Crustacea. 35 



stay in open places, or where the light is strongest, or does it 

 seek sheltered places? 



Feeding. Offer a crayfish various kinds of food, bread, meat, 

 cheese, vegetables, etc. Find what it prefers. Learn how it eats, 

 what organs are used and how they are used. Does a crayfish 

 eat much or little? Is it a rapid or a slow eater? Examine the 

 mouth parts in this connection. (In these experiments be careful 

 not to let the water become foul.) 



The Water Currents to the Gills. While a crayfish is at rest 

 in shallow water, carefully introduce a few drops of ink near the 

 bases of the hinder legs. Where is it drawn in and where does it 

 reappear? Try placing the ink at various points along the edge of 

 the carapace. Place a crayfish in a candy jar. Watch it from the 

 front and below to see the vibratory motions of the outer branches 

 of the maxillipeds. Their motions indicate the rate of movement 

 of the gill scoop, or gill paddle, within. Count the vibrations for 

 a minute. How is it that a crayfish, while breathing by gills, can 

 live so long out of water? 



Senses Note the range of motion of the eyes. Can an 

 enemy approach a crayfish from any direction without being seen ? 

 Can a crayfish see small objects as well as large ones? Does it 

 notice slow motions as readily as quick ones? Does a crayfish 

 see where it is going when it is frightened and darts backward by 

 swimming? 



What advantage is there in having the eyes on movable stalks? 

 What disadvantages? In what ways is the eye protected? Touch 

 one of the eyes. What follows? 



With a straw, broom-straw, or feather, test the sense of touch 

 over all the outside of the body. Where does the crayfish seem 

 most sensitive to touch? Is there any special reason for having 

 two pairs of " feelers " ? 



Which reach farther forward, the big claws or the antennae? 

 Can the antennae extend back as far as the tip of the tail fin ? 



Make noises near the crayfish to test its sense of hearing. In 



