42 STUDIES OF ANIMAL TYPES 



fresh eggs the embryo may be seen, occupying only a very 

 small portion of the egg. As they grow older the embryo 

 enlarges, the shell begins to form, and the young snail keeps 

 constantly turning round and round in the egg. At this 

 stage, note the two dark spots the eyes on the head. 



Make a drawing showing a young egg with its nearly 

 spherical embryo, and then an older stage, with the more 

 advanced embryo. 



If possible, keep the eggs until the young snails come out. 

 Note the time it took the eggs to hatch. 



These snails have the power of spinning a thread, which 

 they may use to ascend or descend in the water. 



Limnsea lays its eggs later in the season in much the 

 same situations. Limna3a should be fed daily on cabbage 

 or lettuce leaves. On these leaves, areas that have been 

 scraped by the lingual ribbon of the snail may be seen. 



XIV. THE CRAYFISH 



Materials. Specimens of crayfish (alive and preserved in for- 

 malin), dissecting instruments, dissecting pan. 



Directions. (The crayfish is common everywhere in brooks, 

 ponds, and pools. In many localities, especially in the southern 

 states, some species dig holes in the ground and build clay chimneys 

 about the mouths of the tubes. Crayfish may be caught by hook 

 and line or scooped up with a net.) 



A. STUDY OF A LIVING CRAYFISH. It will be of great 

 interest to observe them in their natural homes. Note the 

 manner of movement in the water. In how deep water do 

 they live? In what character of soil do those live that 

 build chimneys ? How deep are the holes ? Determine, if 

 possible, what is at the bottoms of the holes. Do the cray- 



