34 A FIELD GUIDE IN NATURE-STUDY 



Put the crayfish in a small glass dish and offer it some scraps of raw 

 meat. Observe how these are handled. Notice how the jaws move. 

 There are accessory jaws in the crayfish as there are in the cricket. Feed 

 the turtle with some scraps of raw meat. How does he secure it? He 

 probably will help himself to fish, frogs, or other similar animals that are 

 left in the aquarium with him unless he is well fed. Confine a toad in a jar 

 with some earth at the bottom. Then put in some house flies, crickets, or 

 other insects. Can you observe how he captures them ? Look up in some 

 book the method of attachment of the toad's tongue in its mouth and draw 

 a diagram to show this. 



Snails. See specimens of the following different sorts of water snails: 

 Campeloma (or Vivipara), Limnaea, Pleurocera, Goniobasis, Physa, 

 Planorbis, and sketch the shells of those written in italics so as to bring 

 out the distinguishing features. 



Many of the snails live on the land seeking, however, very moist situa- 

 tions. The commonest large sorts in our region belong to the genus 

 Polygyra. Specimens of this or of the edible snail to be had at the fish 

 market may be kept in a moist glass jar and fed lettuce or cabbage leaves. 

 Note the sense organs, the antennae, and the eyes of the snails. The eyes 

 are carried on stalks that can be withdrawn. How does the snail shell serve 

 the animal? Note how the opening of the shell is closed in the living 

 animal by the horny operculum in such a snail as Vivipara. Note the lines 

 of growth on the shell. Where is the new material deposited as the shell 

 grows ? Can you see the original shell ? 



Snail eggs will often be found as translucent spheres imbedded in jelly- 

 like material that forms masses as large as beans on the sides of the aquaria 

 or on the water plants. 



Clams. Put a live clam in the aquarium with sand on the bottom. 

 Note the white fleshy foot at the front end by means of which it crawls. 

 When the animal is at rest the breathing- and feeding-tubes or siphons are 

 seen at the hind end. Dust a little finely powdered chalk into the water 

 so it will sink near the siphons. Can you tell which one takes the water 

 in and which serves as an outlet ? See the museum specimens of dissected 

 clams to see the gill chamber into which the siphons lead. Take the clam 

 from the water. How tightly does he close his shell ? Can you pull it open ? 

 Examine a clam shell to see the lines of growth on the outside. How are 

 they arranged ? Can you see the original tiny shell ? The two halves are 

 fastened together along a hinge line. Note the furrows and ridges on the 

 inside of the shell that interlock so the valves or halves of the shell will 

 come together accurately. Note the impression made on the inside of the 

 shell by the attachment of the strong muscles that hold the valves together. 



