THE HOUSE THAT THE OYSTER BUILT. 87 



Nor is this all : a minute mechanism may be traced in 

 each layer. On the outer surface of oyster shells there are 

 fine thread-like lines, or streaks, which are called stria. 

 And on this fact Lord Brougham and Sir C. Bell remark : 

 " We should be inclined to say that the earthy matter 

 crystallises, were it not that the striated or fibrous appear- 

 ance differs in the direction of the fibres in each successive 

 stratum, each layer having the striae composing it parallel 

 to one another, but directed obliquely to those of the layer 

 previously formed, and the shell exhibiting a strong texture 

 arranged upon well known mechanical principles." 



The oyster does not adorn its shell, as some of the 

 soft-bodied creatures do theirs. Their various and often 

 splendid hues are owing to glands situated on the margin 

 of the mantle, and curiously endowed with the power of 

 depositing colouring matter. In many instances an 

 accordance is observable between the patterns or tracings 

 on the shell and the colours as arranged in the organ that 

 secretes them. Thus, in the Banded Snail, there are just 

 as many coloured spots on the edge of this organ as there 

 are zones on the shell, and when a part of the shell is 

 removed, for the sake of experiment, the piece reproduced 

 is brown exactly opposite the dark portion of the organ, 

 and yellow in the other parts. The oyster, however, 

 diffuses throughout its exterior only a brown tint. I need 

 hardly remind the reader that the latter sentence is used in 

 a general sense, and only applies to the mollusc as seen in 

 our " sea-girt Isle ;" for the oyster bears different names 

 in accordance to the localities in which it is found, whether 

 on rocky ground, mud, or sand, and has different colours 

 in different places. In Spain, oysters are found of a red 

 and russet colour ; in Illyria they are brown, but the fish is 

 black, and in the Red Sea, of the colours of the rainbow. 



