434 



be called an "invagination cavity" resulting from the infolding of the 

 ectoderm which carries with it the layers of chiten it secretes. 

 Where the trabeculae unite with the surface cuticula, its colorless 

 peripheral layer becomes continuous with the inner laminae of the 

 surface cuticula, while the yellowish axial core, with its circle of 

 lacunae, is continued directly outwards through almost two-thirds the 

 thickness of the surface cuticle, — hg. 4. 



When the chitenous network forms a rather thin layer, as in the 

 eye region and elsewhere on the thoracic shield, the innermost 

 trabeculae unite to form a nearly continuous layer perforated by 



Pig. 4. 



pores that lead into the irregular sinuses above them. Such a section 

 is shown in fig. 4. We may divide the whole shell in such places 

 into several layers: — 



1) The thick outer cuticula strongly laminated and traversed 

 by two or three kinds of rather large canals which contain ducts 

 of mucous glands or nerve termination connected with minute spines 

 or hairs. All these canals reach the surface. Between them are 

 innumerable canaliculae (pore canals of authors). They are extremely 

 minute (not represented in the figure) and extend in a finely wound 

 spiral almost to the outer surface. The most superficial layer is thin, 

 colorless, vitreous, and devoid of canaliculi. Although very hard and 

 polished under some conditions it is easily destroyed; it constitutes 

 the ganoin layer. In surface views it is seen to be divided by 

 minute raised ridges into polygonal facets. The divergence of the 

 pore canals beneath the periphery of the facets helps to increase 



