LIFE HISTORY OF FLUKE -WORMS. 



149 



about in its place and secreting a slime, a cyst is gradually 

 formed, with a spherical shell. This constitutes the " pupa " 

 state of the Cercaria. Steenstrup thinks that the Cercaria 

 casts a thin skin. In this state the body can be seen through 

 the shell of the cyst, as in Fig. 98, C, where the circle of 

 spines embedded around the mouth is seen. The encysted 

 Cercarias remain in this state from July and August until 

 the following spring ; and during the winter months, in 

 snails kept in warm rooms, they change into Distomas (Fig. 



98, D], the mature fluke differing, however, in some im- 

 portant respects from the tailless larvae. In nature they 

 remain from two to nine months in the encysted state. 



" Now," asks Steenstrup, " whence come the Cercariae ?" 

 Bo janus states that he saw this species swarming out from the 

 " king's yellow worms," which are about two lines long and 

 occur in great numbers in the interior of snails. From these 

 are developed the larval Distomes, and Steeiistrup calls them 

 the " nurses " of the Cercarise and Distomes. They exactly 

 resemble the "parent-nurses" (Fig, 98, A, and 100), and, 

 like them, the cavity of the body is filled with young, which 

 develop from egg-like balls of cells. Steenstrup was forced 

 to conclude that these nurses originated from the first nurses 

 (Fig. 98), which he therefore calls " parent-nurses." Here 

 the direct observations of Steenstrup 

 on the Cercaria echinata came to an 

 end, but he believed that the parent- 

 nurses came from eggs. The link in 

 the cycle of generations he supplied 

 from the observations of Siebold, 

 who saw a Cercaria-like young (Fig. 



99, B] expelled from the body of the 

 ciliated larva of Monostomum muta- 

 bile. Steenstrup remarks that " the 

 first form of this embryo is not un- 

 like that of the common ciliated pro- 



geny of the Trematoda, as they have *< nurse 

 been known to us in many species for a long time, and it 

 might at first sight be taken for one of the polygastric in- 

 fusoria of Ehrenberg, which also move by cilia ; whilst in 



B 



Fig. 99. Development of 



