l84 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The soft body of the animal, lying at the posterior portion of 

 the shell, occupies only about a third of the interior. The body 

 wall gives off two folds or mantles, one fitting closely to and 

 secreting the pedicle valve, the other secreting the brachial 

 valve. Prolongations of this mantle, the caeca, fitting into 

 minute pores (tubules) in the valves, probably supply nourish- 

 ment to and take waste from the non-calcareous part of the 

 shell. Any marked injury to a mantle is necessarily reflected 

 in the shell. If something injures a mantle edge, the first pro- 

 cess in healing is a puckering up of the mantle around the 

 injured place (as in the healing of an injury to a man's skin) 

 which causes a like puckered appearance in the shell at that 

 place ; as the mantle becomes healed the growth lines of the 

 shell become more and more regularly spaced. But since an 

 injury to one mantle causes a lessened vitality, even if very 

 slight, of the entire animal, the opposite mantle likewise displays 

 a crowding and general interruption of regularity in the growth 

 lines. It thus follows that the life history of the individual 

 can be read from the beak forward, not only in relation to the 

 shape and size of the shell, but to its injuries, social crowdings 

 and general health. (See also Fig. 4.) 



Most of the inner space between the mantles is filled with the 

 tentacle-bearing lophophore ; this is supported by the calcare- 

 ous ribbon or skeleton, the brachidium. Those portions of the 

 lophophore which diverge arm-like from the two sides of the 

 mouth are called the hrachia. 



The food consists of diatoms, infusorians, etc., as well as of 

 microscopic organic fragments. Upon each branch of the 

 brachia there is a groove bounded on each side by lines of ciliated 

 tentacles, and which extend from the tip of each arm to the 

 mouth, — a slit-like opening in the middle of the lophophore. 



The motion of the cilia upon these tentacles and within the 

 food grooves (Fig. 73 a, E) causes a current of water, and with it 

 any near-by food particles, to sweep into the grooves and along 

 these grooves to the mouth. 



