194 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
which may occupy a sixth or a seventh of the thickness 
of the whole, which is more transparent than the rest, 
and 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 1-500th of an inch thick, but in the inner part they 
become very thin, and the lines which separate them 
may be not more than 1-8000th of an inch apart. 
Fine, parallel, close-set, vertical strie (¢) 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 magnifying power is employed, it is seen that 
these striz, which are about 1-7000th 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 stris 
‘being taken by threads of air of such extreme tenuity, 
that they may measure not more than 1-30,000th of an 
inch in diameter. It is to. be concluded, therefore, that 
