How Animals Work. 



it seems to shrivel up, and with extraordinary rapidity 

 disappears within its protecting shell. 



The shell-builders may be broadly divided under 

 two headings: namely, those which construct a single 

 shell univalves, as they are called; and those which 

 construct double or twin shells called bivalves. While 

 the bivalve Mollusc can close its twin shells, and thus 

 be cut off from intercourse with the outer world, the 

 univalve has a single permanent opening to its shell, 

 through which the head and foot can be pushed forth 

 when the Mollusc wishes to go for a stroll. Most 

 univalves, however, have a kind of door that fits quite 

 tightly into the opening in the shell when the animal 

 retires within, and this is called the " operculum." It 

 is a thin, shelly, or horny plate, usually carried fixed to 

 the upper surface of the foot of the Mollusc, so that 

 the animal may be said to open or close the front door 

 of its house by the single expedient of pushing forth 

 or drawing back its foot. The operculums of many 

 Molluscs are very pretty things, delicately coloured, 

 and marked with lines running round and round in 

 the form of a spiral. 



It is among the univalves that many of the most 

 beautifully shaped and coloured forms are to be found, 

 while the inhabitants of univalve shells are nearly always 

 curious or interesting in one way or another. The 

 " Screw-shells " to be found on rocky coasts are made 

 up of a number of rings or coils, and perhaps represent 

 the most perfect development of a spire-shaped shell. 

 The largest coil or " whorl " forms the base of the 

 shell, and each succeeding whorl is smaller until the 

 apex of the spire is reached. This spiral form is very 



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