164 ANIMAL ACTIVITIES. 
Can a snail leave its shell and return ? 
’ Can it close its shell ? 
Examine different kinds of snails for this charac- 
teristic. 
Do all the snails you have seen breathe in the same 
way? 
(Can a snail swim ? 
Can it walk on the surface of the water ? 
Is the snail bilateral ? 
Compare a land-snail with a water-snail. 
Study several shells and notice the swtures, the 
apex, the spire, the whorls, the lines of growth, the 
aperture, and the “~. Compare these parts in snail- 
shells of different kinds. 
Notice all other differences and tabulate your obser- 
vations. The columella is the axis of the spire. The 
piece which closes the snail’s shell is the operculum. 
Hold the shell with the apex upward. Is the aper- 
ture toward the right (dextra/) or the left (s¢zzstral) of 
the columella ? 
Is the aperture xotched or entire ? 
Describe a shell from observation of the specimen 
and compare your description with that found in a’ 
standard work. 
Summary of Drawings. (a) A pond-snail as it 
appears while crawling xX 3. 
(4) A shell of litorina or similar shell. 
(c) A shell of purpura or a similar shell. 
(Z) A shell of a limpet. 
(e) Right valve of an oyster. 
(7) Various shells showing different structures. 
Snail and Mussel. A comparison of the snail and 
mussel shows on the part of the snail a distinct advance 
in the collecting of organs of sense at the anterior por- 
tion of the body and the growth of a head there. The 
possession of a head with a brain and active sense- 
organs is apparently associated with the matter of food- 
supply and feeding. So long as a soft-bodied animal 
