38 ILLINOIS BIOLOGICAL MONOGRAPHS [484 



shaped anterior end with a crown of thirty-seven spines. Well extended 

 toto mounts have an average length of 0.38 mm. and a width of 0.12 

 mm. The tail is large and powerful in proportion to the size of the 

 body, and in a state of average extension has a length of 0.50 mm. 

 When contracted it may have a length considerably less than that of 

 the body and when the cercaria is swimming it often reaches to two or 

 three times that length. It ends in a sharp point and at the tip for 

 about 0.05 mm. to 0.06 mm. it is narrow and is composed only of the 

 muscle layers, lacking the parenchymatous core. 



The oral sucker is almost exactly spherical and has an average 

 diameter of 0.043 mm. The acetabulum is a little larger, being 0.049 

 mm. in diameter, and is situated two-thirds of the distance from the 

 anterior to the posterior end. 



The body from the anterior end to the acetabulum is set thickly 

 with rows of small spines, only visible under the highest powers of the 

 microscope. 



The crown contains thirty-seven spines of equal size, arranged in 

 two alternate rows, broken in the middle of the ventral surface. They 

 are arranged regularly except for the two or three nearest the mid-line 

 on the ventral side, which point in. 



The body from the oral sucker to the attachment of the tail is filled 

 with cystogenous glands. They are unicellular and club-shaped and all 

 open on the dorsal surface. 



The oral sucker is followed by a typical prepharynx about 0.022 

 mm. in length. The pharynx is round, on the average 0.017 mm. in 

 diameter. The esophagus and intestinal ceca are not yet functional, 

 but appear merely as columns of granules enclosed in membranes and 

 containing irregular spaces representing the beginning of the lumina. 

 The esophagus is long, reaching almost to the acetabulum, and the ceca 

 reach nearly to the posterior end of the body. 



Cercaria trivolvis (Fig. 39) has the typical echinostome excretory 

 system. The portion in the tail was made out with great difficulty and 

 is apparently not functional. A vessel passes back from the excretory 

 bladder for one-fifth or one-sixth of the length of the tail and sends out 

 two lateral branches which open to the outside. Dorsally at the base 

 of the tail is the adult excretory pore which apparently at this stage 

 gives passage to the outside for the waste material, since altho the 

 vesicle kept filling and emptying the vessels of the tail did not change 

 their caliber. In the oldest specimens the crura are large and distended 

 from the bladder to the region of the pharynx with regularly-shaped, 

 highly refractive granules. For the most part these are round or oval, 

 but some of them appear to be compounded of from two to four of the 



