612 
GILBERT E. JOHNSON. 
majority of individuals are smaller and slenderer. Its average 
length is -5 mm. (PI. 37, fig. 4). The two forms appear 
identical in structure and proportions. But, in the unde- 
veloped condition in which they exist in the worm, they do 
not exhibit features sufficiently distinctive to make identifica- 
tion possible, the different species of Rhabditis being very 
much alike in the larval stage. The sexually mature males 
provide the most important means of discriminating the 
different species, the disposition of the papillm or rays of the 
bursa (PI. 37, fig. 9) being the most useful diagnostic 
character. 
In order, then, to determine the identity of the nephridial 
form it was necessary to rear it from the larval to the adulc 
condition, and with this object, as well as with that of inves- 
tigating the sexual phenomena, cultural methods were em- 
ployed. 
Cultural Methods. 
The researches of Maupas ( 10 , 14 ), extended by those of 
Potts ( 15 ), on the free-living nematodes have shown the 
possibility of rearing different species of Rhabditis, Diplo- 
gaster, Cephalobus, etc., in artificial media. The method 
employed by them consists in the use of watch-glasses, pre- 
ferably of the “ solid ” kind, in which the nematodes are kept 
in drops of water to which is added a small quantity of the 
nutritive medium. To prevent evaporation of the medium 
the watch-glasses may be closed by glass covers fastened 
down with vaseline, or a number may be placed together, 
without covers, in a humid chamber. The latter alternative 
is employed in cases where the decomposition of the medium 
is so intense that the nematodes would succumb to the effect 
of the gases liberated in putrefaction, were they confined in 
the small space afforded by the cavity of a single watch-glass. 
B}" this method of culture nematodes can be maintained in 
conditions favourable to growth and reproduction, and, given 
a suitable medium, generation after generation can be reared 
