[Illustration: FIG. 18.—ELECTRIC ORGAN OF A SKATE IN LONGITUDINAL SECTION—A, SLIGHTLY, B, HIGHLY MAGNIFIED.
A shows the compartments into which septa of fibrous tissue divide the organ. In the centre of each compartment is a disc formed from a modified muscle-fibre. Nerves ramify in abundance on its anterior surface. B, a minute portion of a disc. At the top are seen nerve-fibres in delicate nucleated sheaths; then follow the nucleated layer with which they come in contact, the contorted laminæ which represent the striations of the muscle-fibre, the granular nucleated substance of its posterior layer, some connective tissue, a capillary bloodvessel containing oval nucleated corpuscles. In a tissue space, a single coarsely granular leucocyte is to be seen.]
The usefulness of a torpedo’s electric organs is unmistakable. They are powerful enough to paralyse every animal that touches its back, whether foe or little fish suitable for food. But of what service is its feeble battery to a skate? This and the allied question as to the advantages which can have accrued to the ancestors of the torpedo who first began to change innocent muscle into a weapon of offence are usually answered by pointing to the liability of flat fish lying on the bottom of the sea to become resting-places of parasites, corallines, and other fixed growths. Very mild shocks would suffice to disturb the peace of would-be settlers. In the same way, the electric organs of fresh-water fish may, when rudimentary, have protected the skin from invasion by moulds.
If it be difficult, when considering the dispersal of energy as mechanical work, heat, or electricity, by living tissues, to bring the phenomena into line with those of which physics takes experimental cognizance, how are we to approach the problems involved in the generation of light? Yet the photogenic property of protoplasm is widely distributed. Protozoans and various other invertebrate animals cause the so-called phosphorescence of the sea. The abysmal depths of ocean are lighted by forests of luminous polyps, and traversed by fishes whose heads are furnished with lamps. By her own light the female glow-worm enables her winged mate to keep his tryst. Fireflies (Lampyrus) flash amongst the orange-trees of Italy, and blaze (Pyrophorus) beneath the mangoes of Ceylon.