978 THE AMERICAN NATURALIST. [VoL. XXXV. 
stilet basis is slightly constricted medially. (6) There are ten 
instead of nine longitudinal nerves in the proboscis. 
These differences do not concern the general appearance, 
so that the figure of S. ez/hardi given by Montgomery (95, 
Taf. VIII, Fig. 17) might serve equally well as a general dia- 
gram of the structure of S. asensoriatum. 
OCCURRENCE AND DISTRIBUTION. 
During the early autumn of 1899 Stichostemma was found 
in great abundance in one of the lagoons in Jackson Park, 
Chicago, which has free communication with Lake Michigan. 
Since that time it has been present in the lagoon at all sea- 
sons of the year. Large numbers are found in masses of 
Spirogyra or other filamentous alga, and often, though less 
commonly, the animals are found among Elodea. They are 
most numerous along the shore where the water is only a 
few inches deep. Their presence here is probably due to 
the higher temperature of the shallow water near the shore 
and the consequent more luxuriant growth of the vegetation 
which they frequent. Their food, which consists of species of 
Nais, is very abundant in the lagoon under the same condi- 
tions, and probably also, directly or indirectly, constitutes a 
factor in the local distribution. 
The distribution about the lagoon is rather irregular and 
uneven. In some spots the worms are so abundant that hun- 
dreds may be taken from a single liter of the aquatic vegeta- 
tion. In other spots, apparently equally well suited to these 
forms, only a few individuals or none at all are found. While 
these areas are rather definite during a single season, the worms 
do not always reappear in the same spot in succeeding years. 
It is usually necessary to hunt for the favorable localities anew 
each year. 
The seasonal distribution is very definite. During the later 
summer and autumn the animals are usually very abundant, 
young and old together. They do not disappear from along 
the banks until actual freezing occurs. When the ice forms, 
many of the worms are undoubtedly destroyed, though a large 
