II.] THE CRAYFISH AND LOBSTER. 231 



in sea-water or salt solution and examine with one- 

 inch objective. 



a. The gritty matter will be seen to consist of a 

 number of irregular earthy particles, largely, if 

 not wholly, foreign to the organism; they are 

 functional as otoliths. Carefully remove them 

 with a camel-hair brush. 



ft. The white line will be seen to answer to a ridge, 

 on the summit of which is a row of large setae 

 (auditory seta or hairs), and both on the brown 

 patch and on the opposite side of the main row 

 will be seen scattered groups of smaller setae. 



f. Examine with ^ obj. 



a. Each auditory seta is now seen to be covered over 

 its whole surface with numerous very delicate 

 secondary setae ; these are shortest near the base 

 of the primary one. Towards its base each of the 

 primary setae is constricted and then dilates into 

 a bulbous enlargement for articulation upon the 

 wall of the sac. 



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

 a single layer of polygonal epithelial cells con- 

 taining pigment granules. 



y. By focussing through this epithelial layer a num- 

 ber of parallel fibres will be seen passing up, one 

 to the base of each seta of the main row. 



8. If a perfectly fresh auditory sac be put in i per 

 cent, solution of osmic acid for half an hour, and 

 be then well washed in distilled water and ex- 

 amined, each of the fibres mentioned above (ul- 

 timate ramifications of the auditory nerve) will be 



