232 ELEMENTARY BIOLOGY. [CHAP. 



seen to terminate, near the free end of the seta, 

 in an elongated end organ. 



The auditory sac in the Crayfish is very similar 

 to that in the Lobster, and may be examined in a 

 similar way. It is, on the whole, not so good, 

 chiefly on account of its smaller size. 



2. Examine, in the Crayfish, the exopodite of the an- 

 tennule; it exceeds the endopodite in calibre and 

 length, and is carried, during life, upwardly directed. 

 (Cf. Sect. B. 4 m.) Remove it from the rest of the 

 appendage and examine in water, under a low power. 



a. Its setce ; short filaments, developed from the an- 

 terior borders of the several joints. 



b. Its sensory setce; developed in tufts, from the under 

 face of the filament. There are usually two sets 

 for each segment and they slope obliquely down- 

 wards and forwards. 



c. Examine under \ or \ obj. Each seta is seen to be 

 subdivided into two segments ; a shorter basal one 

 and an elongated terminal one: in the ordinary 

 seta? the latter is pointed, in the sensory ones it 

 is expanded and spatulate. 



More or fewer of the terminal segments of the 

 sensory setae are, not unfrequently, globular and 

 abnormal. 



3. The visual organ. Place the eye of a Crayfish for 

 four or five days in 0-5 per cent, solution of chromic 

 acid and then for twenty-four hours or more in 

 alcohol. 



a. Strip off a portion of the cornea and examine under 

 a \ obj. It is marked out into a great number of 



