67 ] LIFE HISTOR Y OF TREMA TODES—FA UST 67 



The Diplostomulum (Fig. 49) measures 0.48 mm. to 0.63 mm. in length 

 by 0.17 mm. to 0.37 mm. in width, and about 30yu thick in the fleshly portion 

 of the body. The oral sucker is small but powerful, and is directed strictly 

 anteriad. Behind this oral region the esophagus is enlarged into the pharynx, 

 about 40;Li in section. Behind the pharynx is an equal portion of the esopha- 

 gus which is non-muscular, posterior to v/hich the ceca rise, spreading out 

 into a broad furculum. 



The acetabulum is large and circular; it is situated somewhat posterior 

 to the middle of the body. At times of extreme contraction the acetabulum 

 becomes narrowed antero-posteriorly, with a transverse wrinkling. This disc 

 measures 70/x in diameter. The primitive genital pore, situated just in front 

 of the acetabulum, has lost its connection with the genital system and has 

 become modified into a muscular sucking disc. 



The excretory, genital, and nervous systems have been treated on pages 

 37, 45, 54, as types for the hemistome larva. A comparison of these data 

 with v. Nordmann's observations on Diplostomulum cuticula, D. volvens, and 

 D. clavatum, and with the work of Blanchard (1847) on Eemistomum alatum 

 (Goeze) shows the fundamental conformities and differences of the excre- 

 tory systems of the group. However, the nervous system (Fig. 53) is worked 

 out thoroly in this paper for the first time in the Hemistomidae. The genitalia 

 bear a fundamental resemblance to those of the adult species, as described by 

 Brandes (1891), but differ in size, shape and position of the respective organs. 

 This difference may be accounted for in part by the immaturity of some of 

 the organs, but there are undoubtedly specific differences, such as the lamellae 

 of muscular nature in the genital pouch and the glandular cells emptying into 

 the pouch. 



The larva Cercaria ptychocheilus was taken from mesentery cysts of Pty- 

 chocheilus oregonensis Richardson, caught in the Bitter Root River in April 

 1915 in the vicinity of Stevensville, and Carlton, Montana. Thousands of 

 cysts were found. The cysts are much larger than the larvae (Figs. 50, 51) 

 and are filled with a limpid milky fluid which bathes the larva and serves as 

 a liquid cushion. The cyst is oblong, and flattened. It is composed of a 

 thin, tough membrane, and it is attached to the mesentery by a discoid annulus 

 in the middle of one of the flattened sides. Within the cyst the worm works 

 around and grows, so that it comes to fill the cyst in later Ufe. At frequent 

 intervals there is extruded from the excretory bladder a considerable quantity 

 of granules which pile up at the posterior end of the larva within the cyst, 

 but are soon dissolved and absorbed by the fluid medium. 



The encysted animal when placed in a normal saline solution soon increases 

 its activity and bursts thru the cyst. This rent usually occurs at one end 

 of the membrane. The larva then crawls out with a "measuring worm move- 

 ment. " After several hours of activity it settles down on the bottom of the 

 container and remains quiescent, altho slight mechanical disturbances activate 



