FUNCTIONAL ORGANIZATION OF THE GIANT FIBER SYSTEM OF LUMBRICUS

MORE OR LESS elaborate systems of nerve fibers much larger than any others in the same animal have been evolved several times, apparently independently, in the animal kingdom. They exist among certain but not all groups of the cestodes, nemerteans, annelids, crustaceans, cephalopods, enteropneusts, cephalochordates, and vertebrates. They exhibit a diversity of organization. Some are confined to the central nervous system, others are peripheral; some are simply very large ordinary neurons, others consist of compound fibers representing the fused axons of several cells. Complex combinations of these conditions exist and of course a variety of synaptic