100 



ANIMAL BIOLOGY 



FIG. 192. CROSS SECTION 

 OF MUSSEL. (Diagram, 

 after Parker.) 



the pallial line, or the line to which the mantle extended 

 on each shell when the animal was alive ? A free portion 

 of the mantle extended like a fringe below the pallial line. 



The shells were held together by 

 two large adductor muscles. The 

 anterior adductor (Fig. 193) is near 

 the front end, above the foot. The 

 posterior adductor is toward the rear 

 end, but not so near the end as the 

 anterior. Can you find both muscle 

 scars in the shells ? Are they nearer 

 the ventral or dorsal surface ? The 

 points of attachment traveled down- 

 ward and farther apart as the ani- 

 mal grew (see Fig. 191). Higher 

 than the larger scars are small scars, or impressions, where 

 the protractor and retractor muscles that extend and draw 

 in the foot were attached. 



The muscular foot extends downward in the middle, half- 

 way between the shells (Fig. 193). On each side of the 

 foot and behind 

 it hang down 

 the two pairs of 

 gills, the outer 

 pair and the in- 

 ner pair (Fig. 

 192). They may 

 be compared to 

 four V-shaped 

 troughs with 

 their sides full of holes. The water enters the troughs 

 through the holes and overflows above. Is there a marked 

 difference in the size of the two pairs of gills ? A kind of 



POST? ADO" MUS. , 



FIG. 193. ANATOMY OF MUSSEL. (Beddard.) 



