Among protozoa, in certain forms at least, it is easy to observe that luminescence is connected with globules or
granules which were considered by the earlier observers to be oil droplets. Thus, in Noctiluca (Figs. [17] and [18]), when the animal is violently stimulated or in the presence of reagents which slowly kill it, the whole interior appears a mass of starry points of light which can be traced to minute granules along the strands of protoplasm (Quatrefages, 1850).
Fig. 17.—Noctiluca miliaris, showing photogenic granules in cytoplasm. n, nucleus; c, cytoplasmic strands containing photogenic (large) and other (small) granules; p, pharynx; f, flagellum; o, oral groove; t, tentacle; s, spines at base of tentacle; v, vacuoles. Drawn by E. B. Harvey.
Fig. 18.—Noctiluca miliaris as it appears during luminescence (after Quatrefages). Upper left and middle, low power; below, high power; upper right, a crushed fragment still luminescent.
Turning to the multicellular forms, we find the simplest development of luminosity in those animals which possess gland cells producing a luminous secretion. These cells may be scattered over the surface of the animal as in Chætopterus ([Fig. 19]) or Cavernularia, or restricted to certain areas [Pholas, ([Fig. 19]),] or more definitely localized to form an isolated group of gland cells as in Cypridina. True multicellular glands also occur. In every case, however, we find that the luminosity of these uni- or multicellular glands is connected with the presence of granules. They are often spoken of as luciferine granules, although it is not certain whether they are made up of luciferin or luciferase (see Chapter IV) or both. They are most similar to the zymogen granules found so abundantly in gland cells and thought to be the precursors of various enzymes. According to Dahlgren (1915), the luciferine granules stain blue-black by iron hæmatoxylon after fixation at the boiling point, and photogenic cells can be detected by this method of selective staining. Dubois (1914, book), who regards them as examples of bioprotein, comparable to the chondriosomes and handed on from one generation to another, gives them the name of vacuolides or macrozymases. In some forms he has described their transformation into crystals and believed at one time that animal light was a crystalloluminescence. His figures of the