Many Siluridae are armed with powerful and often serrated dorsal and pectoral spines which are certainly capable of inflicting dangerous wounds, and not a few of them possess a sac-like organ with an external opening in the axilla of the pectoral fin. It is possible that the sac secretes a poison for anointing the spine, but at present there is no evidence that such is the case, or that the sac produces any poisonous secretion at all.[154] Among the Elasmobranchs the Eagle-Rays (_Aëtobatis_),[155] and the Sting-Rays (_Trygon_), have barbed or serrated spines on the tail, which inflict wounds far more severe than those caused by {178}mere mechanical laceration; but, except the mucus secreted by the gland cells of the skin, which may possess venomous properties, no special poison-forming glands in connexion with the spines are at present known. PHOSPHORESCENT ORGANS.[156] In common with many other animals of similar habitat, phosphorescent organs (photophores) are highly characteristic structures in many deep-sea Teleosts belonging to widely different families (e.g. _Stomiatidae_, _Scopelidae_, _Halosauridae_, and _Anomalopidae_). These organs probably had their origin in local aggregations of the gland cells of the epidermis, which had acquired the power of secreting a luminous slime. Luminous organs vary greatly in number and in their mode of distribution in the skin. Usually they are found on the sides and ventral surface of the body and head, very rarely on the dorsal surface, and they often present the appearance of brightly glistening jewels set in the skin. A very frequent method of arrangement is in one or two longitudinal lines along the lateral and ventral surfaces, sometimes extending continuously from the head to the end of the tail (Fig. 371, A, and Fig. 379), but occasionally interrupted and limited to portions of the body and tail; and in a few a distinctly metameric disposition is obvious. On the other hand, the very numerous and simple organs of _Opostomias_ are disposed in many transverse bands along the sides of the Fish. In addition to these organs, which are usually numerous, and whose arrangement is linear, specially large and often structurally complex luminous organs are present on different parts of the head and body. In _Opostomias micripnus_ there is a phosphorescent organ on a median barbel depending from the chin. _Sternoptyx diaphana_ has one on the lower jaw. The presence of one or two organs beneath the eyes (Fig. 96) is characteristic of several species (e.g. _Opostomias micripnus_, _Astronesthes niger_, _Pachystomias microdon_, _Scopelus benoitii_, _Malacosteus indicus_). _Opostomias micripnus_ has a luminous organ on the isolated and elongated first fin ray of the pectoral fin, while in certain deep-sea Angler-Fishes (e.g. _Ceratias_) there is one on the anterior cephalic fin-ray of the dorsal fin. The Scopelid _Ipnops {179}murrayi_[157] (Fig. 371, C) has a singular organ, probably luminous, beneath the