transparent superficial bones of each side of the roof of the skull. Another member of the same family (_Scopelus benoitii_) is interesting in having a phosphorescent organ in the middle of the back, which is directed backwards. An American genus of Batrachidae (_Porichthys_) has about 350 photophores in relation with the lateral sense-organs of each side of the head and body.[158] The existence of luminous organs has also been noticed in the Haddock (_Gadidae_).[159] A primitive form of photophore, distributed in considerable numbers on the head and trunk, either in lines or diffused over the surface, exists in eleven species of Selachii (_Spinacidae_), of which some are known to be luminous.[160] [Illustration: FIG. 96.—_Pachystomias microdon_, showing the two rows of phosphorescent organs along the side of the body, and the anterior and posterior suborbital luminous organs. (After Günther.)] [Illustration: FIG. 97.—_Opostomias micripnus_. Median section of a simple phosphorescent organ. _g_, Radial gland tubes. (After Lendenfeld.)] Diversity of structure is equally marked. The essential part of each luminous organ is always a collection of gland cells, usually disposed so as to form the lining of a series of radially arranged gland-tubules in the deeper part of the organ, which also contains ganglion cells, and is supplied with nerves from contiguous spinal or cranial nerves. The simplest form of phosphorescent organ consists of little more than these essential elements. In the more complex organs an investing pigment-sheath, reflecting and lens-like structures, and an iris diaphragm, either singly or in combination, may be added. Fig. 97 represents one of the simplest types of phosphorescent organ, which, in groups of {180}50 to 100, are arranged in transverse bands on the sides of _Opostomias micripnus_, and appear as small white spots on the otherwise black skin of this Fish. Each organ has the shape of a biconvex lens, sunk to about half its thickness in the skin. The inner half is formed of radially-arranged gland tubes filled with small granular cells, and converging towards the centre of the organ. Into the connective-tissue walls of the tubes extend blood-vessels and nerves. External to the gland tubes there is a layer of long slender cells arranged perpendicularly to the surface, and more externally still a layer of ganglion cells. There is evidence that these organs multiply by division. Such simple phosphorescent organs as these differ little from the groups of epidermic gland cells, which probably formed the evolutionary starting-point in the development of these singular structures. [Illustration: FIG. 98.—_Pachystomias microdon_. Section of the anterior suborbital organ. _g_, Irregular gland tubes; _g^1_, radial gland tubes; _i_, iris-like diaphragm; _l_, lens-like body; _p.s_, pigment sheath; _s_, layer of light-reflecting spicules. (After Lendenfeld.)]