238 THE BIOLOGY OF THE SEA-SHORE 



(Loeb, 1905). That contact stimuli are not responsible for 

 this phenomenon has been demonstrated by the following 

 experiment. A Cerianthus is placed upon a piece of wire 

 netting resting on the sides of a glass vessel in the aquarium. 

 The animal gradually forces the base through one of the 

 meshes until it is in a vertical position with the tentacles 

 upwards. If the piece of wire is now turned completely 

 over, the animal does not pull itself out again, but the foot 

 near the tip turns vertically downwards until it reaches the 

 wire net when it is again pushed through as far as possible. 

 By reversing the wire net twice in this way a Cerianthus has 

 been made to push its foot through the meshes three times 

 (Loeb, op. cit.). 



Gravity is of fundamental importance in controlling the 

 orientation of the organism in space, and it is supplemented, 

 as we have seen, by light and by contact stimuli acting on 

 the nerve endings of the skin (stereotropic phenomena). 

 A great number of animals possess some form of organ 

 which has the special function of securing equilibrium, 

 viz. the statocysts of worms, crustaceans, and molluscs, 

 and the ear (semicircular canals) of vertebrates. The 

 stimuli leading to the maintenance of equilibrium are 

 produced, in invertebrates, by the contact of small particles 

 with the sensory cells lining the statocyst. The classical 

 proof of this was afforded by Kreidl. It is well known that, 

 when a crustacean moults, the lining of the statocyst is 

 shed, it being simply inturned cuticle. At the same time the 

 sand-grains are also got rid of, and must consequently be 

 renewed by the animal itself. Kreidl placed specimens of 

 Palcemon during the moult among a quantity of fine particles 

 of iron, and these particles, in default of anything more suitable, 

 were eventually transferred by the prawns to their statocysts. 

 By the use of a magnet Kreidl was then able to overcome the 

 force of gravitation and any inhibitory effect of vision, and 

 to orient the animal as he chose. 



Animals which are naturally devoid of otoliths, never- 

 theless show geotropic phenomena. In addition to the case 

 of hydroids, and of the anemone Cerianthus already quoted, 



