1. The land crab, Gecarcinus lateralis (Freminville), was selected as the principal subject for this study of anatomical relationships existing between neurosecretory centers in the eyestalks and brain of brachyuran Crustacea. Observations were also made on ten other species of crabs.2. Techniques useful in clarifying crustacean endocrine structures have been described.3. It has been found that the sinus gland is actually a mass of swollen nerve fiber endings, arranged in the form of an inflorescence and bearing secretory material. The histological structure of regenerated sinus glands, which appear after sinus gland removal, resembles that of normal sinus glands.4. Nerve fibers, the endings of which compose the sinus gland, originate in neurosecretory cells of the brain, the eyestalk ganglia, and possibly the thoracic ganglionic mass. X-organ fibers contribute their endings to the sinus gland.5. Neurosecretory products of the living preparation appear as bluish-white material in the cell bodies, nerve tr...