VARIETIES OF OYSTERS. 13 I 



with the bilateral symmetry of the Conchifers, while it 

 leaves that of the Brachiopods untouched. 



In the Brachiopods the double shell is flattened as if 

 the back and front of the animal had been squeezed 

 together, and this kind of flattening is technically called 

 depression. 



In Conchifera, the double shell is usually flattened as 

 though the creatures had been squeezed by pressure applied 

 to its sides, and this flattening is called, in the language of 

 Comparative Anatomy, compression. 



Now, as these passive creatures, whether fixed or free, 

 usually rest on the floor of the sea, it follows that they 

 must lie, not on the edges, but on the flats of their shells ; 

 and when thus lying, they rest habitually on one shell, and 

 this shell is often so modified in relation to the other as to 

 suit the lying posture. 



The Brachiopods rest on their ventral shell, with the 

 back or dorsal shell uppermost, and therefore this modifi- 

 cation does not interfere with the symmetry or equality of 

 the two halves ; but it does interfere with the symmetry of 

 the Conchifera, because they rest on their sides. A similar 

 instance of this effect of habit on the two-sided arrange- 

 ment of the body is seen in the soles, turbot, &c., which 

 constitute the family of fishes called Pleuronectidae, as 

 contrasted with the equally flat rays. The depressed rays, 

 lying with their backs uppermost, are quite symmetrical ; 

 while the soles, resting on their sides, are quite distorted 

 in shape, and the two sides differ in colour. 



Notwithstanding this tendency to one-sidedness in the 

 Conchifera due to habit, most of them have nearly equal 

 valves ; and in none is the internal arrangement of organs 

 much interfered with, (a] 



(a) " Cassell's Popular Educator." 



E 2 



