86 FORMATION OF SHELLS OF ANIMALS, ETC., 
the other. These lines exist in all parts of the shell of 
crustaceans, if only of sufficient thickness ; as, for instance, 
in those embedded in muscular fibres, in which they can 
have no communication with any cavity, so as to make 
them analogous to the fibres of dentine; but they are 
merely, as in the artificial products, indications of the 
directions in which attraction is exerted, with variable 
effect, upon the rows of molecules, producing alternate 
lines of strong attraction and feeble attraction, or of 
strong cohesion and weak cohesion; and thus this kind 
of arrangement of the molecules of carbonate of lime is 
altogether unconnected with any particular or specific 
function performed by the shell, as shown by its existence 
in all parts of it—the horny, as well as the calcareous— 
and where the shell answers altogether different purposes, 
as, for instance, where it serves for a covering and where 
it answers the purpose of bone; but these lines are merely 
the necessary consequence of the manner in which shell is 
formed, and, under such circumstances, could not have 
been otherwise. Dr. Carpenter observes, that “the 
calcareous layer of the crab-shell is composed of a sub- 
stance exactly analogous to ivory, being very transparent, 
and apparently homogeneous, when cut into very thin 
slices, and being perforated by an immense number of 
minute sinuous tubuli, which run nearly parallel to one 
another, from one surface of the shell to the other. This 
arrangement may be seen by making a thin section of any 
part of the shell; but it may be shown particularly well in 
the end of the claw, which is thicker and of denser 
texture than the rest. A transverse section of this shows 
the tubes radiating from the central cavity towards the 
external surface, and would, I feel assured, be regarded by 
the most experienced observer as the section of a tooth, if 
