172 EVENINGS AT THE MICROSCOPE. 



general surface of the body, and with long, delicate, mem- 

 branous filaments (seta, often improperly called cilia), 

 which are larger and much more delicate in structure 

 than the ordinary hairs. 



The basal joint is greatly enlarged : if it be carefully 

 removed from its connexion with the head, and broken 

 open, it will be found to inclose in its cavity a still smaller 

 chamber, with calcareous walls of a much more delicate 

 character than the outer walls. This internal cell is con- 

 sidered by Mr. Spence Bate to be a cochlea,* from its 

 analogy, both in structure and supposed use, to the organ 

 so named in the internal ear of man and other vertebrate 

 animals. It is situated, as has been said, in the cavity of 

 the basal joint of the internal antennae, and is attached to 

 the interior surface of its wall farthest from the median 

 line of the Crab. It has a tendency to a spiral form, but 

 does not pass beyond the limits of a single convolution. 



If this interior cell does indeed represent the cochlea 

 of more highly-constructed ears, to which it bears some 

 resemblance both in form and structure, then it seems to 

 identify, beyond dispute, these inner or upper antennas as 

 the organs of hearing. 



Now with this conclusion agrees well the manner in 

 whichthe living animal makes use of the organs in ques- 

 tion. The Crab always carries 

 them erect and elevated ; and 

 is incessantly striking the water 

 with them, with a very peculiar 

 jerking action, now and then 

 vibrating, and, as it has been 

 called, "twiddling" them. These 



,„ ,vb FE « ,m., f antenna, therefore, appear to 



be always on the watch : let 



* Cochlea, the Latin for Snail-shell, is the name given to one of 

 the cavities of the ear, from its resemblance to the interior of a. snail- 

 shell. 



