Note 35 — Chapter VI · LAMP SHELLS.

Page 40. Pholas. A clam-like mollusk. Several species are found on Nahant beaches. P. dactylus is a European form. The genus Zirphæa is found from New England to Great Britain. All are more or less borers. A small species bores in hard mud on the Nahant beaches. Others are known to bore into hard wood and into stone.

Pholas dactylus will be seen to have photogenic or light-emitting structures and substances almost concealed in the tissues of the animal. The light-emitting portions are, according to Panceri, “two parallel cords containing an opaque white matter extending down the anterior siphon, two very small spots at its entrance, and finally an arched cord corresponding to the superior edge of the mantle, reaching to the middle near the valves. The white color of the cords, which stand out in relief, distinguishes them; and, although they are only elevations of the subcuticular tissue, they contain special cells, or rather epithelium, which produces the phosphorescent matter. The whole surface of the Pholas is covered with ciliated epithelium, which dips down into all the parts of the animal; but the special epithelium differs from this. It is nucleated and crammed with granules, and the cells are very refractive. The cells are very fragile, and allow their contents—i.e., granular nuclei and refractive granules—to escape readily. These are soluble in ether and alcohol. Under ordinary circumstances this photogenic apparatus is hidden; but violence readily displaces the special cells, which burst, and their contents are carried all over the surface by the water, assisted by the general ciliation. The white substance, fat-like, retains its luminosity, when spread out on paper, for hours; but the light does not appear to be accompanied by an evolution of heat. When it is placed in carbonic acid gas, the light pales and ceases. On the other hand, the photogenic substance, when barely luminous, is rendered so by physical contact. Agitation, and the addition of fresh or salt water, develop the light, and the same effect is produced by electricity and by heat. The light is monochromatic, and has a constant place in the spectrum as an azure band from E to F, that is to say, in the green.”