124 NOTES ON NEMAS 



are taken, it will be found that fine distinctions can be made with 

 such precision as to dispel all doubt as to the existence, side by side, 

 in the same cell, of bodies of quite different character that it other- 

 wise would be either impossible or exceedingly difficult to distinguish 

 from each other. 



The use of an ordinary apochromatic objective as a condenser ne- 

 cessitates the use of a special object slide, consisting essentially of 

 a carrier, and two cover glasses. The object is mounted between the 

 cover glasses. Such a slide is shown in the accompanying illustration. 

 The substage of the microscope should have a centering arrangement 

 and a rack and pinion, or screw focusing adjustment. A little ex- 

 perience with an apparatus of this sort, in which all known precau- 

 tions are taken to remove color from the optical system leads one 

 to distrust the ordinary Abb6 substage condenser when fine distinc- 

 tions are to be made between colors in the microscopic object, espe- 

 cially if the colors are of similar character. 



NEMATODE POPULATION OF BEACH SAND 



Through the courtesy of the U. S. Bureau of Fisheries I was able to 

 make quantitative observations during the summer of 1916 on the 

 nemas of ordinary beach sand, between tide-marks, at Woods Hole, 

 Buzzard's Bay, U. S. A. Nemas were plentiful; it was calculated 

 that on one beach in the top 3 inches of sand there were at least 527 

 millions per acre. On another beach there were at least 1040 millions 

 in the topmost inch of sand. The nemas varied from a fraction of 

 a millimeter to 10 millimeters in length, averaging 2 to 3 millimeters. 

 On muddy shores where organic matter is more abundant, the riema- 

 tode population is much more dense, thousands of millions per acre. 



Many of these nemas were strictly vegetarian, and fed on micro- 

 scopic plants present in the beach sand, plants both green and color- 

 less. Experiment showed that a considerable amount of light pene- 

 trates average beach sand to a depth of f inch, thus rendering possible 

 the growth of green protophytes and various algae at slight depths 

 in the sand. Some of the nemas were strictly carnivorous, feeding on 

 protozoa and other small animals. 



On trial it proved that the top layers of sand, between high and low 

 tides, under ordinary circumstances, afford the nemas a habitat of 

 considerable stability, since the shifting of these layers during the 



