100 
THE TEARL OYSTER 
bitten througli by a crab or sbrimp. Tliis cementing of tbe 
end of each fibre is seemingly a provision to allow of the 
animal being at rest wMle adhering to the rock, no undula- 
tion dislodging it that cannot tear it Kmb from limb ; and 
that the wave could hardly do, for the surface of shell to be 
acted on is limited, and the power of throwing out extra 
threads is practically unlimited, one oyster having as many 
as fifty at a time. The strength of the hyssus is also not 
insignificant. Testing it with a spring balance, I found 
that a young oyster measuring only Ifths of an inch 
along the hinge line and Ifths of an inch from hinge line to 
margin withstood steady tension till it reached 2flb. when 
the bi/ssus came away at the root. Others of about the same 
size yielded at a tension of I| and 2 lb. 
I conclude that a pearl oyster is not likely to be dis- 
lodged by the force of wave action or cui'rent, and that, if 
it moveSj it moves voluntarily. 
The Shell 
The peculiar formation of the shell alluded to above 
merits close attention, and affords confirmatory evidence of 
the migratory habits of its tenant. RefeiTing to the irregu- 
larly concentric circles of regularly radiating flattened over- 
lapping spines, we must consider first the object of their 
formation, and then the object of their retention. 
It will be remembered that the mantle, which it is 
that secretes the shell, is the part of the oyster which is 
nearest to the contour of the shell, the thin darkly-colored 
membrane which lies between the shell and the brancMce or 
gills, which are a specialized portion of the mantle, and ai-e 
commonly known ia the edible oyster as the beard. 
Bearing in mind that the shell of conchifers is merely 
the outward expression of the animal it contaius and protects, 
the external skeleton not distinct from, but incorporated with, 
the animal by being covered with an epidermis ; bearing in 
