396 TEGUMENTARY ORGANS 



resembling those under the epiderm on the outer surface. Where the 

 nacreous appearance commences, these ares disappear, becoming- 

 obscured by an opaque white substance, which is marked by elevations 

 and depressions, corresponding with, though less prominent than, the 

 principal ones upon the external surface. 



If, now, a section perpendicular to the surface and to the concentric 

 lines be taken, and viewed in the same way by reflected light, the 

 cause of the various appearances which have been described will become 

 obvious. 



It will be seen that the thick middle of the shell is composed of 

 three substances; of a very thin external brown layer, the "epidermis," 

 and of two other layers more or less equal in thickness ; an external,, 

 composed of minute polygonal prisms or columns set perpendicularly 

 to the surface, and an internal, which looks structureless, with a 

 fracture like loaf sugar. The outer prismatic layer preserves its 

 thickness as far as the " brown zone " above described, and then 

 gradually thins out into the flexible marginal membrane. The inner 

 nacreous layer, on the contrary, gradually thins out, and ceases at 

 the commencement of the brown zone. The ends of the prisms are, 

 therefore, bare in the brown zone, whence the polygonal areolation 

 observed in it ; while its colour arises partly from the brown epidermis 

 shining through, partly from a slight tinge of the same kind which 

 runs through the prismatic substance, and renders it distinguishable,, 

 even to the naked eye, from the intensely white nacreous layer. 



Thin vertical sections of these shells present the following 

 appearances under a high magnifying power. The external edge is 

 constituted by a delicate brown band, the " epidermis," in which no 

 structure of any kind can be detected. Within this is the prismatic 

 layer, a dense transparent substance marked by strong parallel lines 

 which run perpendicularly to the surface and either extend completely 

 through the layer, or terminate by joining some other within it. In 

 the former case, the spaces which they enclose appear like the sections 

 of prisms (of -j-^ of an inch, more or less, in diameter) : in the latter, 

 they resemble longer or shorter cones whose bases are turned 

 outwards. A number of such short cones are usually interposed 

 between those ends of the prisms which are in contact with the 

 epidermis. 



Internally, or at the line of contact of the prismatic with the 

 nacreous layer, the lines either remain parallel or converge. 



The prisms are readily broken away from one another, and in this 

 case, or in a sufficiently thin section of the whole layer, they are seen 

 to be traversed by very closely set parallel transver.se lines about 



