362 



PHYSIOLOGY OF THE INVERTEBRATA. 



This proves that the sense of smell is developed in Astacus 

 and that the olfactory organs are the antennulae. 



Auditoiy organs have been observed in the higher 

 Crustacea, especially in the Decapoda. In Astacus (Kg. 

 69, D), there is an auditory sac lodged in the basal joint 

 of each antennule. The upper face of the basal joint has a 

 snaall oval aperture, the outer lip of which is invested by 

 hairs directed inwards. This aperture leads into a wide 

 delicate sac, which contains a fluid in which minute sandy 

 particles (otoliths) are suspended. A ridge, formed of the 



Fig. 69. — The Olfactoky and Auditory Organs of Astacus. 



A = Antennule, with setae at d. C = d enlarged, showing olfactory 

 nerve (e), B = Antennule complete : a = exopodite ; b = endopodite ; 

 c = protopodite. D = auditory organ : / = nerve ; g = auditory 

 hairs (setse) ; A = otoliths. 



posterior and inferior wall of the sac, projects into its interior. 

 Each side of this ridge is covered with a series of delicate 

 setEe (auditory setse), which project into the fluid. An 

 auditory nerve, which enters the sacs, breais up into fine 

 fibrils that are distributed to the setse. A fibril passes 

 through the base right up to the summit of each seta, where 

 it terminates in a peculiar rod-like body. The sonorous 

 waves, transmitted through the water in which Astcccus lives 

 to the fluid and sandy contents of the auditory sac, are taken 

 up by the delicate nerve-endings and conveyed through the 

 auditory nerve to the brain or supra-cesophageal ganglion. 



