FREE-LIVING NEMATODES 



479 



material into a second watch glass, from which they are removed on 

 a very fine-pointed needle and placed in a drop of clear water on a 

 microscope shde. These operations arebest performed on the stage of 

 a dissectingmicroscope, under a lens magnifying five to ten diameters. 

 To collect specimens in large numbers it is best to make use of 

 more elaborate methods. A coarse sieve with meshes two to three 

 miUimeters across is used to remove objects larger than nema- 

 todes. To gather the nematodes, the material that comes through 



Fig. 773. Measuring the length of 

 the camera lucida drawing of a nema- 

 tode. The head end of the drawing 

 lies near the left-hand cuff. The 

 pharynx is shown, and near it, next 

 the knuckle of the Httle finger, is the 

 oblique nerve-ring. The cardiac con- 

 striction lies this side of the end of the 

 forefinger, and the vulva on the 

 farther side. Mention should be made 

 of the presence of the error resulting 

 from the attempt to measure a curved 

 line with a straight measure. The 

 aim should be to reduce this error so 

 much that it can safely be neglected. 

 One means of reducing this error may 

 here be mentioned, namely, reducing 

 the "step" of the divider legs in pro- 

 portion to the sharpness of the curve 

 to be measured. Another method 

 may also be mentioned, but it is to be 

 used with caution, and only as the 

 result of experience. By a number of 

 careful trials it will be found that a 



measurement nearer the truth can be obtained by following a path somewhat on cne outside of the curves 

 of the median line on the drawing or image being measured, but care must be exercised in adopting this 

 method not to overshoot the mark. Where the curve is sharp it is of course safer to go always a Utile on 

 the outside of the curve. I consider it to be sufficiently accurate after a little practice to dispense with 

 actually drawing in a median line on which to measure It is easy to keep sufficiently near the middle 

 by eye. Of course, with a reliable map-measure all these difficulties disappear. The map measurer, ao 

 instrument to be had from most dealers in drawing instruments, has a small milled wheel that may be so 

 rolled along a crooked line as to measure its exact length. 



To obtain the percentage figures used as terms of the formula simply divide each of the various trans- 

 verse and longitudinal measurements by the total length. Using a slide rule these divisions occupy 

 only two to three minutes. (After Cobb.) 



this coarse sieve is passed through sieves of finer and finer mesh 

 until the Hmit of fineness is reached. About the finest mesh ob- 

 tainable is that of the finest miller's bolting silk (0.25 to 0.5 mm.), 

 which, when stretched over appropriate rings made of bottomless 

 dishes will allow fine mud to pass through while it will retain all 

 but the smallest nematodes. By successive siftings practically all 

 the nematodes can be secured. 



The sifting can be supplemented by gravity methods. Aquatic 

 nematodes are Hghter than sand and heavier than water. If the 

 water containing the nematodes be violently agitated and then be 

 allowed to rest for a few seconds the sand will have subsided to the 

 bottom, and the nematodes may be decanted off if the pouring be 

 managed expeditiously. Then, if the nematode-containing water 



