35 



On some Histological Features in the Shells of the 

 Crustacea. By Professor W. C. Williamson, F.R.S. 



In the Report of the British Association for 1848, Dr. 

 Carpenter called attention to the fact that the tegumentary 

 shell of the common crab chiefly consists of a tubulated 

 structure closely resembling dentine. Since the publication 

 of that report, the subject has been further investigated by 

 Professor Quekett ( f Lectures on Histology/ vol. ii, 1854), 

 and by Professor Huxley (' Cyclopaedia of Anatomy and 

 Physiology/ art. Tegumentary Organs, 1855-6). Each of 

 these later writers have contributed much to our previous 

 stock of information on this subject. But as discrepancies 

 exist between the conclusions at which they have arrived, a 

 new investigation of the matter became desirable ; hence the 

 present communication. 



Dr. Carpenter has described the shell of the crab as con- 

 sisting of three layers. " 1st, a horny structureless layer 

 covering the exterior ; 2d, a cellular stratum ; and 3d, a 

 laminated tubular substance." Mr. Huxley points out in 

 addition the existence of a soft uncalcified laminated struc- 

 ture, lining the inner surface of the shell ; consisting, in fact, 

 of similar layers to those external to it, but not yet calcified. 

 My examination of the shell of the common crab confirms 

 Professor Huxley's conclusion that it consists of four hori- 

 zontal textures. Both Professors Carpenter and Huxley de- 

 scribe the outermost one as horny, but it is obviously calca- 

 reous, disappearing much more completely under the action 

 of solvent acids than any of the others. The second or 

 " cellular" layer of Dr. Carpenter is that respecting which 

 the chief difference of opinion exists. Professor Huxley 

 denies the " cellular" character attributed to it by Carpenter 

 and Quekett, affirming that the cell-like areolae result from 

 a peculiar additional deposit of calcareous matter in the 

 uppermost layers of the shell. With this conclusion I 

 thoroughly agree, since oblique sections of the layer demon- 

 strate that the areolae, which in their superficial aspect so 

 closely resemble epithelial cells, sink deeply into the sub- 

 stance of the shell, gradually becoming less distinct and 

 definite as they descend into its lower laminae. 



Dr. Carpenter and Lavalle have designated the outermost 

 of these layers " the structureless epidermis ;" but I would 



