CITTICIJLAR TISSUE. 195 



the strife are the optical indications of parallel undulating 

 canals which traverse the successive strata of the cuticle, 

 and are ordinarilj^ occupied by a fluid. AVhen this dries 

 up, the suri'ounding air enters, and more or less com- 

 pletely fills the tubes. And that this is really the case 

 may be proved by making very thin sections parallel with 

 the face of the exoskeleton, for these exhibit innumerable 

 minute perforations, set at regular distances from one 

 another, which correspond with the intervals between the 

 striae in the vertical section ; and sometimes the contours 

 of the arese which separate the apertures are so well 

 defined as to suggest a pavement of minute angular 

 blocks, the corners of which do not quite meet. 



When a portion of the hard exoskeleton is decalcified, 

 a chitinous substance remains, which presents the same 

 structure as that just described, except that the epios- 

 tracum is more distinct ; while the ectostracum appears 

 made up of very thin laminae, and the tubes are repre- 

 sented bj^ delicate strife, which appear coarser in the 

 region of the dark zones. As in the naturally soft parts 

 of the exoskeleton, the decalcified cuticle mny be split 

 into flakes, and the pores are then seen to be disposed 

 in distinct arese circumscribed by clear polygonal borders. 

 These perforated arefe appear to correspond with indi- 

 vidual cells of the ectoderm, and the canals thus answer 

 to the so-called " pore-canals," which are common in 

 cuticular structures and in the walls of many cells 

 which bound free surfaces. 



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