202 



ANIMAL. PARASITES AND MESSMATES. 



it slightly on the stage of the microscope, we easily dis- 

 cover the ovary, the matrix, and oviduct full of eggs. 

 By isolating some of the eggs, and crushing them gently 

 to break the shell, we set free the worm (Fig. 44), quite 

 different from the mother (Fig. 43). The former has 

 two eyes surrounded by a ciliated mantle, and by means 

 of this ciliated envelope, the monostome swims freely in 



Fig. 43 Monostomum Fig. 44.— Monostomum mutabile. Ciliated embryo with 



mutabiie (adult). sporocyst and young cercarise, greatly magnified. 



the water. If we compress it slightly, we see that in the 

 interior of the ciliated covering, there is still another 

 animal, without eyes, without cilise, and of an entirely 

 different form, which in its turn encloses a whole progeny. 

 The embryo, having long ciliae in front, and in the 

 interior a sporocyst already full of young cercariae, is 

 shown in Fig. 44. It is this latter creature which the 

 ciliated embryo must confide to the care of others ; this 

 she puts out to nurse with some mollusc or other, until 

 it is fit to provide for itself in its turn. "We have still to 

 discover the train by which the parasite must travel, in 



