40 WILLIAMSON, ON SHELLS OF CRUSTACEA. 



ing the free margin of any of the segments of the body 

 after being treated with caustic potass, these granular con- 

 cretions will be seen in various stages of development 

 (fig. 10). Those nearest the free margin are small and isolated. 

 As we recede from the margin the granules coalesce and 

 become larger, receiving external additions, which cement 

 them together, and finally producing the structure just 

 described. The mode of calcification of the small areolae 

 and of the larger discs, though at first glance appearing so 

 different, are thus demonstrated to be identical in all their 

 essential features ; though the process resulting in the large 

 discs is obviously a secondary one, being preceded by that 

 which forms the small concretions. The essential difference 

 between the two lies in the greater extent of the calcific 

 deposition in the former instance than the latter, as is proved 

 by the effects of heat in occasioning contraction of those 

 parts from which the larger discs are absent. 



On decalcifying a portion of the integument thus treated 

 by means of hydrochloric acid, it presents precisely the same 

 appearance as the ordinary examples that have not been so 

 treated. Two distinct granulations are now visible ; one on 

 the extreme outer surface of the membrane, apparently 

 belonging to the pellicular layer, and the other in the 

 areolated layer, if not also in the subjacent corium; the 

 second being the tissue which I believe to be tubulated. The 

 concretions represented in fig. 9 are absent from the points 

 into which the tegumentary hairs of the shrimp are planted. 



Many of the smaller species of decapod Crustaceans reveal 

 modifications of structure throwing additional light on these 

 tissues so interesting to the physiologist. A few of the more 

 important varieties require notice. 



In the British Hyas araneus* we again meet with the four 

 layers of integument. In the specimens examined, the 

 pellicular layer alone presented a deep-red colour, and ex- 

 hibited indisputable evidence that it was comjioscil of Numerous 

 exceedingly thin parallel lamina. The areolated layer was of 

 a pale-yellow hue, and afforded equally clear proof that each 

 areola is the dome-shaped extremity of a six-sided prism, the 

 sides being contiguous with those of its neighbours, whilst 

 its substance is traversed by longitudinal tubuli. Owing to 

 the dome-like shape of the outer extremities of these prisms, 

 the outer non-tubulated pellicle appears to dip down between 



* 1 am indebted to my valued Friend, Mr. Bean, of Scarborough, as well 

 as to a still dearer relative, whoso name lias been Ion? associated with 

 the same place as one of its indefatigable naturalists, for the numerous spe- 

 cimens of British Crustacea upon which I have operated. 



