120 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



from the general curvature of the inner face, but usually 

 nearly coincides with it. 



When a longitudinal or a transverse section is taken 

 through the whole eyestalk, the optic nerve (fig. 28, 

 A, op) is seen to traverse its centre. At first narrow 

 and cylindrical, it expands towards its extremity into 

 a sort of bulb (B, ^), the outer surface of which is curved 

 in correspondence with the inner surface of the cornea. 

 The terminal half of the bulb contains a great quantity 

 of dark colouring matter or pigment, and, in section, 

 appears as what may be termed the inner dark Z07ie (/). 

 Outside this, and in connection with it, follows a white 

 line, the inner ivhite zone (e), then comes a middle dark 

 zone (d) ; outside this an outer pale band, which may 

 be called the outer ivhite zone (c), and between this and 

 the cornea {a) is another broad band of dark pigment, the 

 outer dark zone (b). 



When viewed under a low power, by reflected light, this 

 outer dark zone is seen to be traversed by nearly parallel 

 straight lines, each of which starts from the boundary 

 between two facets, and can be followed inwards through 

 the outer white zone to the middle dark zone. Thus the 

 whole substance of the eye between the outer surface of 

 the bulb of the optic nerve and the inner surface of the 

 cornea is marked out into as many segments as the 

 cornea has facets ; and each segment has the form 

 of a wedge or slender pyramid, the base of which is 

 four- sided, and is applied against the inner surface of 



