o'uoke: larval trematodes. 173 



ures four-fifths the length of the body. A stylet protrudes from 

 the region of the oral sucker. This stylet measures .037 mm. long 

 and has a width at its base of one-fifth of the length. It tapers 

 to a point and has no enlargements anywhere along its length. 



The oral sucker is .04 mm. in diameter and the ventral sucker 

 measures .03 mm. In longitudinal section, both suckers open 

 into pouches wider than the external openings. There are two 

 layers of cuboidal cells in the wall of the ventral sucker. 



The muscular layers comprising the outer wall of the cercaria 

 average .003 mm. in thickness. 



The esophagus is exceedingly narrow, averaging .0015 mm. in 

 diameter. It is grounded by a ring of deeply staining cells cor- 

 responding to the muscular pharynx seen in other forms. The 

 esophagus broadens into a median digestive tract .03 mm. long 

 and .0125 mm. wide. 



Anterior to and dorsal to the ventral sucker are numerous uni- 

 cellular glands. These are probably stylet glands, as their posi- 

 tion is different from that of the cephalic glands described for 

 Cercaria inversa, for instance. No ducts could be found leading 

 from these glands. 



The only excretory tract found consists of irregular spaces 

 lying against the dorsal wall of the ventral sucker. In transverse 

 section of the tail, four large central cells can be seen surrounded 

 by a ring of muscle fibers. In some places the walls between 

 these central cells are made up of smaller narrow cells. 



The anlagen of the reproductive organs are in two masses dorsal 

 to the ventral sucker. These masses are connected by a narrow 

 band of similar cells. 



Cercaria haskelli is found in sporocysts. They are rounded 

 elongate sacs containing germ balls, developing cercarise and 

 mature cercariae. The sporocysts are from .25 mm. to .42 mm. 

 long and are from one-third to one-half as wide. 



Of twenty-three specimens of Planorhis trivolvis collected at 

 Cherryvale, Kan., October 16, five were infected with Cercaria 

 gregaria. The cercarise emerged from the snails by thousands 

 and had the peculiar habit of massing together in the water. They 

 would lose their tails and form such compact masses that they 

 could not be separated without tearing the tissues apart. These 

 masses contained from fifty to five hundred individuals each. 

 The cercariae remained alive in this condition for eight hours. No 

 encystment was seen, and very few cercariae remained for any 

 length of time without joining with one of the masses. 



