A nerve ring, underneath the epithelium of the subumbrella, passes from near the origin of each pedalium at the margin to the origin of the peduncles of the sensory clubs, a little above the margin, giving off a branch to each club. Eight ganglia are found in the course of this nerve. The four pedal ganglia lie near the bases of the pedalia, and are hence interradial; the four radial ganglia lie near the bases of the peduncles of the clubs, and are perradial. A small nerve, radial nerve, can be traced a short distance upwards from each radial ganglion. Underlying the epithelium of the frenula and the suspensoria are ganglion cells and nerve fibers in larger numbers than elsewhere (excepting the ganglia mentioned) in the subumbrella. Otherwise, ganglion cells and nerve fibers underlie the epithelium of the subumbrella, including the inner surface of the velarium, as also do muscle fibers, except in the perradii and in the region of the nerve, where the latter become interrupted.
PHYSIOLOGICAL.
Charybdea.
Light and Darkness—Experiments [1-9], [10], [33], [34].—As already stated in the Introduction, a part of Conant’s experiments were performed in a photographer’s dark-room, with the animals in a deep glass jar. In the dark a fair proportion of the animals became nearly quiescent on the bottom, but upon lighting a lamp many started up immediately, while others took a longer time to come to the surface and swim. These experiments were tried a number of times and on different occasions with very similar results. Some medusæ, however, tried immediately after being brought in, seemed not to react so well upon being placed in the dark-room, nor would they become quiescent. This, probably, was due to the fact that the animals had not yet recovered from the effects of being caught and placed in new surroundings. (Experiments [1], [2], [3].)
Other experiments ([4-8], [33], [34]) were tried by carrying the jar with the animals from the weaker light of a room into the more intense light of outdoors or into direct sunlight. The usual result was an inhibition of pulsation and a settling to the bottom, while the medusæ immediately became active again upon returning with them to the room. These results were so marked that no doubts can be entertained as to their cause, though some exceptions occurred in which animals placed in the sun continued to swim on the surface or soon recovered pulsation. In some experiments, too, no animals responded to the inhibitory stimulus of the brighter light or all very soon recovered. (See, however, Temperature.)
Reducing the light by placing a coat over the jar produced the same effect in some experiments ([8], [9], [10]) as did reducing the light in other ways, while removing the coat produced the same effect as exposure to brighter light. In these instances it appears to be the transition from weaker to stronger light that inhibits pulsation, rather than the actual intensity of the light; and vice versa. It must be noted, too, that when left for some time in any one place the animals changed, some coming to the surface and others going to the bottom.
These experiments show beyond doubt that Charybdea is sensitive to light, and that it is moderate light that stimulates the animals to activity, while darkness and strong light inhibit activity. While the individual exceptions, as Conant himself suggests, are well explained on the supposition of individual diversity, yet it appears that other conditions, such as the time of day, temperature, etc., may have been responsible for some of the exceptional experiments in which no animals responded as expected.
While light of any intensity seems to have stimulated Romanes’[I] Sarsia and Tiaropsis (Hydromedusæ) to activity, we note that it is moderate light that stimulates Charybdea. This fact is evidently correlated with the circumstance that Charybdea usually lives upon or near the bottom.