The light emitted by phosphorescent organs is probably of use to deep-sea Fishes in enabling them to seek and detect their prey in the sunless depths which they frequent. The position of the organs on the sides and ventral surface of the body, and the frequent presence of special luminous organs in the vicinity of the mouth, render them admirably adapted to light up the water in front of and beneath the Fish, while the existence of optical accessories for intensifying the luminous beams, and for regulating their distribution, combined with an abundant nervous supply, suggests that the emission of light is under the control of the Fish, and may be varied as the occasion requires. That these organs may also be defensive, in some instances at all events, seems not improbable. A flash-light from the dorsal luminous organ or "stern-chaser" of _Scopelus benoitii_ would probably dazzle and frighten an enemy in hot pursuit of the _Scopelus_. The use of phosphorescent organs as baits or lures for enticing prey has already been alluded to. There is some evidence that the colour of the emitted light differs in different Fishes; and as there is considerable variety in the precise disposition of the organs, it seems probable that in deep-sea Fishes recognition lights may take the place of the recognition colours and sounds of those whose lot is cast in a sunnier habitat. {182}CHAPTER VII THE SKIN AND SCALES The skin of the Cyclostomata and Fishes consists (1) of the epidermis, formed of several layers of epidermic cells, which are constantly being recruited by the division of the cells of the basal layer; and (2) of a stratum of connective tissue with intermingled unstriped muscle-fibres, blood-vessels and nerves, which constitutes the deeper layer or dermis. From the epidermis are formed the various unicellular or multicellular glands with which the skin is provided; and from one or both of the skin layers originate the different calcareous structures which constitute the hard exoskeleton. In the Cyclostomata the epidermis is particularly rich in goblet-shaped, mucus-secreting, gland-cells. The Myxinoids also possess numerous pockets of so-called "thread-cells." In each of these cells the protoplasm secretes a long spirally-coiled thread, and under the influence of appropriate stimuli the thread is shot out and unwound to a great length. The threads and the mucus are so abundant that one of these animals will convert a bucket of water into a thick mass of jelly. No scales or other hard exoskeletal structures are present in any of the Cyclostomata.