362 HENRY LESLIE OSBORN. 



run along below the cyst, their course entirely uninfluenced 

 by the presence of the cyst. A comparison of Fig. 9 with 

 Fig. 2 shows the difference of habit in this respect at a glance. 

 The preparation from which Fig. 9 was drawn shows five cysts in 

 an area half an inch wide by a quarter of an inch across. It is 

 removed from the right half of the body wall of the frog shown on 

 page 361. The whole piece was stained and mounted in balsam 

 and one of the cysts drawn in Fig. 9. The cyst is much larger 

 than the worm and there are spaces within unoccupied by the 

 worm. The worm in most cases is bent once, only the ventral 

 surface being turned outward. In some cases the bend is not 

 in the center, and then the longer end may be bent slightly as 

 in the one shown in Fig. 9. The frog cysts show the presence 

 of a rich network of capillaries spread out over their surface. 

 These are derived from vessels which come from large and con- 

 spicuous vessels running in the space between the peritoneum 

 and the muscle layer. The ordinary surface of the peritoneum 

 does not possess these capillaries, which are evidently a growth 

 developed as a result of the presence of the cysts. 



The specimens of Clinostomum seen in the frogs are all virtually 

 fully matured, having the full size of the heron specimens. -Their 

 inner organization too is identical in appearance with that of 

 specimens from the fish, and excepting as to the vitellaria with 

 that of the heron. There is thus no room for an hypothesis as 

 to the frog being the first host and the medium by which the 

 fish is reached, which hypothesis we might be tempted to frame, 

 knowing that the frog is one of the foods of the bass, and other 

 predaceous fishes. We are thus left to suppose a first host, 

 possibly an invertebrate, followed by a second intermediate host, 

 the fish which serves as a medium of transmission from the un- 

 known first host to the heron. The case of a second host which 

 serves to transmit a parasite from a first host where it develops 

 to a final host where it matures and where sexual reproduction 

 takes place has been recognized for a number of trematodes. 

 A list containing 28 species is given by Braun (93, pp. 864-866), 

 in which this species is not included. In some of these the frog 

 or one of the fishes serves as the medium by which the final 

 host is reached. 



