CONTRACTILITY AND PHOSPHORESCENCE. 179 



secretion " for the whole organism, so the yolk-granules in the 

 egg-cell are the products of "internal secretion " for the uni- 

 cellular stage of the same organism. 



The cytological significance of secretion, then, whether in a 

 single cell or in a multicellular gland, is essentially the same, 

 and the method of the secretory process is wholly independent 

 of the ultimate disposal or use of the products. Secretion, 

 then, consists essentially in the production of specific sub- 

 stances, with the help of, or at the expense of, the protoplasm ; 

 and whether that material be expelled from, or consumed in, 

 the organism is a question of secondary importance. 



Bearing in mind, then, the general sense in which the term 

 "secretion " has been defined, let us examine luminous organs in 

 different organisms and see how the light-giving material is 

 disposed of. According to the way in which it is used, lumi- 

 nous organisms may be divided into three groups. I am here 

 speaking of the true phosphorescent organisms which produce 

 light by the activity of their own tissues, and not of those which 

 assume a luminous appearance by the habitual or accidental 

 incorporation of other luminous organisms in their own body. 



{a) The first group of luminous organisms includes those in 

 which photogenic substance is thrown out of the body, from a 

 special gland, in the form of liquid, as Pholas, Copepods, etc. ; 

 or from the external surface of the body epithelium in the form 

 of fine refringent granules, as in CJicetopterus, luminous earth- 

 worms, myriapods, etc. The secreted material assumes its 

 luminous appearance when it comes in contact with the sur- 

 rounding media, the air or sea-water, as the case may be. 



{b) The second group embraces those in which photogenic 

 material produced by the secretory activity of the cell is never 

 thrown out to the external world. Oxidation of the material is 

 accomplished by the oxygen taken with the air, or with the 

 oxygen dissolved in the body fluid. In many cases elaborate 

 mechanisms are developed in connection with the luminous 

 tissue to facilitate the oxidation of the photogenic material. 

 Various devices to intensify the luminous effect may be su- 

 peradded, making them wonderfully like the visual organs, 

 with which, indeed, they have often been confounded. This 



