Anatomy of the Food Mollusks 19 



for existence among organisms that popularly is very 

 little appreciated. It is that the battle, which never ceases, 

 is, in almost all cases, nicely drawn so delicately bal- 

 anced through long periods of time, that any slight ad- 

 vantage on one side or the other may result in the more 

 or less complete extinction of one, or even of several 

 interdependent species. At one time newspaper reports 

 informed us that ordnance and projectiles had become so 

 perfected that the armor of war vessels afforded little 

 real protection. Later it was stated that armor-plates had 

 been made so hard and tough that they could be pene- 

 trated or broken only with great difficulty. Yet improve- 

 ments in both go on, and the layman understands that 

 there is a nicely balanced contest for supremacy between 

 them. Everywhere in nature, also, weapons of defense 

 and offense are slowly being perfected, but in the test of 

 actual warfare. A harder armor in the oyster and other 

 mollusks might possibly deprive drunifish and certain 

 marine snails of so much food that their ranks would at 

 least be reduced; and stronger and harder jaws in the 

 drumfish might result in the annihilation of oysters liv- 

 ing in the warmer waters of our coast. This balance in 

 the struggle among organisms sometimes is upset, and 

 fossil remains show that, as a consequence, many great 

 races, both of animals and plants, have suddenly declined, 

 and then completely disappeared from the earth. 



The Mantle. This structure has been referred to as a 

 flap or fold of tissue that grows out from each side of the 

 body, expanding so as to line the inner surface of the 

 shell. Figure i shows the edge of the mantle lying 

 parallel to the margin of the shell ; but a much better idea 

 of it is given by Figure 3, which represents the body of 



