The statocyst of octopods is a sack containing endolymph, lying in a larger cavity, containing perilymph and crossed by strands containing blood vessels. The arrangement shows remarkable parallels with vertebrates. The crista runs a course in three planes, each section being innervated by a separate nerve. It thus presumably serves as a receptor resolving angular accelerations. Each of the parts is further divided into three subsections, with its own flap of massed, loaded hairs. In front of the vertical crista is a rigid plate of cartilage, the anticrista, perhaps serving to prevent stimulation of that part of the ridge when the octopus suddenly accelerates during an attack. The crista contains large hair cells at its centre, there being alternately sections with one row and two rows of these. Flanking the large hair cells are inner and outer rows of smaller hair cells. These show characteristic differences and are asymmetrical above and below. All the hair cells are held to be neurosensory cells, carrying axons. In addition there are present multipolar cells, which besides carrying hairs also have many dendrites, making contact with other hair cells. Some of these multipolar cells probably lack hairs and thus form true peripheral neurons, stimulated by the overlying hair cells. The latter still, however, continue to be primary receptor cells with their own axons. If this is so there are two channels from these hair cells to the C. N. S. Each fires through its own axon and several together fire through each multipolar cell. A third set of nerve fibres is present in the crista, having cell bodies within the C. N. S. and endings around the apical parts of the hair cells and around the nerve cells. These are probably efferent fibres. The macula is placed vertically on the antero-medial wall, with a statolith hanging on it. It contains hair cells with short hairs embedded in the statolith. Multipolar cells lie below the hair cells and have axons but probably no hairs. There is a plexus of presumably efferent fibres. The membranous wall of the statocyst contains muscle fibres, a plexus of nerve fibres, and receptor cells with hairs and (sometimes) dendritic processes. These may be pressure receptors recording changes of pressure within the mantle and body cavity. The outer wall of the statocyst, though mainly cartilaginous, has a membranous region. This plexus and its receptors are especially well developed in a protruding posterior sack. Kölliker’s canal lies in the wall of the statocyst and opens into its cavity, the other end being a closed tube. Its cells are ciliated. It may serve to regulate the endolymphatic pressure by secretion or absorption, assisted by the action of the muscle fibres and perhaps by the receptors of the wall of the sack.
