416 • PHYSIOLOGICAL LIGHT. 



of its envelope numerous retractile and excitable prolongations. It is 

 in this last tract that we see formed rounded granulations having a 

 special refractive power that we shall henceforth find in all photogenic 

 elements. 



Mechanical, physical, and chemical stimuli cause the interior of the 

 Noctilucce to shine, and they then appear to the naked eye like little 

 stars emitting a steady light. But with sufficient magnification this 

 apparently uniform glow is resolved into a multitude of little, brilliant 

 points or sparks that, by their form and distribution, correspond to 

 the refringent granulations of which we have already spoken. 



The photogenic function is widely distributed in the class of Ccelen- 

 terata. There is, it is true, but one luminous species known among 

 the sponges, but they are numerous among the Cnidaria and have 

 been well studied in the Anthozoa of the family of the Pennatulidce. 



In abyssal regions numerous polyps with horny or calcareous axes, 

 such as Isis, Gorgon, or Mopsea, form veritable luminous forests, pro- 

 ducing a truly fairy -like effect. 



Among the Pennatulidce the photogenic function is already localized. 

 It has its seat in the eight cords which adhere to the external surface 

 of the gastro-vascular cavity of polyps and zooids and extends as far as 

 the buccal papillse. The light originates in cells containing an adipoid 

 material and very numerous rounded, albuminoid granulations. In 

 these cells, as with the Noctilucce, mechanical, electrical, and chemical 

 stimuli induce a luminous explosion that is transmitted from one place 

 to another in a very regalar manner, from the foot of the polyp toward 

 the extremity of the arms or inversely, with more or less extensive 

 generalization, according to the intensity of the stimulus. 



The granulations of the luminous cells seem to be formed under 

 stimulation by a phenomenon analogous to that of the formation of 

 crystals by the agitation of a saturated solution; at lea.st that is what 

 occurs in the ectodermic cells of Hippopodius gleba. That elegant 

 ccelenterate is composed of a series of horseshoe-shaped segments, 

 transparent as crystal when the animal is not stimulated, but, when 

 the ectoderm is touched, the cells which compose it become at once 

 opalescent or milky by the formation of a multitude of granulations, 

 and emit a magnificent, azure-blue light. The simultaneous occur- 

 rence of these phenomena is very striking in this particular case. 



Stimulation of the ectoderm also produces light in certain Medusce, 

 such as Cunina albescens and Pelagia noctiluca. In the latter the pho- 

 togenic function is localized in the epithelium of the external surface, 

 the radial canals, and the genital glands. The tesselated cells that 

 compose this epithelium have a nucleus that is often masked by a mul- 

 titude of fine and very refringent granulations, varying in color from 

 yellow to orange. 



When these cells break down they form a luminous mucus that 

 adheres to the fingers and continues to shine for a considerable time. 



