If phosphorescent organs produce a substance essentially a secretion to which their characteristic activity is due, they might without impropriety he classed as glands, but if they are thus classed, it must he remembered that the majority of them are so placed that they have no access to cavities or the exterior; hence they would be in the nature of ductless glands. In one respect, however, they differ even from ductless glands; the substance that they produce is not carried away from them even by the blood-stream hut is used locally for the production of light. Hence though phosphorescent organs may be in many important respects like glands, they differ in certain ways from all ordinary glands.
Whether phosphorescent organs are under the control of nerves or not is a question of some uncertainty. The fact that many highly specialized phosphorescent organs have a rich innervation indicates that they are under nervous influence, but even this may be of the indirect kind such as has already been indicated for glands and not a direct control. In ctenophores Peters (1905) has shown that a few paddle-plates will glow on mechanical stimulation precisely as the rows of plates in the normal animals do. He has also shown that the primitive nervous system of these animals plays no direct part in this phosphorescence. This instance seems to me to be a perfectly clear case of phosphorescence not under the control of nerves, though in an animal with a nervous mechanism.
In the common firefly the relations are not so well understood. Thus Bongardt (1903), though he describes an intimate nervous plexus in the luminous organ of this animal, believes that its rhythmic photogenic activity is not under even indirect nervous control. He maintains, on what, however, is not really strong experimental evidence, that the firefly can not extinguish its light through nervous action and he believes that the phosphorescent rhythm is due to totally different factors. This case merely shows the fragmentary nature of our knowledge of this phenomenon even in so well-known an example as the firefly.
As a good instance of nervous control over phosphorescence the brittlestar, Ophiopsila, recently studied by Mangold (1907), may be quoted. On mechanical stimulation the ventral surfaces of the arms of this animal glow for a short time. The phosphorescence begins in the stimulated part and, if this be an arm, it may spread over this arm to the disk and thence to the other arms. The course that it follows is that of the radial and circular nerve-strands. If any of these are interrupted by being cut, the phosphorescence does not pass beyond the cut, thus showing that it is probably controlled by the nerve.
These instances, few and confessedly fragmentary as they are, indicate that phosphorescent organs, though in many important respects like glands, are in reality a separate class of effectors and that in some
