322 SYNOPTIC COLLECTION. 



new sporocysts which differ from the original sporocyst 

 by having a mouth with a sucker and an intestine, but 

 no anus. The new sporocysts are called Rediae or 

 "parent-nurses" (fig. 3). The wall of the sporocyst 

 breaks and the Rediae migrate to the other organs of 

 the snail, especially the liver, and there develop into 

 Cercariae (fig. 4) which are provided with tails (fig. 5, a 

 single Cercaria showing the anterior and ventral sucker). 

 In this stage the Cercaria works its way through the tis- 

 sues of its host and seeks the water, where, however, it 

 stays only a brief time. It fastens itself to the plants on 

 the shores of the pond and becomes encysted. In this 

 state it is swallowed by sheep ; in the stomach of this 

 animal the cyst is dissolved and the freed worm, finding 

 its way to the liver (fig. 6, a young fluke in which the 

 intestine has begun to branch), develops into the sexu- 

 ally mature Distomum hepaticum (fig. 7). The eggs find 

 their way through the bile duct to the intestine and 

 escape. 



One of the most remarkable instances of acceleration 

 in development is found in Gyrodactylus elegans (PI. 797). 

 Not only do the eggs of this worm develop into sexually 

 mature embryos while within the body of the parent, but 

 the embryos thus formed also develop embryos which in 

 turn produce eggs in process of development, so that four 

 generations are represented by a single specimen. 



Certainly no better example of acceleration in sexual 

 maturity need be offered than these viviparous " young," 

 as they are called, containing offspring which are them- 

 selves in the act of developing eggs. 



The figure shows the adult with one embryo enclosed. 

 The esophageal bulb of the .mother and embryo are seen ; 

 also the posterior sucker with the two great hooks. 



