

VARIETIES OF OYSTERS. I2Q 



saucer-shaped pieces, that fit more or less closely together 

 along their edges, and which, therefore, when drawn to- 

 gether, can completely protect the animal, that lies wholly 

 between them, from all injury. 



These are called bivalve molluscs. The shells are 

 usually so united at one point in their circumferences as 

 to play upon that point as a hinge, while the remainder of 

 the two shells can be separated so as to gape more or less 

 widely on the side opposite the hinge. In this manner 

 the creatures can keep open house when their guests are 

 likely to be those upon which they can prey, and can shut 

 their folding doors when they are themselves likely to be 

 victimised. 



These shells are usually thick and heavy, especially in 

 those species which are marine, for the wear and tear of 

 the sea is greater, and the predatory creatures more power- 

 ful than those in fresh water. Moreover the box or house 

 must be tolerably capacious, otherwise the creatures could 

 not breathe while they were in a state of siege, and must 

 surrender at discretion to the expectant lobster or other 

 freebooters of the deep. It follows that this arrangement 

 is not well suited to locomotion ; and not being locomo- 

 tive to any great degree, they are not endowed with those 

 perfect organs of sense that must be possessed by those 

 animals which chase their prey. When organs of sense 

 are possessed, they are usually collected on a protruded 

 part of the body, and placed above the mouth, which 

 opens at the front part. Such a projection, which supports 

 the eyes, feelers, ears, and smelling capsules, and contains 

 a nervous centre conveniently and closely situated to these 

 gateways of knowledge, is usually called a head. 



Now these bivalve molluscs are distinguished from the 



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