150 ELEMENTARY BIOLOGY. [XII. 



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 chi- 

 tinous transparent sac will readily be found in it, 

 among the muscles &c.; this is the auditory sac 

 and is about J 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 objective. 



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. 



c. Examine with J- obj. 



a. Each of the hairs seen with the lower power is 

 now seen to be covered over its whole surface 

 with innumerable very fine secondary hairs ; 

 these are shortest near the base of the primary 

 hair. Towards its base each of the primary 



