MOLLUSKS 



99 



FIG. 190. MUSSEL crawl- 

 ing in sand. 



Notice the lines on the outside of the shell (Figs. 188 

 and 190). What point do they surround ? They are lines 

 of growth. Was each line once the 

 margin of the shell? If the shell 

 should increase in size, what would 

 the present margin become ? (Fig. 

 191.) Does growth take place on 

 the margin only? Did the shell 

 grow thicker as it grew larger? 

 Where is it thinnest ? 



Draw the outside of the shell from 

 the side. Draw a dorsal view. By the drawings write the 

 names of the margins of the shell (p. 98) and of other parts 

 learned, using lines to indicate the location of the parts. 



Study the surface of the shell inside and out. The 

 inside is called mother-of-pearl. Is it of lime ? Is the 

 deeper layer of the shell of lime ? (When weak hydro- 

 chloric acid or strong vinegar is dropped on limy substances, 

 a gas, carbon dioxid, bubbles up.) Compare the thickness 

 of the epidermal layer, the middle chalky layer, and the 

 inner, pearly layer. 



Anatomy of the Mussel. What parts protrude at any 

 time beyond the edge of the shell ? (Fig. 190.) The shell 

 is secreted by two folds of the outer 

 layer of the soft body of the mus- 

 sel. These large, flaplike folds hang 

 down on each side, and are called 

 the mantle. The two great flaps 

 of the mantle hang down lower than 

 the rest of the body and line the 

 shell which it secretes (Fig. 192). 

 The epidermis of the mantle secretes the shell just as the 

 epidermis of the crawfish secretes its crust. Can you find 



FIG. 191. DIAGRAM. 

 Change of points of attach- 

 ment of muscles as mussel 

 enlarges. (Morgan.) 



