118 DEFENSIVE POINTS. 



weak that it no longer affords any real protection against the natural 

 enemies of that species, which are accustomed to devour the animal, shell 

 and all, when they attack it — so that they would nowise be deterred by 

 a thicker shell or the presence of an operculum. 



In the same way an operculum would be of no service to such cling- 

 ing forms of mollusks as the limpets or the abalone (Haliotis) ; yet, if an 

 abalone or a slipper limpet — " deckhead," as the fishermen call it-— be ex- 

 amined, the coiling will be plainly seen. 



Protected by its shell, and anchored immovably, the adult oyster sets 

 at defiance the. great majority of enemies which it would have good rea- 

 son to fear were it naked. Its egg-stage and embryonic career were beset 

 with perils which it had no means to guard against, and escaped wholly 

 by chance. During its young life it was at risk of being eaten by a large 

 number of creeping things and swimming things, against whose predatory 

 teeth its thin new tests were an insufficient safeguard. But now, in adult 

 age, with feet well planted, muscles strong to hold its doors closed against 

 intrusion, and a triple-plated, rough-coated shield of limestone over its 

 head, the foes able to prevail against it are reduced from a whole sea full 

 to a small catalogue, the principal members of which the reader will find 

 described in ensuing chapters. 



