156 ELEMENTARY BIOLOGY. [CHAP. 



wanting and there the membrane is seen to 

 possess oval nuclei. 



24. 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 joins in front 

 at an angle a rounded anterior portion. It bears 

 several tufts of hairs: one of these is very small and 

 lies at the inner side of the flattened surface, just at 

 the angle where it meets the rounded part ; among 

 these hairs 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. A 

 chitinous transparent sac will readily be found in 

 it, among the muscles &c. ; 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 aperture to the exterior. 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 streaks : mount in sea-water or sodic 

 chloride solution and examine with one inch ob- 

 jective. 



a. The white line will be seen to answer to a 

 ridge on the apex of which is a row of large 

 hairs, and both on the brown patch and on the 

 opposite side of the main row will be seen 

 scattered groups of smaller hairs. 



