CHAPTER IV. 



MULTIVALYES AND BIVALVES, 



FOR the purposes of identification, the other mollusca may be 

 divided into multivalves, bivalves, and univalves. This is, 

 perhaps, a little too suggestive of classifying the races of mankind 

 according to the clothes they wear, and the more so as many of the 

 mollusca, like many of the men, never or hardly ever wear any 

 clothes at all. But as we are dealing with shells, and most of us 

 are more likely to become acquainted with the shell than with the 

 wearer, it is advisable to begin in this way. 



First, then, for the multivalvular -s^hell ; and it will not detain us 

 long, as there is only one genus accepted as British which has a 

 shell of the kind. That genus is Chiton, and its species can be 

 recognised at once by the resemblance they bear to the ordinary 

 woodlouse or garden armadillo. 



In appearance the Chitons differ from any other mollusca, and it 

 is at first sight difficult to believe that they belong to the sub-kingdom, 

 or that they can be grouped with the Gastropoda as they now are. 

 The shell is built up of eight plates, which overlap from behind 

 forward, like the tiles of a house, so that the animal, when alarmed, 

 can roll itself up into a ball. A head-piece of the shape A, a tail- 

 piece of the shape C, and half-a-dozen plates of the shape B, are 

 held together by a muscular girdle so as to form the shape D : which 

 is distinctive enough to be left to speak for itself. 



B 



THE PARTS OF A MULTIVALVE SHELL (Chiton discrepant}. 



Having got rid of these unexpected forms, we can give our 

 attention to the bivalves and univalves, taking the oivalves first, as 

 being more easily dealt with, and less in number. 



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