STRUCTURE OF LUMINOUS ORGANS 73 



granules wMcli 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 (Quatre- 

 fages, 1850). 



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 

 Chcetopterus (Fig. 19) or Cavernularia, or restricted to 

 certain areas [Pholas, (Fig. 19),] or more definitely local- 

 ized to form an isolated group of gland cells as in Cypri- 

 dina. True multicellular glands also occur. In every case, 

 however, we find that the luminosity of these uni- or multi- 

 cellular 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 

 lucif erase (see Chapter IV) or both. They are most simi- 

 lar 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 haematoxylon 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 hio protein, comparable 

 to the chondriosomes and handed on from one generation 

 to another, gives them the name of vacuolides or macro- 

 zymases. In some forms he has described their transfor- 

 mation into crvstals and believed at one time that animal 

 light was a crystalloluminescence. His figures of the 



