allee] 



SMALL CRUSTACEANS 



75 



produced each season. The young of ah the hroods normally live 

 past the first winter, at least; many of them live about eighteen 

 months. As with the isopods, these animals are usually found in 

 shaded water. Thus in Gary Brook hundreds may be taken with 

 a single dipping of a net made under the overhanging bank, while 

 a similar collection from the exposed part of the stream will 

 give only a few individuals. This reaction is in part a negative 

 reaction to light. Like the isopods, the amphipods give a daily 

 vertical migration, coming nearer the surface in the evening and 

 when the weather is cloudy than at noon or on bright days. 

 Amphipods are also strongly positive to tactile stimuli. This is 

 shown by the fact that they are found under gravel grains or bur- 

 rowing under sand or silt of stream bottoms or among leaves and 

 other vegetation in ponds. In the school room they have a 

 marked tendency to collect in corners of the containing vessel. 

 Stream amphipods are more sensitive than stream isopods 

 and after collection they should be kept in running water al- 

 though they can gradually be acclimated to quiet water. Care 

 must be taken to avoid a sudden change of temperature. Aquaria 

 containing amphipods should be provided with a few sprays of 

 water plants, such as elodea, to furnish resting places and food 



Figure 4. — The Shrimp, Palaemonetes. (After Herms.) X 4. 



