INTEGUMENT 



363 



that bounds their proximal surfaces, and here, accordingly, we see no such 

 strength and straightness of the fibrils. They are somewhat branched 

 in these cells and of much more delicate for- 

 mation. 



This epithelium is also interesting because 

 of the fact that connective-tissue cells have 

 wandered among the bases of the cells, and 

 because it is an example of an epithelium 

 without a basement membrane. The lateral 

 boundaries of the cells are very thin and very 

 difficult to see. As in the case of other epi- 

 thelia that are used to form a shell of lime, 

 the nuclei of the cells are nearer the distal end 

 of the cell than those of the majority of other 

 epithelial cells. This latter fact is particularly 

 true of the plecypod mollusks. 



The cuticle that we find in the earthworm 

 is represented in the lobster by a thick struc- 

 ture, the shell, which is a real cuticle of or- 

 ganic material, stiffened and hardened by the 

 deposition of salts of lime. The organic base 

 of the shell consists of a square-meshed reticu- 

 lum built up of fibrils (see Fig. 329). In the 

 meshes of this reticulum is a more weakly devel- 

 oped groundwork of organic substance, the 

 whole plastic structure being combined with 

 the lime salts much as it is in bone. 



The specimen that we are studying is a 

 section of the integument of a lobster that 

 had just shed its shell, and had not yet had 

 the lime salts deposited in the new one. 

 Thus it may be assumed that we have the 

 best view obtainable of the organic structure 

 of the shell, and it can be seen that it is 

 stratified, with the strata somewhat thicker 

 and less distinct the farther they lie from 

 the cells. The darker parts of the substance 

 that are thinnest, and which serve to separate the strata from each 

 other, are the horizontal lines and meshes of the reticulum, and are much 

 easier to distinguish than the vertical lines. The layers thus seen are 

 to be grouped into a number of regions in the thickness of the shell. The 

 best grouping of these layers seems to be into an outer and very thin region 

 that is denser and darker than the rest of the shell, a somewhat thicker 



If 



FIG. 329. Slightly schematic 

 figure of the structural 

 groundwork of a lobster's 

 shell. The figure is discon- 

 tinuous at x, and incomplete 

 on the upper surface for 

 want of room. (After 

 SCHNEIDER.) 



