68 ILLINOIS BIOLOGICAL MONOGRAPHS [68 



it again. In a modified Ringer's solution ninety per cent of these larvae were 

 kept alive for forty-eight hours. 



It seems probable that Cercaria ptychocheilns is in an intercalated host. 



DISTOMATA 



Xiphidiocercariae (Stylet Larvae) 



The xiphidiocercariae are grouped together because of their possession in 

 common of a larval stylet. The writer believes that the features of the geni- 

 tal and excretory systems of the group are more fundamental characters 

 which will hold the members of the group together. 



Cercaria crenata Faust 1917 



Cercaria crenata is a delicate larva, with an ovate bodily outline and a 

 short lanceolate tail (Fig. 55). The body measures 0.25 mm. in length by 

 0.13 mm. in width, and the tail, 0.15 mm. to 0.16 mm. in length by 20^ to 

 30/i at the base. The entire body except the tail is covered with minute hair- 

 like spines. A large median spine, the stylet organ (Figs. 56, 57) lies in the 

 dorsal wall of the oral hood. This organ is about 30/^ long by Sfx in width at 

 its base. It has the general shape of a quill pen, with reinforcements at its 

 base and also in the distal portion toward the acute point. The distal third 

 of the stylet is bent ventrad about 20 degrees. There are two prominences 

 in the anterior portion of this organ, one where the shaft joins the quill and 

 a less prominent one-half way between this position and the quill point. 



The oral sucker is relatively large, 20/^ in diameter, while the acetabulum, 

 situated three-fifths the way from the anterior end, measures just half that 

 diameter. The tail is inserted in the posterior caudal pocket which has no 

 spinous projections. 



Cercaria crenata was found in large numbers in 13.6 per cent of Lymnaea 

 proxima Lea, taken from the springs at Fort Missoula, Montana, in October 

 1916. It occurs in oval sporocyst sacs, 0.5 mm. in length and 0.35 mm. in 

 diameter. At one end the germinal epithelium, is localized and from this end 

 the ova are proliferated. Only cercariae have been found to develop within 

 the sporocysts. 



When the cercaria is mature its breaks thru the wall of the sporocyst and 

 swims thru the surrounding medium. The tail is retained for a consider- 

 able time, and encystment is slow. This seems to indicate a considerable 

 period of free-swimming life. 



The internal structure of Cercaria crenata is such as to distinguish it readi- 

 ly from the other stylet cercariae. The excretory system is characterized pos- 

 teriorly by a subspherical vesicle, deeply crenate. It measures 20/x long and 

 30jU wide. Anterior to the bladder a bicornuate trunk empties into the ves- 

 icle thru a common median tube. The horns of the U are widely separated. 

 At the place where each main lateral turns forward there is given oflf a small 

 dendritic tubule, directed posteriad. Some distance ahead of the acetabulum 



