CUTICULAR TISSUE. 193 



pulled with needles in the direction of its depth, it 

 stretches to eight or ten times its previous diameter, 

 the clear intervals between the dark bands becomins 

 proportionally enlarged, especiallj^ in the middle of the 

 slice, while the dark bands themselves become apjoarently 

 thinner, and more sharply defined. The dark bands 

 may then be readily drawn to a distance of as much as 

 l-300th of an inch from one another; but if the slice is 

 stretched further, it splits along, or close to, one of the 

 dark lines. The whole of the cuticular layer is stained 

 by such colouring matters as hematoxylin ; and, as the 

 dark bands become more deepl}^ coloured than the inter- 

 mediate transparent substance, the transverse stratifi- 

 cation is made very manifest by this treatment. 



Examined with a high magnifying power, the trans- 

 parent substance is seen to be traversed by close-set, 

 faint, vertical lines, while the dark bands are shown to 

 be produced by the cut edges of delicate laminae, having 

 a finely striated appearance, as if they were composed 

 of delicate parallel wavy fibrillse. 



In the calcified parts of the exoskeleton a thin, tough, 

 wrinkled epiostracum (fig. 56, B, a), and, subjacent to 

 this, a number of alternately lighter and darker strata 

 are similarly discernible : though all but the innermost 

 lamin£e are hardened by a deposit of calcareous salts, 

 which are generally evenly diffused, but sometimes take 

 the shape of rounded masses with irregular contours. 



Immediately beneath the epiostraciun there is a zone 



