359 



observed by Nitzsch and afterwards by Siebold. 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. 164, C, where 

 the circle of spines embedded around the mouth is seen.* The 

 encysted Cercarise 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. 164, D) the ma- 

 ture fluke differing, however, in some important respects from the 

 tailless larvae. In nature they remain from two to nine months in 

 the encysted state. 



" Now," asks Steenstrup, " Whence come the Cercaria? ?" Boja- 

 nus 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 Distomas, and Steenstrup calls them the "nurses" of the 

 C'ercarhe and Dislomata. They exactly resemble the "parent 

 nurses" (Fig. 164, A) 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. 164) 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 observa- 

 tions of Siebold, who saw a Cercaria-like young (Fig. 165, B) 

 expelled from the body of the ciliated larva of Monostomum 

 mutabile (Fig. 165, A, a, nurse developing from ciliated larva ; m, 

 mouth ; b, eye specks). Steenstrup remarks that " the first form of 

 this embryo is not unlike that of the common ciliated progeny of 

 the Trematoda, as they have been known to us in many species 

 for a long time, from the observations of Mehlis, Nordmann and 

 Siebold, and it might at first sight be taken for one of the poly- 

 gastric infusoria of Ehrenberg, which also move by cilia ; whilst 

 in the next form which it assumes the young Monostomum bears 

 an undeniable resemblance to those animals which I have termed 

 'nurses' and 'parent nurses' in that species of the Trematoda 

 which is developed from the Cercaria echinata" 



Thus the cycle is completed and the following summary of 



