192 THE BIOLOGY OF THE ANIMAL 



Find also opposite the upper end of the nn- 

 pression of the anterior adductor muscle 



11. The scar of the anterior retractor muscle. — 



Compare with the preceding. 



Opposite the lower end of the impression of 

 the anterior adductor muscle find 



12. The scar of the protractor pedis muscle. — 



Compare with the above. 



Break a valve in two, examine the broken edge with 

 a hand-lens, and note the structure of the valve. 



Make a drawing of the outside and inside of each 

 valve, and of a transverse section through the thickest 

 part of one of the valves. 



Example 3. — Pond Snail (Lymncsus Sp.) 



Material. — This snail is abundant in ponds and slow- 

 flowing streams. Its large size makes it especially favor- 

 able for the study of the univalve shell. For compara- 

 tive study use the shells of any land or water snails that 

 may be had. A penknife or dull scalpel and a hand- 

 lens will be needed. 



Method of Examination. — Before removing the body 

 from the shell (which may be done by dropping the snail 

 into hot water for a few minutes, then picking out the 

 body with a pin or needle), closely observe the manner 

 in which and the places where the body is connected to 

 the shell. Notice also how far back in the shell the 

 body lies when completely contracted. 



"With the penknife prepare longitudinal and cross sec- 

 tions of the shell to show the structure of the colu- 

 mella. When applicable use the questions given for 

 Uoiio or Anodonta ; also notice 



