282 ELEMENTARY BIOLOGY. [CIIAP. 



border or peritrcmc. It is thrown into a spiral of 

 four wliorls. 



h. Note its texture and colour (cf p. 272). As the 

 apex (first formed part) is approached, a smooth 

 friable texture and a pearly lustre are assumed. 



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

 wards you, the shell be carefully opened up with 

 scissors, the columella will be seen as a central 

 pillar or axis. Cut into this with care ; it w^ill be 

 found to be hollow, and closed in below by an 

 overgrowth of the peritreme — the shell is thus a 

 spirally coiled tube. 



In young shells the columellar cavity opens freely 

 below, by an aperture or iinibilicus. 



d. The cohtimilar 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.) 



■f. The hybeniaculum or epiphragm. Examine a dor- 

 mant specimen, and note that the mouth of the 

 shell is completely closed by this. Remove it and 

 examine under a low power ; note the perforation 

 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, which 

 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 organ. Insert a scissors 



