The absence of pigment is sometimes protective. The surface-swimming larvae of many Teleosts have no chromatophores, and therefore no obvious pigmentary colours. Their bodies are so translucent that they can be seen through, and hence are visible only with difficulty. The transparency of the body may even be increased by the absence of the red haemoglobin of the blood, as is the case with the pelagic _Leptocephalus_-larvae of the Eel.[149] The iridocytes of the reflecting tissue may also disappear under the influence of changed surroundings. The larvae of various species of _Onus_ (Gadidae) are silvery in hue during their pelagic career, owing to the presence of iridocytes in the skin, but on becoming mature they change to a dull dark colour, and live under stones or in holes and crevices in the rocks. During the change of habit the reflecting tissue (argenteum) is lost, and the needful chromatophores are acquired.[150] Instances of the loss of pigmentary colours, owing to the cessation of the controlling influence of natural selection, are to be found in the absence of chromatophores on the white under surface of the Flat-Fishes, where such colours are useless but not necessarily harmful, and in the colourless, cave-inhabiting Fishes, of which the Blind-Fish (_Amblyopsis_) of North America may be taken as an example. {176}POISON GLANDS OF FISHES. A few Teleosts are provided with weapons of offence or defence in the shape of poison-glands, probably derived from the epidermis, and associated with spines on the gill-covers, or in connexion with the dorsal fin, or with both. [Illustration: FIG. 95.—The opercular spine of _Trachinus draco_ and its poison-glands. _ar_, Articulation of the opercular bone with the hyomandibular; _gl.gl_, the two poison-glands; _op.m_, opercular membrane; _op.s_, opercular spine; _r_, outer ridge of the spine; _sh_, sheath of the spine. (From W. Newton Parker.)]