II.] THE CRAYFISH AND LOBSTER. 203 



of chitinous material, deposited in layers, alternately 

 denser and less dense. 



b. Compare a similar section of any calcified portion 

 of one of the appendages, ground down on a hone 

 (see Appendix E.). A similar longitudinal striation 

 is obvious. 



Examine the peripheral area; the striae — but a 

 tenth the diameter of those seen in the main mass — 

 are closely aggregated ; the free surface is invested 

 in an uncalcified epiostracum. The whole is per- 

 meated by a close set series of wavy pore-canals^ 

 usually filled with air in the process of manipula- 

 tion, and rendered thereby highly refractive. 



Compare tangential sections, made in a similar 

 manner. The pore-canals will be obvious as minute 

 black-dots, closely (in places irregularly) aggregated 

 together. 



An aggregation of the strijs, identical with that 

 described above, is frequently met with on the inner 

 side also. 



c. Cut a longitudinal vertical section of a piece of the 

 exoskeleton, together with the underlying in- 

 tegument, which has been decalcified by treatment 

 with i§ chromic acid solution. Note, in addition 

 to the structural features already described — 



a. The setcB ; each is an uncalcified outgrowth, 

 usually arising at the base of a shallow pit. 

 They vary greatly in size and detailed structure in 

 different parts of the body ; the commoner forms 

 end in a pointed extremity, fringed with lateral 

 hairlike filaments. 



