THE KING CRAB'S GRANDFATHER. 193 



genus, for it has been in existence on the earth from the epoch 

 of the Potsdam Sandstone to the present. In every formation 

 are some species of lAngida ; and living species may be found 

 along our Atlantic coast, clinging by their fleshy peduncles to 

 the wharves and other supports. Another remarkable fact 

 about lAngula is this : Its shell is composed largely of bony 

 substance phosphate of lime while the shells of ordinary 

 molluscs are composed of stony substance carbonate of lime ; 

 and this peculiarity of constitution has clung to this little type 

 through all the ages. 



It is a peculiarity of Brachiopods to have the two valves 

 unequal; one is more convex than the other. The more con- 

 vex valve has also the more projecting beak. But each is 

 symmetrical taken by itself. That is, if you lay it down on 

 the side, you see the beak in the middle, and on each side of 

 it, the outline of the valve presents the same shape and curve. 

 Now, the clam and river mussel are quite different. In these, 

 the two valves are equally convex; and, if you consider one 

 valve by itself, it is not symmetrical. That is, if you lay a 

 valve down on its side, you find the beak nearer one end ; 

 and the slope of the shell-outline is not the same on each side 

 of the beak. Shells of this sort belong to the class La-md'-li- 

 branchs. All the difference in the forms of these two classes 

 arises from the position of the animal in the shell. In the 

 Lamellibranchs, one valve is on the right side and the other 

 on the left. So the principle of bilateral symmetry makes one 

 valve the counterpart of the other. In the Brachiopods, one 

 valve is on the back and the other over the abdomen. So 

 the principle of bilateral symmetry does not operate between 

 the two valves ; but the right and left sides of each valve 

 separately are symmetrically developed. By bilateral sym- 

 metry we mean the law or principle which causes every fea- 

 ture of the right side of an animal to have a corresponding 

 feature on the left side. This principle runs through the 

 whole animal kingdom. Even among the star-fishes, crinoids, 

 corals or other so-called "radiate" animals, we can draw a 

 line which will separate right and left sides. Try it in a star-fish. 



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