STRUCTURE OF LUMINOUS ORGANS 75 



diffusion and convection movements. The most beautiful 

 examples of luminous secretions are found among the 

 ostracod Crustacea. 



In Cypridina hilgendorfii the luminous gland is situ- 

 ated on the upper lip near the mouth. It is made up of 

 elongate (some 0.7 mm. in length), spindle-shaped cells, 

 each one of which opens by a separate pore with a kind 

 of valve. The openings are arranged on five protuber- 

 ances. Muscle fibres pass between the gland cells in such 

 a way that by contracting the secretion can be forced out. 

 In the sea water the secretion luminesces brilliantly 

 and the Japanese call these forms umi hotaru, or marine 

 fireflies. Fig. 22 is a diagram showing the structure. 

 Watanabe (1897), who first studied this form, and also 

 Yatsu (1917) have described two kinds of granule-con- 

 taining cells, one vnth. large yellow globules, 4-10/x in 

 diameter (Fig. 23), the other with small colorless granules 

 0.5, in diameter. I have observed in the living form these 

 two types and also large colorless globules of the same size 

 as the yellow globules. All dissolve when extruded into 

 the sea water. Dahlgren * has described from sec- 

 tions four types of cells containing (1) large globules, 

 (2) small granules, (3) a fat-like material, (4) a mucous 

 material. Just what the significance and nature of these 

 types of substance is cannot be stated at present. At 

 least one, probably two, are concerned in light production 

 The others may possibly form digestive fluids which act 

 on the food of the animal. 



Turning now to the animals possessing light cells with 

 intra-cellular luminescence Ave find in general that such 

 light cells are localized to form definite light organs and 



* Private communication soon to be published. 



