II.] THE CRAYFISH AND LOBSTER. 23 1 



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. 



y8. The white line will be seen to answer to a ridge, 

 on the summit of which is a row of large setae 

 {auditory setce 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. 



c'. Examine with i 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. 



p. 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 



