14 ILLINOIS BIOLOGICAL MONOGRAPHS [460 



free in the livers of the infected snails, show that the cercariae as 

 they develop are continually making their way out of the rediae to 

 finish their growth in the liver of their host. The most highly differ- 

 entiated cercaria found in the redia (Fig. 11, 14, cr) was 0.3. mm. in 

 length and 0.1 mm. in width and had a tail 0.22 mm. in length. The 

 two lateral eye-spots were developed and the pigment was present 

 to a considerable extent around them, but was not further scattered 

 or concentrated to form the so-called intermediate eye-spot. Outlines 

 of the longitudinal vessels of the excretory system and the anlage of 

 the reproductive system could be clearly distinguished. The oral sucker 

 was well defined but none of the rest of the digestive system could 

 be distinguished. 



The mature Cercaria urbane nsis (Fig. 5) varies greatly in shape 

 being at greatest contraction nearly round, about 0.27 mm. in length 

 and 0.20 mm. in width and when extended 0.54 mm. long and 0.11 mm. 

 wide. When not in motion the tail is contracted being about 0.2 mm. 

 long and 0.05 mm. wide at its base. At times of greatest movement 

 the tail becomes attenuated to about one-half its usual diameter and 

 often reaches a length of 1.2 mm. It is weakly attached to the dorsal 

 mid-line of the posterior end of the body and tapers to a sharp point. 

 There is an inverse ratio between the contraction of the body and the 

 tail for when the body is most contracted the tail is most extended and 

 vice versa. 



In view of the great power of movement of Cercaria urbanensis 

 the histological structure of its tail is of considerable interest. Just 

 inside of the thin cuticula is a layer of circular muscles arranged as 

 strands separated by about twice their length from each other. Inside 

 of this is a very strong layer of longitudinal muscle fibers each 0.0026 

 mm. in diameter. Next comes a single layer of parenchymatous cells 

 somewhat irregularly elongated with nuclei 0.005 mm. in diameter. Ex- 

 tending the length of the tail and forming a core are two rows of long 

 cells which are close together and have their long axes parallel with the 

 length of the tail. These cells vary in size having a length from 0.028 

 mm. to 0.035 mm. In a cross section of a tail 0.075 mm. wide one of 

 these cells measured 0.026 mm. in thickness and 0.014 mm. in width. They 

 are full of heavily staining granules and their nuclei are 0.007 mm. 

 in diameter. There is nothing suggestive of a possible function for 

 these cells. Figure 9, a cross section thru a tail, shows the structures 

 described above. 



At the posterior lateral angles of the body are projections which 

 may be extended to aid the animal in locomotion. The tips of these 

 projections are made firm by structures which appear to be infoldings 



