230 ELEMENTARY BIOLOGY. [CHAP. 



Composed of slender fibres of variable size, each 

 consisting of a structureless outer wall, surrounding 

 a finely granular or obscurely fibrillated central 

 axis. Nuclei ; seen at intervals. 



b. Tease up in water, a ganglion which has been 

 treated with osmic acid. 



Composed of large oval branched cells, each 

 consisting of a granular mass in which lies a clear 

 round nucleus, containing a nucleolus. 



K. The sense organs. 



i. The auditory organ. This lies in the basal joint of 

 the antennule and is best examined in the Lobster. 

 The upper surface of this basal joint is flat posteriorly 

 and arched in front. It bears several tufts of setae : 

 one of these is very small and lies at the inner side of 

 the flattened surface, just at the angle where it meets 

 the arched part ; among these setae is the opening into 

 the auditory sac, through which a bristle can easily be 

 passed. 



a. Take a fresh antennule from a Lobster and cut 

 away the under surface of its basal joint, with a 

 scalpel. A transparent chitinous sac will readily be 

 found in it, among the muscles ; this is the auditory 

 sac and is about ^ of an inch long. Carefully dissect 

 it out. 



b. If this sac be held up to the light a little patch of 

 gritty matter will be seen on its under surface near 

 the external aperture. Behind this can be seen a 

 curved opaque line; behind this, and concentric 

 with it, a shorter brownish streak. Cut out carefully 

 the part of the sac which bears these marks : mount 



