282 ELEMENTARY BIOLOGY. [CHAPJ 



border or peristome^ It is thrown into a spiral o< 

 four whorls. 



b. Note its texture and colour (cf. p. 272). As th< 

 apex (first formed part) is approached, a smooth] 

 friable texture and a pearly lustre are assumed. 



c. .The colutiiella. If, with the peritreme directed to- 



wards you, the shell be carefully opened up witl 

 scissors, the columella will be seen as a central! 

 pillar or axis. Cut into this with care ; it will be 

 found to be hollow, and closed in below by ai 

 overgrowth of the peritreme — the shell is thus 

 spirally coiled tube. 



In young shells the columellar cavity opens freelj 

 below, by an aperture or umbilicus. 



d. The cobwiellar muscles. Remnants of the glistening 

 white tendons of these are often to be found, at- 

 tached to the upper end of the shell axis. {Cf| 

 Sect. E. I a.) 



e. The hybernaculum or epiphragm. Examine a dorj 

 mant specimen, and note that the mouth of th( 

 shell is completely closed by this. Remove it am 

 examine under a low power ; note the perforatioi 

 of its central area. 



C. The pulmonary sac and its associated structures. 



I. Examine the pulmonary sac from above. It consists 

 of a membranous expansion of the body-wall, whicl 

 overlies the entire antero-dorsal region of the visceral 

 hump. Large blood-vessels are developed within its 

 walls; and there is visible through it, on the right 

 side, the yellow excretory orgafi. Insert a scissors 



