THE CUTANEOUS SENSE-ORGANS.—These organs, the most remarkable and certainly the most characteristic of the sense-organs of Cyclostomes and Fishes, are bud-like groups of epidermic cells in relation with the ends of sensory nerve fibres. Each consists of a central core of sensory cells, provided with terminal cuticular sensory hairs, and surrounded by a zone of supporting and mucus-secreting cells which leave the hairs exposed at the apex of the bud. Two kinds of these organs can be distinguished, which differ in their innervation and in their position in the skin. Of the two, the so-called _end-buds_ are the more primitive. They occupy a superficial position in the epidermis, and their sense-cells are as long as the supporting cells. They are present in Cyclostomes and Elasmobranchs, and especially in Teleosts, where they are irregularly distributed over the surface of the body, on the fins, lips, and barbels, and also in the epithelium of the mouth and pharynx. In the Dipnoi they are limited to the oral cavity, and in the higher Craniates they become taste-buds.[450] Their somatic sensory nerves[451] are derived from the vii., ix., and x. cranial nerves, and the lateralis accessorius. In the second type, usually called "_nerve-eminences_," the sensory cells are shorter than the supporting cells, and they are always innervated by the lateralis system. When first developed in the embryo they are quite superficial, like end-buds, but later the epidermis in which they lie sinks inwards so as to line a series of pits, closed sacs, tubes, open grooves, or closed canals. _Pit-organs_, so abundant on the head and trunk of Teleosts (Fig. 220), are simple epidermic pits with insunken nerve-eminences, disposed in groups or in {384}lines (accessory lateral lines) or irregularly distributed. The "_Spalt-papillen_" of Elasmobranchs are pit-organs in which the orifice of the pit is reduced to a slit. The more deeply-seated _Savi's vesicles_ on the ventral surface of the Torpedo, and the _nerve-sacs_ of Ganoids, are similar organs converted into closed sacs and pinched off from the rest of the epidermis. _Lorenzini's ampullae_ or mucus canals, which are found in definitely located groups on the lateral and upper surfaces of the head in Elasmobranchs, may perhaps be compared to pit-organs prolonged inwards to form subcutaneous tubes, each of which terminates in a radially-septate, chambered dilatation or ampulla, containing groups of sensory cells.