From the phosphorescent slime secreted by a simple skin gland to the most complicated eye-like phosphorescent organ, we have a series of intermediate forms that are quite sufficient, even in the imperfect state of our knowledge at the present day, to enable us to understand the outlines of the evolution of these peculiar and interesting organs.
We can distinguish two kinds of phosphorescent organs in the deep-sea fish. There are the curious eye-like or ocellar organs situated usually in one or more rows down the sides of the fish’s body, forming as it were a series of miniature bull’s-eye lanterns to illuminate the surrounding sea (fig. [6]); and various glandular organs that may be situated at the extremity of the barbels or in broad patches behind the eyes or in other prominent places on the head and shoulders.
Ocellar organs have been known for many years to occur on the sides of the interesting pelagic fish, Scopelus. Most of the species of this genus live in the open sea at moderate depths, coming to the surface only at night, but other species are found in almost every depth down to 2,000 fathoms of water.
Fig. 6.—Opostomias micripnus; 2,150 fathoms. (After Günther.)
In Opostomias micripnus, a dark black fish living at a depth of over 2,000 fathoms, there are two rows of ocellar organs running down the sides of the body from the head to the tail. In the living animal they are said to shine with a reddish lustre. In addition to these, the conspicuous organs, there are groups of fifty, a hundred, or even more very much smaller organs situated on the sides and back of the fish, each of which is lenticular in shape and consists of a number of short polygonal tubes containing a granular substance with rounded bases resting on the subjacent tissue. The whole organ is covered by a simple continuation of the cuticle of the body-wall. The granular substance contained in the tubes is most probably the seat of luminosity.
Fig. 7.—Head of Pachystomias microdon (after Von Lendenfeld). A, anterior sub-orbital phosphorescent organ; B, posterior sub-orbital phosphorescent organ.
As a type of the glandular organs we may take one of the sub-orbital organs found on the head of Pachystomias microdon.
In this fish there are two very conspicuous white organs immediately below the eye. The anterior one, which lies below and in front of the eye, is oval, with its upper margin slightly concave. In section it is seen to be surrounded by a thin layer of black pigment, and to consist of a reticular glandular substance in which is embedded a hammer-shaped lens-like body. Between these two structures there is interposed a thick layer of light reflecting spicules.