12 INTRODUCTION. 



eating away the stone. A section of these form the family of 

 " LithophagidcB," or stone-eaters^ of Lamarck. Others^ again, 

 takc'up their parasitical abode in the bodies of animals, and 

 feed upon their substance ; as for instance, the Stylifer, which 

 is found in the vital part of star-fish, and Coronula, and 

 Tubicinella, found buried in the skin of the whale. 



LOCOMOTION— ^Wac^ed/, Unattached. 



A much more subordinate source of distinction arises from 

 the freedom or attachment of the shells. Some of them float 

 or walk freely in their natural element ; others are fixed or 

 attached to foreign bodies. Among those which are attached, 

 there is again a difference as to the mode of attachment. Some 

 are united to foreign bodies by means of a glutinating sub- 

 stance, secreted by the animal, and joining part of the surface 

 of the shell to that of the stone, coral, or other substance. In 

 this way shells are fixed to each other in groups ; this is the 

 case with the Spondyli among bivalves, and the Serpulae 

 among univalves. M. de BlainviUe applies the term " Fixes" 

 to these shells. Others are kept in a particular place by 

 means of a Byssus or Tendinous fibrous line or bunch of silky 

 hairs, acting as a cable, and allowing the Mollusc to ride as it 

 were at anchor. This Tendon is connected with some part of 

 the animal from which it passes through an opening or hiatus 

 in the shell, as in the Terebratula and the Mytilus. 



