12 ILLINOIS BIOLOGICAL MONOGRAPHS [458 



The methods of locomotion of Cercaria urbanensis, either when 

 swimming in open water or when upon a substratum, are very striking. 

 The body when swimming was contracted almost into a round ball and 

 the tail, which was curled ventrally so that it passed across the ventral 

 surface of the body, lashed backward and forward with great rapidity. 

 This method of locomotion was very effective and the cercariae could 

 be seen even with the naked eye in rapid movement thru the water. 

 In spite of the lack of a ventral sucker the cercaria was able to move 

 well on a surface by utilizing two projections which form the posterior 

 lateral angles of the body. In the process of locomotion the cercaria 

 took hold with its oral sucker and the body contracted until it was 

 practically round. Then the sucker let go its hold and the body 

 stretched out, at the same time extending the posterior projections 

 until they became little points digging into the substratum. Again the 

 oral sucker took hold and again the body contracted. By a continued 

 repetition of these movements this cercaria progressed at about the same 

 rate of speed as those having two suckers. After each contraction the 

 posterior projections held the amount gained. During the contraction 

 and at the beginning of the extension of the body the tail kept lashing 

 very rapidly, but during the rest of the movement it was held still and 

 somewhat contracted. 



Some of the freed cercariae after moving around for a while 

 settled down and formed cysts. A cercaria which was moving along 

 on a surface extended and contracted its body more and more slowly, 

 until while retaining a hold with the oral sucker the body 

 became almost round. While in this position the cystogenous material 

 was extruded, and the appearance was soon given of a round cyst 

 with a tail attached. Soon the worm inside freed itself from its con- 

 nection with the tail and squirmed around in the cyst. Finally the tail 

 wriggled loose from the cyst and continued swimming around for con- 

 siderable time, resembling in form and movement a free living nema- 

 tode. The cysts formed in a watch glass were flattened on the lower 

 surface and had much the shape of a chocolate drop. The process of 

 encystment is illustrated by figures 1, 2, 3, and 4. Encystment in the 

 open has been noted for Cercaria ephemera Nitzch. The encystment 

 of this species, which was first reported by Nitzch (1807), was the first 

 described for a cercaria. La Valette St. George (1855:33-34) described 

 and figured the process for the same species, and in one of his figures 

 shows the tail attached to the cyst. Von Linstow (1896:377) for Cer- 

 caria monistomi von Linstow describes the encystment in the same host 

 as the larval generation. 



