II.] THE CRAYFISH AND LOBSTER. 233 



minute facets; mostly square, more rarely hexagonal 

 or irregular. 



b. Tease up in weak glycerine a portion of the con- 

 tents which underlie the cornea. Look for — 



a. Crystalline cones ; elongated angular transparent 

 bodies, each of which may or may not be in con- 

 tinuity with 



y8. A striated spindle or rhabdome; fusiform and 

 about a third the length of a. It is striated in 

 appearance and occurs in relation to a cellular 

 pigmented investment (retinula). 



c. Imbed an eye similarly prepared and cut a number 

 of longitudinal sections from it : mount in glycerine 

 and examine under one inch objective. Look for 

 the sections which have passed through its middle; 

 they will be seen to present a central mass (optic 

 bulb. Cf. Sect. J. 2 a.) from which a number of lines 

 appear to radiate to the facets on the surface. 

 These radiating lines (which are obscured here and 

 there by concentric pigmented layers) are indica- 

 tions of the striated spindles and crystalline cones. 



a. The exoskeleton ; calcined to form two segments, 

 a smaller basal annular one and a larger terminal 

 dome-shaped one, united by an inter-articular 

 membrane (Cf. Sect. B. 4 «.). That of the ter- 

 minal segment is thick and dense, except where 

 it overlies the crystalline cones ; there it is thin 

 and transparent, forming the cornea. 



/J. The muscles; seen on either side, immediately 

 beneath a, they pass between the two segments, 

 as between those of any one appendage. (Cf. 

 Sect. C.) 



