LIFE BISTORT 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 Pig. 98, C, where the circle of 

 ■spines embedded around the mouth is seen. The encysted 

 Cercariae 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 iiake difEjriag, however, in some im- 

 portant respects from the tailless larvse. In nature they 

 remain from two to nine months in the encysted state. 



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

 TBojanus 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. Prom these 

 are developed the larval Distomes, and Steenstrup calls them 

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

 Tesemble 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. 

 S9, 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- Fig. 99.-Deveiopment of 

 like that of the common ciliated pro- ^°ZSh™; te"i't"S.' 

 geny of the Trematoda, as they have Js, nuree.-After siel,oid. 

 I)een 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 



