Mimetridium cryptum is a slender, elongated, New Zealand sea anemone. It shows fast contractions in the retractor muscles of its mesenteries, oral disc radial muscles, and tentacle longitudinal muscles. The nervous system shows considerable regional differentiation in orientation of neurons, range of diameters of nerve fibers, and density of nerve net. Fast-contracting muscles are overlain by relatively dense nerve net, with many nerve fibers of more than 2 ,μ diameter; slow-contracting muscles are overlain by a sparse nerve net whose nerve fibers are about 1μ in diameter. A tendency for nerve fibers to run parallel is marked in some regions. Individual neurons may run from one structure to another, and even pass from the endoderm of the mesenteries to the ectoderm of the oral disc.