36 WILLIAMSON, ON SHELLS OF CRUSTACEA. 



prefer applying to it the name of pellicle, since it only con- 

 stitutes a small portion of the true epidermal tissue of the 

 crab. In like manner I would designate the second, the 

 areolated layer, instead of employing Carpenter's term 

 cellular, which implies what appears to be erroneous. The 

 third, tubulated layer, Avhich all the above writers have shown 

 closely to resemble dentine, may be designated the calcified 

 corium, whilst the innermost layer described by Professor 

 Huxley may be called the uncalcified corium. 



It is unnecessary again to describe the arrangement of 

 these layers. Dr. Carpenter has pointed out the tubulated 

 structure of the corium, and the inflections of its laminae, 

 which at numerous points ascend like flat-topped cylindrical 

 pillars, penetrating the areolar layer, and reaching the 

 pellicle, where they occasion the white spots seen on the ex- 

 terior of the crab's shell. 



The areolae of the second layer, when viewed superficially, 

 present various aspects. Ordinarily they appear in the form 

 of dotted hexagonal spaces separated by translucent lines, the 

 former having defined outlines, which are darkest on the side 

 remote from the light, indicating projection and the forma- 

 tion of a shadow. In other cases the areolae are of a lighter 

 hue than the intervening lines, which are dark and defined. 

 Oblique sections of these latter varieties show that at a very 

 little depth below their outer surface the dark lines pass 

 into others which are lighter than the areolae; and still 

 deeper down, the areolae themselves become so merged, that 

 all distinct areolation ceases to be visible. These facts indi- 

 cate that the areolation is due to some conditions affecting 

 the surface of the areolated layer on the line of junction 

 between it and the pellicle. That this is the case will be 

 demonstrated by an examination of some other forms of 

 Crustaceans. 



When viewed superficially, numerous vertical tubules are 

 seen in the areolae. When a thin vertical section has been 

 quickly mounted in hot Canada balsam, so as to prevent the 

 latter from displacing the air in the tubules, these arc suffi- 

 ciently conspicuous. The entire layer is seen to be composed 

 of very thin laminae which follow the inflections of the sur- 

 face of the subjacent corium, and the direction of these 

 tubules is affected by the curves of the laminae, which they 

 penetrate nearly at right angles. On approaching the pro- 

 jecting pillars of the corium, the areolated laminae bend up- 

 wards, parallel with the sides of the pillars. Whether or 

 not the former are continued across the circular extremities 

 of the latter in a verv thin and non- areolated state I -m\\. 



