86 A SNAIL. 



EXTERNAL FEATURES. The details of structure may be studied in 

 the following order: 



a) Observe first the univalve shell which always conceals a part of the 



body, and into which the entire animal may withdraw when seek- 

 ing safety. In what respects does it differ from the shell of a clam ? 

 The pointed end is called the apex, the opening at the other end 

 the aperture. Between apex and aperture are a number of 

 spiral turns or whorls, the larger lower one termed the body 

 whorl, the other constituting the spire. How many whorls in 

 the spire of your specimen ? The spiral grooves separating the 

 whorls are the sutures. More or less at right angles to these 

 are very many fine striae, the lines of growth, each indicating a 

 former position of the lip or outer rim of the aperture. What 

 part of the shell corresponds to the umbo of the clam ? 



b) Determine the direction of the spiral turns in your specimen. To do 



this hold the shell with apex up and the aperture turned toward 

 you. If the lip is to the right the spire is a right-handed or dex- 

 tral one; if to the left, it is left-handed or sinistral. The in- 

 terior axis around which the spiral coils is called the columella. 

 This axis can be demonstrated by sawing the shell of a large sea 

 snail from apex to aperture. 



c) The fractured edge of a snail shell will show the same three layers 



appearing in the shell of the clam. The outer of these, the 

 periostracum, is tough and horn-like; the inner is the pearly 

 layer, smooth and of beautiful luster. Between the periostracum 

 and the pearly layer is the thick prismatic layer. 



Exercise i. Draw the empty shell of a snail in the position referred to in b) , 

 if necessary enlarging considerably to show detail. 



Exercise 2. Draw the longitudinal section of a sea snail shell furnished you 

 by the instructor. 



d) Turn now to the occupant of the shell and note the foot or broad 



disc on which the animal creeps. In some sea snails there is at- 

 tached to this an oval piece of calcified material, the operculum, 



