42 WILLIAMSON, ON SHELLS OF CRUSTACEA. 



the pellicular and areolar layers. These calcareous concre- 

 tions display a coarse lamination, indicating the direction of 

 the laminae of the uncalcified corium, with which they are 

 parallel. They are also penetrated by myriads of the usual 

 undulating tubules, which appear to be prolonged, though 

 less distinctly, through the uncalcified corium. I see no 

 reason for doubting that the latter are tubuli as well as the 

 former. 



In a thin horizontal section of this integument, made 

 through the upper part of the corium, which has been 

 decalcified (fig. 12), we see delicate circular areas, with 

 defined outlines and a central canal, indicate the position 

 occupied by the calcareous concretions. This is important, 

 since it shows that cell-like appearances may exist in the 

 decalcified membrane, which, nevertheless, are entirely due 

 to peculiarities of calcification ; cells having taken no part in 

 their origin. 



Vertical sections of Portumnus depurator display even still 

 more distinctly than the last Crustacean the dome-shaped 

 cylinders of the areolated layer, and remove every possible 

 doubt that could remain as to the non-cellular origin of this 

 layer. The conclusion is inevitable, that the whole results 

 from peculiarities attending the process of calcification. It is 

 clear that in the Crustacea, two distinct co-existent calcifying 

 processes are very commonly in operation, the one resulting 

 in a more intense calcification than the other ; their greater 

 density and higher refracting power giving to the former 

 the definite outlines seen both in the areolar layer generally, 

 and in the lenticular discs of the shrimp. 



Professor Quekett describes the areolated texture as con- 

 spicuous in the crayfish. I have only examined young 

 specimens, but in them it was very indistinct, if not wholly 

 absent. I found the four layers revealed in the vertical sec- 

 tion (fig. 13), but the areolar layer (fig. 13 b) and the calcified 

 corium (fig. 13 d) occupied the greater portion of the sub- 

 stance, the pellicle (fig. 13 a) and the uncalcified corium (fig. 

 13/; being very thin. That which is the obvious homologue 

 of the areolar layer in other Crustaceans is thick and very 

 distinctly tubulated, the tubuli being distributed with the 

 greatest uniformity. This layer would scarcely be distinguished 

 from the subjacent calcified corium but for its greater trans- 

 lucency. 



An anomalous example of the common lobster furnished 

 new and interesting modifications of crustacean integument. 

 The pellicular layer was so thin as to be scarcely traceable. 

 The areolated layer, the chief seat of colour, was very distinct 



