WILLIAMSON. ON SHELLS OF CRUSTACEA. 39 



my vertical sections, mounted in Canada balsam, appear to 

 oppose this idea. Some, at least, of these lines are filled 

 with air, indicating a tubular structure. 



The very thin areolated layer is not so readily distinguished 

 from the corium in the vertical section as in some other 

 Crustacea, but I have obtained several evidences of its separate 

 existence as in the crab. The most remarkable feature in the 

 integument of the shrimp consists of numerous discs, the 

 result of a secondary calcification, which becomes incorpo- 

 rated with the pre-existing tubulated calcific deposit. Each 

 disc commences as a faint brownish ring, within which the 

 areolae are at first visible, but the centre shortly displays 

 signs of consolidation, as in fig. 4; in which example the 

 deposit has commenced at the root of one of the short hairs 

 so abundant on this tegument* The small discs thus 

 originated increase both in breadth and thickness, but 

 especially the former. Some of them assume the aspect of 

 figs. 5 and 6, where the dark radiating lines indicate a con- 

 dition of the calcareous matter resembling what is seen at 

 the first formation of the disc, and different from what occurs 

 in the intervening translucent parts, which appear to be more 

 consolidated. A translucent crucial figure is often seen in 

 the centre of each disc. But in the majority of instances, as 

 already pointed out by Professor Huxley, the discs assume 

 the aspect of fig. 7. Here the centre consists of numerous 

 small concretionary granules, each commencing as a separate 

 point, but which grow and coalesce by external concentric 

 additions until they unite to form a solid translucent 

 calcareous disc — strongly reminding us of what is seen in 

 the pulp-cavities of cetacean teeth. In fig. 8 some of these 

 granules are detached from the disc. When a portion of the 

 integument of the common shrimp is boiled in a solution of 

 caustic potass, though the soft chitinous element is not 

 destroyed by the process, it undergoes some change, revealing 

 an organic structure which does not hitherto appear to have 

 been noticed. The areolated layer seen in fig. 3 now presents 

 the appearance represented in figs. 9 and 10. It consists of 

 at least two layers of minute irregular concretions. Each of 

 these corresponds in size with one of the unaltered areolae, 

 and I assume them to be identical, though unable to demon- 

 strate the fact. Each concretion appears to be formed by the 

 coalescence of a number of minor concretions. On examin- 



* These hairs, which are tubular, are planted in uncalcified depressions 

 in the areolar layer ; a large branching tubule, ascending from below, pene- 

 trates all the layers of the integument, and, reaching the base of the hair, 

 communicates with the canal running aloug its interior. 



