12 INTRODUCTION. 



stances, then they are named " Petricolce" — some of these 

 latter form the holes in which they live by corroding or eating 

 away the stone. A section of these form the family of " Li- 

 thophagidce" or stone-eaters, of Lamarck. Others, again, take 

 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 Tubici- 

 nella, found buried in the skin of the whale. 



LOCOMOTION— Attached, 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 Serpulse among 

 univalves. M. de Blainville applies the term " Fixa? 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. 



