, (_b_) partially ossified, or (_c_) membrane bones may be developed in connection with it,--if so, there is usually more than one membrane bone developed in connection with each half. There are at least four pairs of branchial arches present during development. The sternum, if present, is not costal in origin. CLASS I. PISCES. The exoskeleton is in the form of scales, which may be entirely mesoblastic or dermal in origin (e.g. _cycloid_ and _ctenoid_ scales), or may be formed of both mesoblast and epiblast (e.g. _placoid_ and _ganoid_ scales). Large bony plates may be derived from both these types of scale. In general fish with a greatly developed dermal armour have the endoskeleton poorly developed; and the converse also holds good. The integument of the dorsal and ventral surfaces is commonly prolonged into longitudinal unpaired fins, supported by an internal skeleton. These fins are distinguished according to their position as dorsal, caudal and anal fins. The dorsal and anal fins are used chiefly as directing organs, the caudal fin is however a most important organ of propulsion. Three types of tail are found in fishes, viz.:-- 1. The =diphycercal=, in which the axis is straight and the tail is one-bladed and symmetrical, an equal proportion of radiale[29] being attached to the upper and lower surfaces of the axis. 2. The =heterocercal=, in which the tail is asymmetrical and the axis is bent upwards, the proportion of radiale or of fin-rays attached to its upper surface being much smaller than that attached to its lower surface. 3. The =homocercal=, in which the tail though externally symmetrical, so far resembling the diphycercal type, is internally really heterocercal, the great majority of the radiale or of the fin-rays being attached to the lower surface of the axis. The cranium in the simplest cases (e.g. Selachii) forms a cartilaginous box enclosing the brain and sense organs; in bony fishes it is greatly complicated. When palatine or pterygoid bones are present they are formed by the ossification of cartilage; in Sauropsida and Mammalia they are laid down as membrane bones. There is no tympanic cavity or auditory ossicle in relation to the ear. There are two principal types of suspensorium by means of which the jaws are attached to the cranium:--