194 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



wliich maj' occupy a sixth or a seventh of the thickness 

 of the -whole, which is more transparent than the rest, 

 nnd often presents hardly any trace of horizontal or 

 vertical striation. When it appears laminated, the strata 

 are very thin. This zone may be distinguished as the 

 ectostracum (b), from the endostracum (c), which makes 

 up the rest of the exoskeleton. In the outer part of the 

 endostracum, the strata are distinct, and may be as much 

 as l-500th of an inch thick, but in the inner part they 

 become very thin, and the lines which separate them 

 may be not more than l-8000th of an inch apart. 

 Fine, parallel, close-set, vertical striae (e) traverse all the 

 strata of the endostracum, and may usually be traced 

 through the ectostracum, though they are always faint, 

 and sometimes hardly discernible, in this region. When 

 a high magnifyiEg power is employed, it is seen that 

 these striae, which are about l-7000th of an inch apart, 

 are not straight, but that they present regular short un- 

 dulations, the alternate convexities and concavities of 

 which correspond with the light and the dark bands 

 respectively. 



If the hard exoskeleton has been allowed to become 

 partially or wholly dry before the section is made, the 

 latter will look white by reflected and black by trans- 

 mitted light, in consequence of the places of the striae 

 being taken by threads of air of such extreme tenuity, 

 that they may measure not more than l-30,000th of an 

 inch in diameter. It is to be concluded, therefore, that 



