STRUCTURE OP SHELL. 553 



ing the lower edge of the section always thicker than the 

 upper. 



Dr. Carpenter describes the shell of the Crab and Lobster 

 as being composed of three layers, viz. the epidermis 01 

 cuticle, the rete-mucosum or pigment, and the coriuin. 

 The epidermis is of a horny nature, being generally more 

 or less brown in colour, and under the highest magnifying 

 powers presenting no trace of structure (fig. 247, No. 2) ; 

 it invests all the outer parts of the shell, and has in many 

 instances large cylindrical or feather-like hairs developed 

 from certain portions of its surface. The rete-mucosum, 

 or pigment cells, consist of either a series of hexagonal 

 cells, forming a distinct stratum, or of pigmental matter 

 diffused throughout a certain thickness of the calcareous 

 layer. (Fig. 247, No. 5.) In the Crab and Lobster it is 

 very thin, but in the Crayfish it occupies in some parts 

 more than one-third of the entire thickness of the shell ; 

 when examined by the microscope, this portion appears to 

 be composed of a large number of very thin laminae, which 

 are indicated by fine lines taking the same direction on 

 the surface of the shell, the number of lines being the 

 greatest in the oldest specimens ; these layers, even in the 

 Crayfish, are covered 'by a thin stratum of very minute 

 hexagonal cells, without any trace of cell matter in their 

 interior. The corium is the thickest layer of the three, 

 being the one on which the strength of the shell depends, 

 in consequence of the calcareous material deposited in it. 

 (Fig. 247, No. 4.) When a vertical section of the shell of 

 the Crab is examined, it is found to be traversed by parallel 

 tubes, resembling those in the dentine of the human tooth ; 

 these tubes extend from the inner to the outer surface of the 

 shell, and are occasionally covered by wavy lines, probably 

 those of growth, shown in a portion of No. 3, fig. 247. If 

 a horizontal section of the same shell be made, so that the 

 tubes be divided at right angles to their length, the sur- 

 face will clearly exhibit their open ends, surrounded by 

 calcareous matter. In Shrimps and very small Crabs, the 

 deposition of the calcareous matter takes place in concen- 

 tric rings like those of agate ; and occasionally small cen- 

 tres of ossification, somewhat like Pinna, with radiating 

 striae, are met with in the Shrimp. If the calcareous 



