No. 3.] PHOSPHORESCENT ORGANS. 669 



or sometimes entirely absent over the dorsal aspect of the body. 

 Each organ, viewed from the surface, appears as a silvery spot 

 more or less circular in outline and varying in size from a point 

 scarcely visible with the unaided eye to .8 mm. in diameter. 

 The larger organs are found on the ventral surface of the 

 fish, the less conspicuous ones on the dorsal surface — a fact to 

 be again referred to later in the paper. They are often sur- 

 rounded by, or bordered on one side by, an increased amount 

 of pigment. This is especially noticeable in organs on the sides 

 of the body where the pigment is much increased in amount on 

 the side of the organ away from the mid-ventral line. 



The structures, indiscriminately designated by Test as "lateral- 

 line organs," "slime glands," "mucous pores," or "pores," are 

 in reality lateral-line sense organs of the kind designated as 

 nerve hillocks by Merkel. Such an organ, viewed from the 

 surface, presents a point on the epidermis free from pigment, 

 the end of the sense organ itself with its immediately surround- 

 ing supporting cells. It is too distinct and characteristic in 

 appearance to be mistaken by even the most casual observa- 

 tion. 



The lines of phosphorescent organs and lateral-line organs 

 are so closely associated in their distribution in the skin of the 

 fish that they may be most economically mapped out together. 

 Test did not discriminate between these two sets of organs. 

 Terms he introduced, referring to lines of organs, will be 

 indicated by quotation. 



II. Distribution of the Phosphorescent Organs and 

 THE Lateral-Line Sense Organs. 



The organs of both systems are arranged in lines or rows 

 and are very constant in their relation to each other in any 

 given row, also the rows are constant in their relative positions 

 in different individual fishes. The number of organs in a given 

 line of either phosphorescent organs or sense organs may, how- 

 ever, vary much in different individuals (see Table). The 

 average number only will in most instances be given in the 

 general description. 



