ANIMAL PARASITES 105 



The periods given for the various operations are subject 

 to wide modification, dependent on the size of the specimen and 

 the convenience of the worker. Minute specimens can be carried 

 through in 10 minutes each, but when using Delafield's hematoxy- 

 lin as a stain it is always advisable to use a weak solution and 

 stain for a considerable period. It should also be noted that if it 

 is necessary to differentiate with acid alcohol after hematoxylin 

 stain, it is desirable to follow by alcohol rendered slightly alka- 

 line by the addition of a few drops of a 0.1 per cent solution of 

 bicarbonate of soda, in order to prevent fading. 



NEMATODA 



Collection. — Adult nematodes are no less widely distributed 

 than are trematodes. Though most commonly present in the 

 alimentary tract, they should be sought in the liver, bladder, 

 kidney, lungs, body cavity, and circulatory system. Larval 

 forms are common in the tissues and organs. 



Inconspicuous forms from the digestive tract are collected 

 by slitting the various sections one at a time and washing their 

 contents into tall jars of water to be repeatedly washed and 

 sedimented until the fluid is clear. The sediment is then poured 

 into shallow trays and carefully examined. In the meantime 

 the wall of the section under study should be closely examined for 

 adhering parasites. It should also be examined over a strong 

 light for evidence of embedded forms. 



Hall and Cram, 1926, point out the value of screening the 

 contents of the digestive tract through a series of metal screens 

 having mesh apertures 6, 12, 24, and 40 to the inch. These are 

 used in a rack which is placed in a sink and washed with a stream 

 of water, after which the material on the screens is examined for 

 parasites. The lungs, urinary bladder, gall bladder, etc., should 

 be slit open in a dish of water and also examined in this way. The 

 method is of course applicable to forms other than nematodes. 



Killing and Fixation. — The specimens found should not be 

 subjected longer than necessary to the action of the water but. 

 should be vigorously shaken to clear them of adhering mucus 

 and then killed in hot, almost boiling, 70 per cent alcohol to 

 which, preferably, has been added 3 to 5 per cent of glycerine. 

 The alcohol may be allowed to evaporate gradually or more 

 rapidly in an oven at ra. 60°C. until they are left in cleared condi- 



