86 OYSTERS, AND ALL ABOUT THEM. 



with a power of secretion : the process of separating cer- 

 tain matters from the nutritious fluids of the body. It is 

 probable that in almost every such act a double purpose is 

 served ; the blood being freed from particles which would 

 become superfluous or injurious, and the fluid separated 

 answering some secondary purpose. The process, com- 

 mon as it is, in all its essential features, to vegetables and 

 animals, is everywhere performed by the same agency, the 

 development of single cells, each possessing its own inde- 

 pendent vitality. It is a part of their regular actions to 

 secrete and withdraw certain ingredients from the nutri- 

 tious fluids, and afterwards to set them free again, either 

 by passing through in perspirable vapour, or the rupture of 

 the cell-wall. 



Now the oyster secretes a calcareous, earthy matter, 

 precisely adapted to the enlargement of its dwelling, by 

 means of its mantle, the outer membranous layer which 

 invests its body. Whatever, indeed, be the form of a shell, 

 (and shells partake of a marvellous variety,) the additions 

 required by growth, according to the law of its kind, both 

 in shape and colour, are ascribable to the wonder-working 

 mantle of the inmate. 



Duly provided for its work by the secretion of the 

 necessary matter, and by the instinct for its proper use, the 

 oyster begins the enlargement of its shell ; for this the 

 margin of the mantle protrudes, and firmly adheres all 

 round the circumference of the valve with which it corre- 

 sponds. Thus the calcareous matter is gradually deposited 

 in a soft state on the extreme edge of the shell, and, becom- 

 ing hardened, it is converted into a shelly layer ; the pro- 

 cess being repeated at intervals, every newly-formed layer 

 enlarges the diameter of the shell. Thus the shell corre- 

 sponds in its increase with the growth of its tenant. 



