Page:The aquarium - an unveiling of the wonders of the deep sea.djvu/219

172 spot or rarely a minute speck; the middle plate two parallel specks also minute. In P. squilla each plate has a roundish or squarish spot of yellow, all equal in size, and forming an angular band of spots. The distinctions drawn from the form and dentition of the rostrum, and the gibbous carapace of P. squilla, I need not speak of particularly, as these are sufficiently appreciable in cabinet specimens.

When viewed with a candle the eyes of the Prawn reflect the light with a glare exactly like that seen in a cat's eyes under similar circumstances. The light, in the case of the Prawn, is seen only when the candle is held between the beholder and the insect, and becomes brighter and larger the more nearly the flame of the candle is brought to the line which unites the observer's eyes and the object. It might seem as if nothing could possibly be discerned when the flame is absolutely in this line, but it is not so; both eyes being open, the line of vision of each eye passes on one side of the candle, and we discern the two eyes of the Prawn, like two little globes of fire.

In the case of the cat the phenomenon is said to be produced by the choroid coat at the bottom of the eye (tapetum lucidum), reflecting from its polished metal-like surface the entering rays, and converging them by its concavity, as if from a concave mirror. This is simple and perfectly intelligible; but I do not see how a similar effect is produced in the compound eyes of the Crustacea, each of which is composed of a great number of conical lenses with apices inward. What is there in this structure that can represent a concave mirror? Yet no one