46 WILLIAMSON, ON SHELLS OF CRUSTACEA. 



tion is very distinct, especially in the anterior parts of the 

 carapace. This tissue obviously represents both the areolar 

 layer and the corium of fig. 16. In some parts of the tail, 

 where I have been unable to detect this tissue, its place is 

 occupied by a structureless pellicle apparently identical 

 with that represented in fig. 16 a : since I have distinctly 

 traced its extension over the granular layer where the latter 

 exists. On tearing a semi-calcified portion of the carapace 

 in which the areolae were but faintly discernible, the torn 

 margin appeared defined and angulated, corresponding with 

 the inter-areolar lines, proving the latter to be lines of less 

 perfect cohesion than the rest of the tissue. On decalcifying 

 the fragment thus torn the areolae almost entirely disap- 

 peared, demonstrating how much they owed to conditions of 

 calcification. 



The above example appears to throw direct light on the 

 genesis of crustacean integuments. It is clear that no cells 

 exist external to the basement membrane ; they all underlie it. 

 Whether or not the basement membrane is cast off along with 

 the exuviated shell has not yet been determined, but probably 

 such is not the case. The cellular derm appears to be the secret- 

 ing organ, producing the tegument, which latter permeates 

 the basement membrane by exosmosis, when in a fluid state, 

 and becomes consolidated into a structureless layer external to 

 the membrane. The laminae first formed are apparently those 

 constituting the pellicle, then those constituting the areolar 

 layer, and subsequently those forming the corium. Calcifi- 

 cation of these layers, both in its primary and secondary 

 varieties, is a process occurring subsequently to the formation 

 of the membranous lamellae, and probably due to some 

 protoplasmic fluid conveyed into the tissues by means of the 

 tubuli with which they abound. Prior to the formation of 

 these laminae some of the cells of the derm or enderon 

 change their form and position. The tubuli themselves pro- 

 bably exist as such prior to calcification ; the latter only 

 rendering them more distinct. 



The process of calcification appears to be identical with 

 what I have elsewhere shown to occur in the genesis of the 

 tooth of the Echinus ('Journal of Dental Science') and 

 in the scales and bones of fishes (' Phil. Trans.') Com- 

 mencing as a granular point, each calcareous atom increases 

 by concentric additions to its exterior, leading to cohesion of 

 separate particles, and consolidation of the entire tissue. As 

 no cells are traceable in the chitinous layers of membrane, 

 the formation of these granules, it must be admitted, goes on 

 independently of any direct cellular agency. 



