354 Miscellaneous. 



especially destined to contain the colouring matter of the shell ; it 

 has a peculiar organization, and always contains the base of the hairs 

 and the corneous tubercles : this is the pigmental layer. The in- 

 ternal layer is much the thickest, and constitutes nearly the whole 

 shell ; in it are found the passages of the hairs, tubercles and spines, 

 as well as a great number of small irregular bodies of an organic 

 nature : this is the dermal layer. 



These two last layers are the only ones in which the carbonate of 

 lime is deposited ; they have a nearly analogous organization. Under 

 a weak magnifier, we ascertain that they are formed throughout of 

 extremely fine and delicate lines, whose general and striking cha- 

 racteristic is that they are constantly parallel. This organization 

 exists in the great majority of cases, and we remark that, when it is 

 absent, or difiicult to detect, the dermal layer presents iridescent 

 tints, often as brilliant as those of the most beautiful shells (the 

 Anomouri). These lines are not produced by independent and super- 

 posed layers, for the shell is not separable into laminae correspond- 

 ing to these lines. By means of a veiy high magnifying power we 

 can perceive that these lines form part of one whole. The intimate 

 organization of the shell therefore presents itself under three principal 

 forms : 1st, we only find extremely thin filaments, joined to one 

 another and directed from within outwards, perpendicularly to the 

 surface ; these filaments, becoming thicker and more opake at simi- 

 lar levels, give an appearance of parallel lines : 2nd, these filaments 

 exist, but are traversed at right angles and along parallel zones, 

 by other bundles of filaments ; from these last issue ramifications 

 which anastomose with the adjacent zones and thus reunite all the 

 bundles : 3rd, the perpendicular filaments no longer exist, and we 

 only meet Vv^ith parallel bands, from which issue very irregular rami- 

 fications which unite with the adjacent bands. 



The hairs of the decapodous Crustacea are simple or barbed ; they 

 never have barbules. They are not a prolongation of the epidermal 

 layer ; they are always in communication with the interior of the 

 shell by a canal which traverses in a straight line the thickness of 

 the carapace, and which is one while empty and at another filled 

 with a matter similar to that which exists in the interior of the hairs. 

 They all have a central canal filled with a marrow analogous to that 

 which is found in the hairs of the higher animals. They all grow 

 from a rounded part, which has the greatest analogy with bulbs. 

 These sorts of bulbs are always situated in the pigmental layer. 

 The irregular bodies which cover certain Crustacea, and in particular 

 Pisa tetraodon, are only hairs, the barbs of which are coherent. 



The spines of the decapodous Crustacea appear to be continued in 

 the epidermal layer, with which they have the greatest analogy in 

 appearance and composition. We find in them a very considerable 

 number of small canals analogous to those of the hairs, and which, 

 like the last, traverse the whole shell to reach the spine. 



With respect to the tubercles often found in the pigmental layer, 

 and each of which has a small canal by means of which they com- 

 municate with the interior of the shell, we can only consider them as 



