MANUAL OF NATUKAL HISTOEY. 147 



BIVALVES. 



The Conchifera, comprising the Bivalve Mollusks, 

 have the sides of their mantle protected by shelly 

 valves, which, however, are not spiral as in the Gas- 

 teropods, but simply concave and united together 

 by a horny ligament. The Conchifera are without 

 any apparent head, nor are they possessed of either 

 eyes or tentacles. There is usually a pair of gills 

 between the mantle and the body on each side ; and 

 the foot, although attached to the belly, does not 

 form a flattened disk as in the Gasteropods, but is 

 laterally compressed, allowing the animals to leap 

 or burrow, but not serving as an organ of reptation. 

 The mantle, in many of the families, is posteriorly 

 prolonged into two siphons or tubes, the upper one 

 of which is anal and excretory, and the lower one 

 branchial or respiratory; in other families the si- 

 phonal tubes are entirely wanting. Some Bivalve 

 Mollusks swim freely about by alternately closing 

 and expanding their valves, as the Pectens, which 

 have hence been termed the "butterflies of the 

 deep;" others, on the contrary, are securely an- 

 chored to foreign bodies at the bottom of the sea, 

 as the Clam-shells and the Oysters ; while others, 

 as the Venuses and Cockles, are enabled to move 

 about on the surface of the sand by placing their 

 bent foot under their shells and suddenly straighten- 

 ing it, causing it to act like a lever. Some Conchifera 

 bury themselves with great facility in the mud or 



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