TREMATODE SPOROCYSTS AND CERCARI^E. 205 



elongated. When much flattened they are usually long ovate, 

 the greatest width being at the ventral sucker, which is near the 

 middle of the length, thence they taper towards each extremity 

 but more towards the posterior than the anterior end. When 

 less compressed they may be long fusiform, or subcylindrical. 

 The body is covered throughout with exceedingly minute spines. 

 When the cercariae are fixed without pressure they may be ovate, 

 short fusiform, always thickish, frequently arcuate, the neck 

 especially having a tendency to be bent ventrad. The suckers 

 are nearly equal but there appears to be some variation. In 



Fig. 7. Sporocyst with cercarise, from Pecten irradians, life, under slight 

 pressure; two cercariae escaping from one end of sporocyst. Length 1.78 milli- 

 meter. 



most cases the ventral sucker is slightly larger than the oral. 

 The apertures of the suckers are variable depending on the state 

 of contraction when fixed. Frequently the aperture of the ven- 

 tral sucker is transverse. In one case it was elongated axially. 

 The aperture of the oral sucker was in most cases nearly circular. 

 The pharynx is subglobular and lies near the anterior border of 

 the ventral sucker. The intestinal rami were not distinctly 

 shown. The anterior spine is not easily seen in the mounted 

 specimens. The stained and mounted material does not usually 

 show more of the excretory system than the posterior vessel 

 which is very conspicuous in the living worms. In some of the 

 mounted specimens this posterior vessel was evident; in others 

 it could not be distinguished. In the flattened, oval individuals 

 it was not seen (fig. 5). In the cylindrical forms it was usually 

 visible (fig. 6). Rudiments of reproductive organs appear in all 

 the older cercariae. The most conspicuous of these are two lat- 



