allee] 



SMALL CRUSTACEANS 



73 



the surface at night and on rainy days than they are in bright 

 sunlight. This means that there is a daily vertical migration that 

 depends, in part, at least, on light conditions. One other factor 

 helps cause them to collect under leaves in the day time, — the 

 fact that they are positive to touch stimuli. This means that, 

 other things being equal, the isopods will come to rest where they 

 can get most of the surfaqe of their body in contact with some 

 object. In pans they collect in corners; in ponds they are under 

 leaves or in the hollow stalk of a decaying plant. In this respect 

 Mancasellus shows an interesting reaction. When the animals 

 are brought into the laboratory they crawl up along the edge of 

 the containing vessel (other than smooth glass) until they are 

 one, two, or even three inches out of the water, and often remain 

 hanging there until they die of suffocation. 



Both species of isopods are easily kept alive and in good 

 condition in the school room. Pint Mason jars are very con- 

 venient for transferring isopods or any other small water animals 

 from the field. The jar should be filled with water from the 

 place in which th,e animals are taken and some of the live or 

 decaying plants on which the isopods are found should be added. 

 If possible stream isopods should be kept in running water. 

 When this is impracticable they may be kept in standing water 

 providing that at first it is changed frequently. After a time 

 the isopods may be acclimated to unchanged water, if the transi- 

 tion is made gradually. A little soil and a few dead leaves, pref- 

 erably those that have been taken from a small pool, should be 

 placed with the isopods to furnish them shelter and food. 



Figure 3. — An Amphipod. (After Emerton.) 

 Amphipods. 



^ X 4. 



Amphipods (Fig. 3) are also flattened crustaceans but unlike 

 the isopods they are laterally compressed. In other words, the 



