140 ■ LOWER VERTEBRATES. 



escent or luminous organs are either larger bodies of an ovoid shape, placed on the 

 head, in the vicinity of the eye, or smaller globular bodies regularly arranged in series 

 along the side of the body and tail, especially near the abdominal profile. The 

 former have not yet been studied. Of the latter class, the number of jsairs is in 

 direct relation to that of the vertebrae, the muscular system, etc. These again are of 

 two kinds. The organs of one kind consist of an anterior, biconvex, lens-like body, 

 vs^hich is transparent during life, simple or composed of rods, and coated with a dark 

 membrane composed of hexagonal cells, or of rods arranged as in a retina. This 

 structure is found in Astronesthes, Stomias, Chauliodus, etc. In the other kind, the 

 organ shows throughout a simply glandular structure, but apparently without an 

 efferent duct {Gonostoma, Myctophum, Maurolicus, Argyropelecus). Branches of 

 the spinal nerves run to each organ, and are distributed over the retina-like membrane 

 or the glandular follicles. The former kind of organs are considered by some natural- 

 ists to be true organs of vision (accessory eyes). 



Although these organs thus differ in structure, there is no doubt that the func- 

 tions of all have some relation to the peculiar conditions of light imder which the 

 fishes provided with them live ; these fishes being either deep-sea forms, or nocturnal 

 pelagic kinds. 



There are three possible hypotheses as to the function of these organs. 



1. All are accessory eyes, or at least sensory in their function. 



2. Only the organs with lenticular body are accessory eyes, and those with a glan- 

 dular structure produce and emit phosphorescent light. 



3. All are producers of light. 



The first view seems very improbable. Many of the sj)eoies with phosj^horescent 

 spots possess very large eyes, which would render these feeble ones unnecessary, 

 while other species of the deep sea, having eyes concealed or imperfect, lack the lumi- 

 nous spots. 



The second supposition seems to be nearer the truth. It is supjDorted by the fact 

 that the glandular organs in Myctophum have actually been observed to shine with 

 "phosphorescent" light, and by the obvious similarity of the organs with lenticular 

 body and retina-like membrane to a true eye. In depths illuminated by phosphor- 

 escent light only, we might well exfiect the development of peculiar organs of vision. 

 But many fishes which inhabit great depths (Macruridte, Tiachypteridae), have large 

 eyes of the ordinary sort, from which it appears that the ordinary organ of vision is 

 sufficient for seeing by phosphorescent light. 



While we must admit that those comjjound organs may prove to be organs of 

 sense, it is not unreasonable to suppose that they too, like the glandular organs, are 

 producers of light. It maybe produced at the bottom of the posterior chamber, and 

 emitted through the lenticular body in particular directions, with the same effect as 

 when light is sent through the convex glass of a "bull's eye." This hypothesis seems 

 to be less bold than the other, which would require us to believe that vertebrate ani- 

 mals, with a nervous centre specialized for the reception of the inij)ressions of the 

 higher senses, should receive them through the spinal coi-d. 



At present, therefore, the existence, in fishes, of ' accessory eyes ' capable of receiv- 

 ing visual impressions, cannot be admitted ns proved, although it is not impossible. 



The SiOMiATiDiE are deep-sea fishes with the body naked, or covered with thin 

 loose scales. The mouth is very large, which fact has suggested the name of the typical 

 genus, Stomias, and the teeth are strong, numerous, and unequal, some of them being 



