124 EXPLANATION Ol 



PLATE IL 



Figs. 1-8. The Development of the Fluke, Distoma 

 pacifica. 



N.B. All the figures, except figure 1, which is 

 taken from Siebold's Memoir on Monostomum muta- 

 bile, in Wiegmann's Arcliiv. f. Naturg., 1836, are 

 from my own drawings. 



Figs. 1-2, Show the ßrst generation in the development of 

 the distoma, — the " parent-nurses." 



Fig. 1. First stage in the development of Monostomum 

 mutahile ; 1 a, an ovum, through the sheU of which 

 is seen the developed embryo, which is represented 

 in 1 h, after it has quitted the egg, and is swimming 

 about at liberty ; 1 c, and 1 d, are the same indi- 

 vidual, after its metamorphosis from an active form 

 into an inactive sluggish creature, which is not itseK 

 a mother, but nourishes within it a progeny from 

 which, in the third generation, a parent animal does 

 proceed. I call this a ''parent-nurse^' {grossamme). 



Fig. 2. Animals which inhabit fresh- water snails, as 

 entozoa, and after a few generations become true 

 Distomata, or resemble them ; on which account, in 

 the text I have termed them their ''parent-nurses." 

 From the analogy in the external form, with the 

 creatures which proceed from the ova of Monostomum, 

 and are afterwards metamorphosed, it must be as- 



