499] LARVAL TREMATODES—CORT 53 



POLYADENOUS CERCARIAE 



In eighteen per cent., of 170 specimens of Planorbis trivolvis col- 

 lected from the drainage ditch north-east of Urbana, Illinois, the livers 

 were filled with elongate, cylindrical sporocysts very much twisted 

 together. The sporocysts were not branching but it was very diflficut 

 to trace out the individual sacs. When this could be done they were 

 found to be of about uniform caliber and various lengths (Fig. 66). 

 Two that were measured were 1.48 mm. and 1.9 mm. in length and 

 varied in diameter from 0.13 mm. to 0.17 mm. The walls were thin 

 and contained flecks of orange pigment, which were very dense in the 

 oldest specimens. Many of the sporocysts contained large numbers of 

 actively moving cercariae which would escape and swim about freely 

 when the liver was teased apart. The wall of the sporocyst was com- 

 posed of a layer of pavement epithelium with flattened nuclei. No 

 thickenings were found in the wall and no traces of germ gland, altho 

 small germ balls were free in the cavity. Cercariae at all stages of devel- 

 opment were found in the sporocysts (Fig. 65). I propose to call this 

 species Cercaria isocotylea from the fact that the acetabulum and the 

 oral sucker are very nearly equal in size. 



Cercaria isocotylea (Fig. 68) is oval elongate, slightly pointed 

 anteriorly, and of uniform width from the region of the pharynx back 

 to the acetabulum. The length and the width varied with the contrac- 

 tion state within rather wide limits. From the measurements of mounted 

 specimens of moderate contraction, the length averages 0.17 mm. and 

 the width 0.06 mm. The cross section is oval and the thickness a little 

 greater than half the width. The tail is small in proportion to the size 

 of the body and set in a groove on the ventral side of the posterior end. 

 When contracted it may be less than one-half the body length, but when 

 the cercariae is swimming it may be extended to greater than that length. 

 Under ordinary circumstances it has an average length of 0.01 mm. 

 and a width at its base of 0.02 mm. 



Cercaria isocotylea moved actively both in open water and on a 

 substratum. When swimming it turned so that the ventral side was 

 up, the body was contracted and bent slightly ventrad. The tail became 

 much extended and lashed rapidly backward and forward. It did not 

 however have the power of moving the animal definitely in one direction 

 for any length of time, and locomotion was very erratic. Whenever 

 while swimming the cercaria came in contact with a surface, the tail 

 ceased its lashing and the body began to stretch and reach around until 

 the oral sucker could obtain a hold. Then the animal would creep along 

 with the aid of its suckers. Sometimes after the cercariae had come 



