254 NATURE-STUDY 



fleshy portion, upon which the creature rests and with which 

 it crawls. This is the foot. At the front end are to be seen 

 some horn-like projections called antennae or feelers. Land 

 snails have four such, the second pair larger and bearing the 

 eyes. If the animal is touched on the antennae it quickly 

 retracts them, turning them inward, as we turn a glove 

 finger inside out. Water snails have only two feelers, and the 

 eyes are at their base. As land snails crawl over the ground 

 and plants they leave a track of slime. 



Snails feed upon algae and other plants. Some of the land 

 snails do . considerable harm to the vegetables in gardens. 

 Watch a snail as it moves along on the glass of an aquarium. 

 On the under side near the front will be seen an opening, the 

 mouth. This is provided with a hard upper lip, and a rough, 

 rasp-like " tongue, " with which the snail scrapes and eats. 



The Fresh-water Clam or Mussel is common on the sandy 

 flats of lakes and rivers. This shell animal burrows, or 

 rather ploughs, in the surface of the sand and makes long 

 furrows. It moves by means of a strong, fleshy foot. 



Clams breathe by means of gills within the shell. Water 

 is kept constantly in circulation through the gills. This 

 circulation may be nicely shown by the following experiment 

 on a clam in an aquarium. Be careful not to disturb the 

 animal lest it close its shell. With a long pipette or fountain- 

 pen filler carefully drop some indigo, carmine, or ink solution 

 just behind the shell, at the end opposite to the foot, near the 

 sand. It will be seen that the colored liquid is drawn in, and 

 then later ejected from another part of the shell a little higher 

 than where it entered. 



The shell is an interesting structure. It has a hinge of 

 elastic ligament, which naturally tends to hold it slightly 



