SHRIMPS, CRABS, AND BARNACLES 103 



resisting surfaces, hinged or jointed to one another, and 

 made to " play " one on the other by the alternate 

 contraction and relaxation of the muscles attached to 

 them. 



The Arthropods differ among themselves in the 

 number of body-rings, the enlargement or dwindling of 

 certain rings, and the fusion of a larger or smaller number 

 of the rings to form a composite head, or a jointless mid- 

 body or hind-body. The successive legs are primarily 

 and essentially like to one another, and each body-ring, 

 with its pair of legs, is but a repetition of its fellows. At 

 the same time, in the different classes included as 

 " Arthropoda " a good deal of difference has been attained 

 in the structure of the legs, and they have in each class 

 a different form and character in successive regions of 

 the body, distinctive of the class, and are sometimes, but 

 not always, absent from many of the hinder rings. All 

 these Arthropods agree in having a leg on each side 

 immediately behind the mouth belonging to a body- 

 ring, which is fused with others to form the head very 

 specially shortened, of great strength and firmness, and 

 shaped so as to be pulled by a powerful muscle attached 

 to it, against its fellow of the opposite side, which is 

 similarly pulled. These two stumpy legs form thus a 

 powerful pair of nippers called " the mandibles." They 

 are jaws, although they were in the ancestors of the 

 Arthropods merely legs. These jaw-legs, or leg-jaws, are 

 characteristic of all the crab class, as well as of the other 

 Arthropods, but no bristle-worm or other animal has 

 them. The jaws of marine worms are of a totally 

 different nature. So are the jaws of snails, whelks, and 

 cuttle-fish. Many of the crab class have not one only, 

 but several, pairs of legs following the mouth converted 

 into jaws. Thus, if you examine a big shore-crab, or ? 



