The statocyst of Aplysia limacina is a rounded vesicle with a diameter of 200-250 μ. Its wall is composed of two kinds of cells. The outer supporting cells are separate cells in fresh tissue; only under the influence of pressure or fixing agents their walls burst and artificial syncytia are created. The inner sense or giant cells are on their inner surface covered with motile cilia. Each statocyst of Aplysia contains 13 sense cells; their nervous offshoots constitute the statocyst nerve which runs towards the cerebral ganglion. The statolith is a cluster of about 1000 loosely aggregated chalk particles (statoconia). It fills the greater part of the statocyst lumen and is lightly moved by the cilia. Special statolith-free cavities or sense hairs, such as are known from animals with a true rotation sense, were not found in the statocyst of Aplysia. Freely swimming Aplysiae perform correction movements with their head leading, when they are brought out of their normal position in space. Likewise, fixed Aplysiae, when lifted up in the water and rolled or tilted about horizontal axes, show compensatory static head reflexes. Rotation around a vertical axis causes no response. Unilateral section of the statocyst nerve causes neither a loss of the position reflexes nor any asymmetry of posture or movement. Bilateral section of this nerve, however, abolishes all correction movements and compensatory reflexes; swimming animals perform somersaults. Tactile stimuli from the underground support the animal's spatial orientation. An orienting influence of light was not observed. After unilateral section of the cerebro-pedal connective a fixed Aplysia only responds when rolled the towards ipsilateral side (with a compensatory turn of the head towards the contralateral side); when rolled 90° towards the controlateral side no reaction occurs. The results of the elimination experiments (Table, p. 49) lead to the following conclusions: 1) from each statocyst two reflex pathways originate, one of which is activated after a roll around the long axis to the left side and causes a head turn to the right, whereas the other one comes into action after a roll to the right side and causes a head turn to the left; 2) the pathways of both statocysts which turn the head to the left run from the cerebral ganglion through the left cerebro-pedal connective towards the left pedal ganglion; both pathways which turn the head to the right run through the right cerebropedal connective towards the right pedal ganglion (diagram, Fig. 7, p. 53). These and other results are discussed in relation to data of earlier investigations. The course of the static nerve as shown morphologically to occur in other gastropods resembles closely the pathways postulated for Aplysia on physiological grounds. With regard to the process involved in stimulation it is assumed that in the statocyst of gastropods, like in other static organs, a shearing force exerted on the cilia represents the effective, physiologically adequate stimulus. Recent findings about the submicroscopical structure of the cilia in the statocyst of gastropods as well as about the mechanical sensitivity of motile cilia give this assumption strong support. © 1969 Springer-Verlag.
