162 



the otoeyst does not become functional until the megalopa 

 stage. 



Hensen and the early observers believed that the 

 otoeyst was an auditory organ. Hensen found that the 

 " auditory " setae were individually sensitive to sound 

 vibrations of different frequency, and concluded that in 

 the lobster the auditory organ had a range of three 

 octaves. 



Kreidl was the first to deny that the otocysts 

 possessed an auditory function. He substituted iron 

 filings for the otoliths, and the experiments led 

 him to believe that this organ served the function of 

 equilibration. 



Bethe, while accepting Kreidl's results, did not reject 

 the idea that the otoeyst was also an auditory organ. 



The researches of Prentiss led him to believe that the 

 otoeyst was a static organ solely. It is probable that in 

 the Brachyura the hooked setae and grouped setae have 

 lost most of their functional activity owing to the absence 

 of otoliths. The thread setae are, undoubtedly, the most 

 important sensory organs of the otoeyst. (For a further 

 discussion of this subject the reader is referred to the 

 paper by Prentiss.) 



The sensory setae of Cancer are of two kinds. 



The tactile setae may be present in various parts of 

 the body. They are found on the antennules and 

 antennae, and on most of the appendages. The setae of 

 the otocysts are probably modified tactile setae. They 

 are characterised by having a long tapering shaft which 

 bears barbs, and each seta is innervated by a single nerve 

 fibre and a single nerve cell. 



The olfactory setae (fig. 74) are short and blunt, and 

 are much more firmly attached to the integument than 

 the tactile setae. Each seta is divided into proximal and 



