The dilated axon endings of the sinus glands of the brachyuran crab, Gecarcinus lateralis, are filled with homogeneously dense granules, each granule being bounded by a delicate membrane. The granules are of two orders of magnitude: 0.05 to 0.1 µ and 0.15 to 0.2 µ in diameter. Each axon ending contains granules of a nearly uniform size. Endings with granules of the larger size range predominate.
Non-nervous cells endogenous to the sinus gland are scattered among the nerve endings. The cell contours are irregular, and cytoplasmic processes ramify between endings.
The axons are unmyelinated, having only thin limiting membranes, and they possess many neurofibrils. Granules in preterminal portions of the axons tend to lie at the periphery of the fiber, and in some cases in chains at the core of the fiber.
The granules appear to be storage and release centers for neurosecretory substances or their precursors.