SENSORY ORGANS AND RECEPTION 347 



positively geotactic, swimming vertically upwards or downwards. When a 

 geo-negative animal is tilted, some of the upper rows of cilia cease beating, 

 and the resultant asymmetrical action of the remaining ciliary rows tends 

 to restore the animal to its normal position. When the statocyst is removed, 

 orientation to gravity is lost, and when the statolith is pushed to one side, 

 some of the ciliary combs are inhibited. 



Positive geotaxis is shown by many burrowing animals, turbellarians, 

 polychaetes, holothurians, brachiopods, etc. Some of these creatures have 

 statocysts, which are responsible for initiating and directing the response. 

 By imposing a centrifugal force it has been possible to change the direction 

 of the geotactic response of Convoluta roscoffensis. This animal has a single 

 statocyst and is positively geotactic. When subjected to rotation, the animals 

 deviate from the vertical axis by an angle which is the resultant of the 

 gravitational and centrifugal forces. Two polychaetes, Arenicola grubei 

 and Branchiomma vesiculosum, which burrow in sand, have statocysts in 

 the head which govern their gravity reactions. Arenicola and Branchiomma 

 always burrow vertically downward and when the aquarium in which they 

 are placed is turned they make a compensating turn in the direction of 

 burrowing. Removal of both statocysts abolishes the reaction. 



Static reactions to turning or tilting are dependent on statocysts in some 

 animals (mysids, decapod Crustacea, the heteropod Pterotrachea, etc.). 

 In the prawn Palaemon the reactions continue normally after extirpation of 

 one of the two statocysts. When the animal is tilted and the statolith is 

 shifted to one side, reflexes causing rotation in one direction begin; and 

 movement of the statolith to the opposite side evokes reflexes in the other 

 direction. Each statocyst possesses a two-way action and is able to control 

 the whole system of righting reflexes. 



Electrical recording from the statocyst nerves of decapods (Homarus, 

 Panulirus, Loxorhynchus) reveals continuous and spontaneous discharge 

 of nerve impulses. In the lobster {Homarus) four types of statocyst re- 

 ceptors have been distinguished. Type I gives a characteristic response for 

 positions maintained about the transverse axis, and serves to detect 

 absolute position (Fig. 8.30). Type II signals static position and move- 

 ment to and away from a given position. A third type is an accelerator 

 receptor, responding to angular displacement about any axis. Accelerator 

 receptors are spontaneously active at rest: when activated, the response 

 consists of a burst at the onset of rostrum-down, side-down, or contra- 

 lateral horizontal rotation, followed by depression at the termination of 

 these movements. Other receptors respond to vibration conducted through 

 a solid substrate. When iron filings are introduced into the statocyst and a 

 magnet moved about the organ, discrete bursts of nerve impulses are 

 recorded. The statocyst thus records static positions and is sensitive to 

 angular movement and acceleration. It is not responsive to water-borne 

 vibrations (26, 27, 28). 



Gravity responses can occur in the absence of statocysts, e.g. Para- 

 mecium, Cerianthus, blastulae of Arbacia, etc. Some animals react to 



