FOOT — ON ANIMAL LUMINOSITY. 51 



Infusoria, sucli as the Noctiluca miliaris ; the other kind of phosphores- 

 cence observed is an appearance upon the sea of sheets or trains of 

 whitish or greenish light, often sufficiently brilliant to illuminate the 

 sails and deck of a passing vessel, to permit of print being read near 

 the stern cabin windows, to show night birds hovering over the water 

 in search of prey. This luminosity is produced by the presence of 

 vast numbers of light-producing Acalephae and Mollusca, by various 

 species of Beroae, Medusae, and Salpae, which are often congregated in 

 such multitudes, that a ship sailing rapidly may continue during the 

 entire night to pass through extensive fields of these floating and 

 glowing organisms. The phosphorescence of the sea has a close con- 

 nexion with the abundance of organic beings ; in parts of the ocean 

 where they are scarce, this phosphorescence is more rarely and less 

 brilliantly seen. Occasionally, when they abound to such a degree as to 

 make the water gelatinous, the sea at night takes the appearance of a 

 plain covered with snow, or of an ocean of milk. Besides these surface 

 animals, there are sources of illumination of the sea often at con- 

 siderable depths, such as the great Medusa pellucens discovered by Sir 

 Joseph Banks, which, with a disc six inches in diameter, shines like a 

 globe of fire. Darwin* mentions that near the mouth of the Plata he 

 observed some circular or oval patches at a considerable depth, from 

 two to four yards in diameter, and with defined outlines shining deep 

 down in the water, with a steady but pale light, while the surrounding 

 water only gave out a few sparks. The appearance resembled the 

 reflection of the moon, or some luminous body, for the edges were 

 luminous from the undulations of the surface. The ship, which drew 

 thirteen feet of water, passed over without disturbing these patches. 

 An animal which is one of the fixed sources of the luminosity of the 

 sea is the conchiferous Mollusk, the Pholas, or stone-borer, edible, and 

 reputed to have an agreeable taste, with a flavour of pepper. One species, 

 P. costata, is sold in the market at Havannah as an article of food. 

 Plinyf was aware of the phosphorescent properties of this shellfish; 

 and in his chapter on the marvellous properties of the Dactylus, 

 remarks that it is the peculiarity of these fish to shine brightly in the 

 dark, when all other lights are removed, and the more moisture they 

 have the brighter is the light they emit. In the mouth even, while 

 they are being eaten, they give forth their light, and the same, too, 

 when in the hands. The very drops, in fact, that fall from them on 

 the ground, or on the clothes, are of the same nature. Beccarius, who 

 tried numerous experiments upon the Pholas, found that of all the 

 liquids into which he put the Pholades, milk was rendered the most 

 luminous. A single Pholas made seven ounces of milk so luminous 

 that the faces of persons might be distinguished by it. He also states 

 that when the animal is preserved in honey, the property of becoming 



* "Naturalist's Voyage round the World, '' p. 163. 

 t "Hist. Nat. Lib." ix., cap. 87. 



