840 FRESH-WATER BIOLOGY 



weeds. The smaller forms may be taken in numbers by transfer- 

 ring water weeds, dead leaves or other rubbish found on the bottom 

 into tubs or dishes, and picking out the specimens with a pair of 

 pincers. The larger forms must be caught by hand, or with a 

 small dip-net (minnow netting). For many forms the seine is a 

 very successful implement. 



In collecting the burrowing crayfishes special efforts are neces- 

 sary. It sometimes happens that the crayfish can be induced to 

 come to the mouth of its hole by destroying the entrance. But 

 generally the collector should not hesitate to go after the crayfish 

 by digging it out Of course, a spade or shovel is most efficient, 

 although often too heavy to be carried along, but a strong garden- 

 ers' trowel is very convenient : the best tool is a so-called pioneers' 

 bayonet. With this the groimd should be loosened aroxmd the 

 hole, and the dirt be taken out with the hands, care being taken 

 always to follow the direction of the hole. By digging deep enough 

 (i to 3 feet), finally the pocket will be reached, in which the cray- 

 fish Uves, and then it may be taken out. 



Preservation should always be in alcohol. Formalin should be 

 avoided, except in cases of necessity. Even then the specimens 

 should never be left in the formalin for a long time: it hardens 

 them too much, makes all the appendages brittle, and renders 

 them unfit for safe handhng. The best results are obtained by 

 killing them in weak alcohol and transferring them into stronger 

 (2 to 3 changes), until they finally are in 75 to 80 per cent alcohol: 

 when so treated all appendages remain soft and flexible as in Hfe. 



For scientific study no special work is required in the case of 

 the larger forms, and all systematic characters may be seen with 

 the bare eyes or by the use of a hand-lens. In the smaller forms 

 it is necessary to study the appendages separately. They should 

 be teased out under a dissecting microscope (using two pairs of 

 pincers) and mounted in the usual way upon microscopic slides. 

 Care should be taken that the appendages are taken out in the 

 proper order, so that they do not become mixed. For the micro- 

 3copic investigation a very low power is sufficient. 



