SENSE ORGANS 



215 



against the hairs, and thus enabUng it to keep its equiUbrium. If 

 the statocysts be removed, the crayfish loses its sense of position 

 and will often swim upside down. Presumably the sand grains 

 falling to the bottom of the sac by gravity stimulate nerve- 



,ex. 



8 



9rn. 



Fig. 146. — The statocyst of the crayfish. 



A , The right antermule, seen from the median side with the basal joint opened and the flagella cut short ; 

 B, basal joint of the left antennule from above ; C, two hairs from the statocyst. — C partly after 

 Howes. 



en., Inner flagellum ; ex., outer flagellum ; grn., sand grains ; n., nerve of the statocyst ; n.f., nerve fibres ; 

 0., opening of the statocyst ; stc. statocyst. 



endings, so that the necessary muscular movements for the 

 maintenance of position are produced. It has been possible to 

 replace the sand grains in the statocysts of a prawn by iron 

 filings, and the animal can then be induced, by a magnet held 

 above it, to turn over on its back. The antennules bear on their 

 outer flagella bristles which are sensitive to chemical substances. 

 Various of the setae, especially those of the antennae, are organs 

 of touch. 



