xii.] THE FRESH-WATER CRAYFISH. 157 



r. Examine with \ 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 

 hairs is constricted and then dilates into a 

 bulbous enlargement which is fixed to the 

 wall of the sac. 



(3. The brown patch is seen to owe its colour to 

 a single layer of polygonal epithelial cells 

 containing pigment granules. 



y. By focussing through this epithelial layer a 

 number of parallel slightly granular bands is 

 seen passing up, one to the base of each hair 

 in the main row on the top of the ridge. At 

 the base of the hair to which it runs, each 

 band is constricted and, entering the bulbous 

 enlargement of the hair, joins a small hemi- 

 spherical swelling within it. 



B. If a fresh auditory sac be put in i per cent, 

 solution of osmic acid for half an hour, and 

 then laid for twenty-four hours in distilled 

 water and examined, each of the granular 

 bands mentioned above is seen to consist of 

 a bundle of fine fibres which swell out into 

 fusiform enlargements at intervals. 



c. A great part of the whole interior of the audi- 

 tory sac of the lobster is covered with very 

 fine hairs which can only be seen with a high 

 power. Epithelium is absent except the pig- 

 mented patch above mentioned. 



