372 FRESH-WATER BIOLOGY 



rhabdocoel intestine with pharynx, an oral sucker, and usually a 

 birth pore. The redia generation may be repeated and either 

 this or the sporocyst generation be eliminated, so that the cycle 

 may become modified in either direction. 



When development within the moUusk is completed and the 

 transfer to the adult host takes place, the transfer may be direct 

 if the moUusk is eaten by a suitable host. Yet this is not the 

 usual method since the ordinary cercaria possesses a well-developed 

 swimming organ in the tail which characterizes this stage and is 

 cast off when the larva reaches a new host or a place of encyst- 

 ment. This swimming tail is reduced in a few types and wanting 

 only very infrequently. In other cases various modifications, such 

 as bristles, folds, branches, lateral membranes, etc., increase its 

 functional value. 



The cercaria usually deserts the snail and actively seeks out its 

 primary host, but after reaching the outer world it may also encyst 

 on vegetation or force its way into a second intermediate host, an 

 aquatic arthropod or small fish, and encyst there. Here it rests, 

 a small immature encysted distome, until the tissue is consumed 

 by a suitable host, whereupon it is set free in the alimentary canal 

 and seeks its final location to attain after a period of growth the 

 adult form and full maturity. Life histories are known among 

 trematodes only in the most fragmentary way and the field offers 

 inviting prospects to the student. 



As appears from the account just given two free-Hving stages 

 recur in the development of most flukes. The miracidium nor- 

 mally depends on active migration through the water to reach 

 and infect the secondary host. In spite of the constant and 

 abundant production of such larvae their occurrence in plankton 

 or other fresh-water collections is not recorded. This may be due 

 to the extreme delicacy of the larvae which go to pieces almost as 

 soon as collected. 



When infected snails are kept in an aquarium, the cercariae 

 swarm out at certain times in great numbers and can be seen 

 cwimming actively about in the water. They conduct themselves 

 under such circumstances Uke true plankton organisms: protozoa, 

 rotifers, and entomostraca in the same aquarium. Yet although 



