THE ELASMOBRANCH FISHES 



29 



Light Organs 



Fig. 32. Transverse section of sting, TJrolophns 

 halleri. (A. M. Paden, orig.) 



e., enamel of sting; e.o., enamel organ; d., den- 

 tine of sting; j).gl., so-called poison gland. 



In deep-sea fishes, in general, gland cells have contributed to a most remark- 

 able specialization : that is, the}- have become converted into light organs or 

 photophores. Such organs have been found in various Selachians, principal 

 among which are Spinax niger, Laemargus, and Etmopierus. In Etmopterus, 

 Oshima (1911) has made out two types of light organs: (1) punctate, cup- 

 shaped organs which in the living specimen have a pearly luster; and (2) 

 linear, semicylindrical organs which apparently result from a fusion of two 

 or more punctate organs. In Et- 

 mopterus these light organs are 

 located in definite patterns, prin- 

 cipally along the sides and ven- 

 trally, but they are jiresent also 

 on the dorsum. 



Johann (1899) gives a section 

 through a luminous organ of 

 Spinax (fig. 33a) which shows 

 that it is formed as a modification 

 of cells in the germinative or 

 basal laj^er of the epidermis (gr. ) . 

 These cells enlarge and, as a cup, 

 sink slightly into the corium. Two 

 types of cells are present, one, the lens cell (I.e.), the other, the light or photo- 

 genic cell (It.c.) . 



The lens cells are few in number and are located toward the surface where 

 they appear as enlarged mucous cells. In their beginning they arise from the 

 basal layer and migrate outward, becoming large and granular. Upon coming 

 in contact with the surface they pour out some or all of their contents and 

 become the lens cells of the adult type. The light or photogenic cells {It.c. ) con- 

 sist of a few cells (Sphiax) or a number of them {Etmopterus) which occupy 

 an irregular position at the base of the cup. 



Under the basal membrane which supports the cup are blood sinuses {h.s.). 

 Usually indenting the walls of these sinuses are masses of enclosing pigment. 

 Figure 33b of Etmopterus shows the arrangement of pigment not given in 

 figure 33a. This heavy band of pigment lines the bowl of the cup and exten- 

 sions pass inward and practically cover the cup as the so-called iris {ir.). 



There is doubt as to how the organ thus described functions. It is possible, 

 however, as in some of the bony fishes, that these basal cells, which are essen- 

 tially mucous cells, form a luminous secretion, the oxidation of which produces 

 the light. That the organ is efi'ective in the production of light has been ob- 

 served through the study of living specimens. Thus in Spinax, Dr. Theodor 

 Beer has observed a strong phosphorescent light given off along the side and 

 ventral region. The light was of variable intensity, glowing for a time and then 



