44 WILLI AM SON j ON SHELLS OF CRUSTACEA. 



depurator and pusillus, and numerous other forms, which 

 illustrate an organization that attains its highest development 

 in the common crab ; whilst in Portumnus depurator, the 

 diversion of the upper laminae of the corium from their 

 horizontality appears at its minimum. 



Thus far it would appear that in all the Podopthalmous 

 Crustaceans, the integument consists of four layers. An outer 

 or superficial, almost structureless pellicle («). An areolated 

 layer (b), but from which the areolae may be occasionally 

 absent. A calcified corium (d), and an inner layer of uncal- 

 cificd corium (/), which may become calcified as newer layers 

 appear within it. Each of these layers varies exceedingly in 

 the relative development attained, but traces of each of them 

 always exist ; each one becoming in turn the predominant 

 element in the integument of the various Crustaceans. The 

 corium appears to be more or less tubulated, whether calcified 

 or uncalcified. The areolar layer is often, though perhaps less 

 frequently so. Areolation does not appear to be confined to 

 the " areolar" layer, since it ocenrs on a small scale in the 

 corium of the crab and lobster ; wherever it exists in the 

 examples which I have recorded, either of corium or areolar 

 layer, it is unquestionably the result of peculiar conditions 

 under which the calcific matter is deposited, and not of cellu- 

 lation. In the calcified corium of Corystes Cassevilaunus , we 

 find an illustration of the secondary calcification to which 

 areolation appears due. The lime exists in two distinct con- 

 ditions ; in the one it is equally diffused through the corium. 

 But within this are large irregular masses of a totally 

 different aspect evidently the result of a secondary process. 



But Professor Huxley has already pointed out that areola- 

 tion exists in the uncalcified membranes of the articulations 

 of the claws of the common crab, where it cannot possibly be 

 due to any process of calcification. In the latter example 

 the areolae appear to me to be tubulated portions of integu- 

 ment, and the intervening reticulate lines to be merely spaces 

 from which the tubules are absent, as is unquestionably 

 the case in the areolae sometimes seen in the calcified corium 

 of the common crab. Areolation appears to me invariably 

 accompanied by tubulation, whilst the variations in the 

 arrangement of these tubules, and the peculiar calcification 

 which renders the areolae so visible, appear to be common 

 results of some single cause. "What that cause is, remains to 

 be ascertained. 



There still remains for consideration the question of the 

 origin of these various layers, and their relations to the 

 deeper portions of the crustacean integument. Of course it 



