XII.] THE FRESH-WATER CRAYFISH. 151 



hairs is constricted and then dilates into a 

 bulbous enlargement which is fixed to the 

 wall of the sac. 



. The brown patch is seen to owe its colour to a 

 single layer of polygonal epithelial cells con- 

 taining pigment granules. 



7. 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. Afc the 

 base of the hair to which it runs, each band is 

 constricted and, entering the bulbous enlarge- 

 ment of the hair, joins a small hemispherical 

 swelling within it. 



8. If a fresh auditory sac be put in 1 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. 



e. 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. 



d. The auditory sac in the crayfish is very similar to 

 that in the lobster, and may be examined in a 

 similar way. It is however not so good, both on 

 account of its smaller size and because the audi- 

 tory hairs, although longer, are collected in a close 

 tuft, which makes it more difficult to se6 the 

 manner of their insertion. 



