60 THE CRAYFISH [CH. II 



In the basal joint of the first antenna is situated the 

 ' auditory ' sac (otocyst), whose aperture is protected with 

 branching ' guard ' setse and whose curved floor bears two 

 rows of hollow, almost simple, sensory setse which are 

 moveably articulated to the wall of the sac. Within the 

 sac are numerous otoliths, which are chiefly, if not entirely, 

 foreign bodies, such as grains of sand. These are scattered 

 by the chelae over the aperture of the sac and thus fall into 

 it. It is highly probable that this organ is of more service 

 as an instrument for informing the crayfish of its attitude 

 and for the maintenance of the equilibrium of the body 

 than as an organ of hearing. Martha Bunting 1 has proved 

 experimentally that extirpation of both otocysts brings 

 about a complete upset of the animal's sense of equilibrium. 

 A crayfish thus mutilated will swim about upon its back 

 for a long while apparently unconscious of the inverted 

 posture. 



Kreidl 2 by a beautiful experiment, which in this 

 country has not received the attention that it deserves, 

 has indeed placed the matter beyond the region of dispute. 

 Remembering that at each moult the otoliths (statoliths) 

 are removed with the cast-off cuticle, and that fresh foreign 

 particles are then put in by the animal, Kreidl conceived 

 the idea of causing some force other than that of gravity 

 to act upon them. Finely powdered particles of iron were 

 accordingly supplied to specimens of Palcemon xiphios 

 and P. squilla (marine Decapods allied to the crayfish) 



1 Pflilger's Archiv f. d. ges. Physiol. LIV. p. 531. 



2 Sitz. Wiener Ak. d. Wiss. en. 1893, Abth. 3; and cf. Hensen, Zeit. 

 wiss. Zool. xin. 



