20 ILLINOIS BIOLOGICAL MONOGRAPHS [140 



but become active during the early part of the night it is probable that 

 migrations in the streams occur at that time. By the middle of May all the 

 worms seem to have accumulated among the roots of grass in or at the 

 edges of rapids. 



It is likely that copulation does not ordinarily take place during the 

 fall migration, as specimens at that time are usually found isolated and seem 

 to remain more or less isolated during the winter. During the spring 

 migration, however, they gather together in large numbers and I have 

 several times found females that still retained the mass of spermatozoa at 

 the posterior end, showing that copulation had taken place not more than 

 three days before. It is soon after this migration that egg-laying begins. 



The process of copulation is not difl&cult to observe. If two fresh worms, 

 male and female, are placed in a glass cylinder about 10 cm. in diameter, 

 filled with water, copulation takes place in a short time and may be 

 observed thru the walls of the cylinder. When specimens are placed in a 

 large open dish they swim about actively for a long time and copulation 

 usually does not take place until the latter part of the night, when they 

 become more quiet, and sometimes not until the second night. For the 

 observations for this report I used specimens that had just emerged from 

 their hosts, but in early spring collections most of the females copulate 

 after being brought to the laboratory and observations could easUy be 

 made on them. In spite of the fact that egg-laying does not seem to take 

 place in the fall, the specimens are mature for copulation when they emerge. 

 I have kept males and females that had just emerged or had been removed 

 from the hosts in open dishes and had copulation take place within 48 

 hours after emergence. 



There seems to be no definitely directed effort on the part of either male 

 or female to seek its mate. There is of course the usual tendency on the 

 part of both to become entangled with the other, but the solid knot that 

 makes observation impossible is usually not formed by two specimens 

 until they have been together for a long time. The two worms merely 

 become intertwined at places and then again disentangled, only to become 

 reentangled again. That process is continued until finally the body of the 

 female comes to lie within the spiral coil formed by the posterior end of the 

 male. This coil soon tightens, the prongs are spread over the body of the 

 female (Fig. 30), and the posterior end of the male with a rotary motion 

 passes backward over the body of the female. The male does not seem to 

 choose the direction in which it is to move except that it tends to move 

 contrary to the motion of the female. The direction taken is nearly as 

 frequently toward the anterior end of the female as toward the posterior 

 end. Usually after several trials the posterior end of the male passes over 

 the posterior end of the female. When the prongs have already passed 

 over the end (Fig. 31) a discharge of sperm takes place. The cloacal open- 



