126 HISTOLOGY 



nation. When the magnification is increased, however, it can be seen 

 that the light is given off from myriads of tiny dots (Fig. 116). These 

 points are probably the same as the granules or photochondria. Any 

 stimulus, as a jar or an electric current, causes both the light to shine 

 and the contractile elements to shorten. 



The Ctenophores and other Coelenterata also show light. In the 

 common Mnemeopsis, careful investigation has shown that the light is 

 produced in the " ribs " on the sides of the body. It was not possible to 

 show which cells were responsible for the secretion of a luciferase, but 

 the substance probably appeared in some contractile elements. As in 

 Noctiluca, the light appeared in response to the same stimulus that causes 

 contraction and cilliary motion. 



A case of light-production in the Echinoderms may be studied in an 

 ophiurian. The genus Ophiura, of these creatures, shows two species, 

 telactes and phosphor es, which are luminous. Again, we find that there 



FIG. 116. Phosphorescence in Noctiluca milwris Sur (QUATREFAGES). A portion of the 

 body is represented with numerous scintillating dots. 



are no exclusively photogenetic cells. The light is produced in some 

 muscle fibers which must secrete, therefore, both myochondria and 

 photochondria. 



The Mollusks have Many Light-producing Tissues. All grades of 

 specialization are shown and we shall mention three, one a superficial 

 organ, and the others organs with an internal secretion. Pholas, a 

 plecypod mollusk, is the possessor of the superficial organ which ap- 

 pears as a glandular epithelium that discharges a luciferase mixed in 

 mucus. The light is given off by an external oxidization of this mate- 

 rial and the slime continues to glow for some time after its discharge. 

 The epithelium is found on two triangular regions on the inside of the 

 mantle and on two cords that ascend into the siphon, as well as in some 

 other epithelia that do not produce it strongly. 



The highly specialized light-organs are found on the cephalopod mol- 

 lusks, especially those which live in deep water. They show a great 

 variety of forms, and we shall present two. One of them, the light-organ 



