348 University of California Publications in Zoology [Vol. 19 



under the cover glass. When free in the water, however, the movement 

 is vigorous. The body, which is concave ventrally between the suckers, 

 is doubled in upon the ventral sucker. The posterior end also is doubled 

 in so that the body is almost in a ball. The tail then lashes vigorously 

 back and forth, usually in one plane, driving the cercaria at a high 

 rate of speed but in no definite direction. The duration of this 

 vigorous movement is usually less than twenty-four hours although 

 longer periods of activity have occasionally been seen. This movement 

 is almost identical with that described by Cort for Cercaria trivolvis 

 Cort (1915, p. 37). 



EXCRETORY SYSTEM OP CERCARIA 



The excretory system of the cercaria of Echinostoma revolutum 

 (fig. 47) proved to be extremely difficult to work out. The difficulties 

 were due principally to two things. First to the fact that the entire 

 body from the region of the pharynx to the posterior end is filled with 

 cystogenous glands which are somewhat opaque. The longer the 

 cercariae are free in the water the more opaque these glands become. 

 The second difficulty was due to the complexity and progressive 

 modification of the accessory collecting tubules from the posterior end 

 towards the oral region. Also the exact place of attachment of the 

 capillaries was hard to determine. The fact that only a few flame cells 

 are likely to be active at a given time, some only during or after the 

 bursting of the body wall, and the fact that a cercaria lasted only a 

 few minutes, also added to the complexity of the situation. However, 

 the abundance of material greatly offset these difficulties. In order 

 to work out this system in this cercaria, one hundred and twenty snails 

 were examined, of which eighty-six contained mature active cercariae. 

 From these eighty-six snails probably over two thousand cercariae 

 wen- studied before the complete ramifications of the tubules and the 

 pattern could be made out. This one system was studied for a period 

 of five consecutive weeks before this pattern was determined. 



Since the limits of the bladder are poorly defined, I shall adopt the 

 following nomenclature. The tubes in the tail I shall call the caudal 

 division of the bladder (la) ; the part commonly called the bladder I 

 shall call the muscular sac of the bladder (lb), which may or may not 

 be divided into two parts as drawn in figure 47 ; the two large winding 

 tubes joining the anterior end of the muscular sac of the bladder I 

 shall designate as the muscular descending tubes of the bladder (1c) ; 



