502 University of California Puhlications in Zoology [Vol. 18 



ACTIVITY OF THE CERCARIA 



The cercaria of S. japonicum when freed from its sporocysts in a 

 watch glass swims freely for a short time and then settles down to the 

 surface where it moves by a looping movement. It has a strong positive 

 thigmotropism, so that whenever any part of its body touches a sur- 

 face the cercaria immediately adheres to it and starts moving on it. 

 In fact, this cercaria is apparently better adapted for locomotion on 

 a substratum than for free swimming. In swimming the movement 

 of the cercaria of ^S*. japonicum consists of a vibration of both the 

 body and the tail which carries the animal either forward or backward 

 in an irregular course through the water. The movement is more often 

 backward than forward. During the vibration the body is slightly 

 contracted, but the stem of the tail is considerably elongated. The 

 posterior end of the body and the point of bifurcation of the tail are 

 relatively constant points, while the anterior end of the body and 

 the middle of the stem of the tail vibrate backward and forward so 

 rapidly that they disappear from view. 



The method of locomotion on a substratum is also very character- 

 istic. It is a modification of the ordinary looping movement common 

 to so many distome cercariae (see Cort, 1915). The cercaria is able 

 to take hold both with the ventral sucker and the anterior tip. Its 

 method of taking hold with the anterior tip is very characteristic. 

 In trematodes, both cercariae and adults, in which the oral sucker is 

 fully developed, the suction which allows this sucker to take hold is 

 produced by the action of the muscles of the sucker in connection 

 with the mouth and buccal cavity. In the cercaria of ^S*. japonicum 

 the sucking action of the anterior end is developed in an entirely 

 different way and has no connection with the mouth or buccal cavity. 

 As long as a cercaria is alive the anterior tip keeps rolling in and out 

 by the interaction of the strong circular and longitudinal muscles of 

 the anterior region of the oral sucker, and the circular muscles of 

 the posterior region. The exact mechanism of this rhythmic action 

 is not clear but the sequence of movements is about as follows : The 

 strong contraction of the circular muscles of the posterior region of 

 the oral sucker presses this organ forward and causes the anterior end 

 to be thrust out with the openings of the cephalic glands and the 

 spines surrounding them at the anterior extremity. This is followed 

 by a roUing-in of the tip for a considerable distance. This rolling-in 



