In the Gnathostomata, on the contrary, there is no buccal funnel, and the mouth, whether terminal or ventral in position, opens directly outwards. The hypophysis is usually carried inwards with the stomatodaeal invagination which in the embryo gives rise to the mouth, and is therefore from the first in relation with the ventral surface of the brain. Biting jaws (palato-quadrate and Meckelian cartilages), formed by the modification of an anterior and primitively gill-bearing visceral arch, are {148}invariably present. The olfactory organs are obviously paired, and they are distinct from the hypophysis. Paired limbs are present. As previously stated, the true Fishes form the second of the six "classes" into which the Craniata are divided. As compared with the higher Craniata, their distinctive characters may be concisely stated as follows:— Fresh water or marine Gnathostomata, which in their shape and in method of breathing are adapted for an aquatic life. Throughout life their respiratory organs are in the form of vascular processes (gills) derived from the walls of the branchial clefts, and supported by a series of branchial arches. The principal organ of locomotion is the powerful muscular tail; in addition, however, there are paired fins, pectoral and pelvic, corresponding to the fore- and hind-limbs of the terrestrial Craniata, and possessing a supporting cartilaginous or bony skeleton ("ichthyopterygium") which cannot readily be compared with the limb-skeleton of the latter. Fishes also possess a system of median fins, supported by a special skeleton of their own. An exoskeleton of dermal spines or denticles, scales or bony plates, is usually present. Except in one group, the Dipnoi, the heart has but one auricle, and receives only venous blood, which it forces, first, through the blood-vessels of the gills, and thence, as arterial blood, through the vessels of the body generally. An air-bladder is frequently present, and serves as a hydrostatic organ or float, but in a few cases it may act as a lung, and helps the gills in the work of respiration. The paired olfactory organs rarely communicate with the oral cavity by internal nostrils. Peculiar cutaneous sense-organs are disposed in linear tracts along the sides of the body (lateral line sensory organs), and on the head, and appear to be specially associated with a life in water. Fishes may be divided into the following "sub-classes," and these in turn may be subdivided into various "orders" and "sub-orders":— (i.) ELASMOBRANCHII; _e.g_. Sharks, Dog-Fishes, Skates, and Rays. (1) Pleuropterygii†; e.g. _Cladoselache_. (2) Ichthyotomi†; e.g. _Pleuracanthus_. (3) Acanthodei†; e.g. _Acanthodes_. (4) Plagiostomi. (_a_) Selachii; _e.g_. many extinct and all living Sharks and Dog-Fishes. (_b_) Batoidei; _e.g_. Skates and Rays. (5) Holocephali; e.g. _Chimaera_ and _Callorhynchus_.